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ejlindahl 29-12-2015 01:11

Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
1 Attachment(s)
I went to put new gear oil in my 3.3 Mercury lower unit and the oil would not go in the lower "fill hole". (I did remove the top "oil level" screw plug to allow air and/or oil to vent out the top.) When I removed my oil pump fill nozzle from the lower fill hole the oil sprayed back out under pressure. Closer inspection revealed the top "oil level" screw hole was clogged with a small amount of slushy, oil and water soaked, partially crystalized, salt. (The old oil I had previously drained was perfect/not milky. It drained very slowly over several days in my cold garage.) I removed the impeller pump housing (see #17 in the schematic below) that contains the bearing cup and bearing (#13) and rubber sealing grommet (#16) and cleaned out the slushy salt crystals in the screw hole. I found that the hole goes straight through to the bearing cup and that the rubber grommet blocks the passage of oil/venting air from the gear case. It seems that this grommet (and the #13 bearing race) should have holes in them to allow venting air or oil to come out from the gear case, thru the bearing and out the "oil level" hole. I am thinking that who ever installed the rubber grommet did not align the grommet's hole, (if there is one), and possibly the hole in the bearing race, (if there is one), with the "oil level" screw hole. (When I look in the hole I can see part of the bearing race as well as the rubber grommet - they completely block any possibility of air or oil to vent out).

I certainly could take the assembly to a shop to press the bearing out of the cup to allow me to inspect and properly align the grommet and bearing race. Or I could take a needle and try and poke through the rubber grommet and into a corresponding hole in the bearing race, (if there is one). But neither is a step I want to take before I seek advice from the forum.

Does anyone have experience with a problem like this or is familiar with the pathway of venting air and/or oil from the gear case to the upper "oil level" hole? My engine is a 1996 Merc 3.3 hp with a forward/neutral gear shift.

This link may be easier to read than the schematic below

Thank you,


mariner36bob 02-01-2016 10:29

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Should only have to vent the gear case once, at the time of filling with oil. Fill as normal, give a couple extra pumps to burp air and close with screw. No "in service" venting is needed or desired, it's a sealed reservoir....

Parts 2, 13, and 15-23 are all working together to keep water in pump housing and oil in the lower end.

Did you by chance determine how much oil came out? You may find some oil/sludge did not drain..

Can you reverse the fill procedure and fill from top to clear it out?

If you can get a little oil in it from the top hole then warm it up, put a little more in, warm it up again, little more etc. until your full enough to heat it up proper. Then drain all that crap oil out warm. I bet it will take the salt clog with it...

Draining cold likely left enough oil in it to not be scared trying to warm it up "empty". Does that make sense?

You've got to get it warm...

Worst one I've ever had was completely filled with peanut butter consistency emulsified oil. Only 1or2 ounces drained out, somewhat clean, the remainder was a white/grey sludge that required forcing it out the bottom hole via low pressure air into the top hole. Gross.

I bet you have a little salt in the vent screw hole is all.

ejlindahl 04-01-2016 11:23

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Thanks for the reply Mariner36 Bob. Great hint about measuring how much oil drained out and checking for sludge. I have removed part #17 and can see down into the gear case and it is pristine, no sludge, just bright clean metal with a remaining film of clear, light brown, gear oil, as if it just came out of a factory sealed oil can. Nice to see that on a 20 year old engine.

However, my problem is (when adding new gear oil, with the upper vent screw removed) there is no way for air or oil in the gear case to vent out through the water pump housing (#17) (and then through to the vent screw hole). The vent passage hole in the pump housing (#17) is mechanically blocked by the outer perimeter case of bearing #13 and the oil seal, (#16) both of which are pressed into the housing #17. When I Iook into this vent passage hole I can see the shiny metal outer bearing case and the soft black rubber seal totally blocking the passage, (there is definitely not a plug of crystalized salt for instance). Just to be sure I jammed the plastic tipped nozzle of my compressed air gun into this vent passage hole and at #150 psi no air passed by the bearing and seal to come out the other side, (this is an athwartship vent passage. I am not referring to the vertical axis where the drive shaft runs through the bearing and where there is and should be a tight seal of the rubber seal #16). The blockage of compressed air proves that venting air (or oil) can not get out of the gear case to the vent screw hole. There must be an alignment issue: I think there is (or should be) a small hole in the outer bearing case that is not lined up with the vent passage hole in the water pump housing, (#17) (sloppy manufacturing??). A small hole in the bearing case would allow oil or air to vent. I am hoping there is someone who is intimately familiar with this particular model of engine (and possibly specific to this inaugural year of the forward/neutral gear shift) that knows how the passage of venting oil/air is supposed to go through this pump housing/drive shaft bearing/seal assembly.

Any one out there familiar with my specific situation please chime in. If not my solution is to button it all back up then fill the gear case with oil before I install the aft sealing collar of the prop shaft. And I will take M36Bobs advice to heat the oil and gear case up to around 100 degrees F to make the oil fills every nook and cranny.

PS: M36bob mentioned a little salt in the vent screw hole. I did have some of that which I previously mentioned as “slushy salt crystals” but I have cleaned that and everything else thoroughly including the hole in the pump base/housing #17 that lines up with the vent screw hole. Also, when I removed the water pump housing (#17) from the drive leg/gear case the drive shaft also came with it. So now I have to disassemble the prop shaft assembly to reinsert the pinion gear onto the driveshaft. A shop manual I later read says you can avoid this by holding down the driveshaft while removing #17 from the gear case. However in my situation the drive shaft seemed pressed onto the bearing and the bearing is pressed into part 17 and in any case it was very hard to remove the very tight fitting 17 from the drive leg/gear case. So I don’t think it is actually possible to do this.


mariner36bob 04-01-2016 17:09

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
I hope there's a Merc tech on here to help you but I am 100% confident you're looking in the wrong place.

Should be no reason to disassemble pump or remove shaft for oil change, but I get it....find the problem. :)

No oil passes through the water pump. Period. Parts 15-23 are all working together to keep water in pump housing and oil in the lower end. (My mistake above post, 2&13 are bearings not seals).

Oil in lower unit simply keeps drive shaft spline to prop shaft bearings lubed, it has no need to go up.

Just to be sure, you've removed both screws #6 right? Both are #3 flat head. No offense I've done it myself, removed the "wash down" bolt thinking it was oil vent...

This sentence makes me ask if you've accidentally removed bolt #8 or #3 which are 10 mil hex head bolts:
"I have cleaned that and everything else thoroughly including the hole in the pump base/housing #17 that lines up with the vent screw hole."

Hope any of this helps.

ggray 04-01-2016 20:28

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Well, this is real interesting.

I have a Merc 3.3 that hasn't been used in some years. But some years ago, I had the same problem. Drained the oil, then couldn't fill through the bottom hole.My motor may be a bit older than yours, also with the F-N shift.

Since it was not being used much, I just forced some oil in. Been a long time and can't remember what I did. Maybe repeated pumping of oil in the top hole, motor lying on its side. Just can't remember.

Again, since it wasn't being used much, I didn't analyze so much as you have.

Seems like now is a good time to study further and we can compare notes.

One other thing. For a short period of time concurrently with ownership of the Merc, I also had a Nissan 2.5. Virtually the same motor. The Merc used a WHOLE lot more fuel, and I finally discovered (or thought I discovered) that the Merc had a final drive that was geared down lower, which seemed strange to me, although it would explain the higher fuel use. Same prop, more horsepower, and turning slower?? Specs called for same rpm, so WTF?

Some time later, when I no longer owned the Nissan so couldn't confirm my earlier "findings", a mechanic on one of these forums with knowledge of these motors said that all these motors had the same final drive. No ifs ands or buts. I found it hard to argue, so I questioned my ratio determination. Still an unanswered question.

But I'd be curious what final drive ratio others find in their Tohatsu clones, of which their were many.

ejlindahl 05-01-2016 17:08

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
5 Attachment(s)


My 3.3 is 1996 which I think is the first year they made it with a neutral gear. I guess previous to that it was just direct drive/forward only. Perhaps my drive leg vent blockage was a “teething” problem that was corrected in later models as I haven't seen any reference to it other than you and maybe one other person that described the same inability to fill the leg. (I think this person’s shop just took off the rear prop shaft cover and dumped in oil to fill it, which is what I will do if nothing else.) (I have had one other teething problem which is the unfinished/unmilled female bearing surface of the plastic shift lever pivot)

I don't know about the gear ratio issues but 2.5 vs 3.3 is 32% more HP so it would use that much more gas. Also, I've seen on this forum a post that only difference between the two is the the slot that the throttle lever travels thru. On the 2.5 the slot in the face plate is simply shorter than the 3.3 and if you file the slot longer so the throttle can be pushed lower down on its travel, the motor will rev up faster and produce more power. This is just internet hearsay so don’t hold me to it.

OK so I finally took some pictures of the issue they are below but here is a link with further description on Photo bucket photo bucket: I wish I knew how to insert the photo individually below each of my descriptions but with my lousy skill set I can't figure it out

If anyone can tell me how to correct this problem of the top gear case vent being plugged (perhaps with a bearing with a small [vent] hole in the outer case), please let me know. Ideally I’d like to find a Mercury mechanic of that era who came across this problem, circa 1996 or a little later. Surely many of these engines were serviced by shops and someone would have found the same problem as mine.

First photo below is of the assembly. The shift rod has been removed. You can see the water pump housing on the drive shaft. The drive shaft is stuck or more likely pressed onto the bearing and the bearing is pressed into the lower cylindrical part of the pump housing. This housing is the bottom part of the water pump.

Next photo below is the top of the drive leg where the water pump housing fits into. You can see the small vent hole in the circular chamber where the drive shaft runs through. One solution would be to gouge/router/grind a channel down from the vent hole to and past the lip in the chamber where the round part of the pump housing goes. This would allow air and oil to vent up to the hole. There is such a tight fit of the round part of the pump housing and the circular chamber above the lip that no oll can pass this way without some kind of channel.

Next below is the pump housing above the drive leg. You can see where the hole in the housing that holds the drive shaft bearing and seal will line up with the vent screw hole on the side of the drive leg. However the drive shaft bearing and rubber seal that is pressed into the housing completely blocks any passage of venting oil or air, even 150 psi of compressed air!

Next below is the pump housing partially inserted into the drive leg, again with the holes lining up.

Last photo below is a close up of the hole in the pump housing. If you click on the image you can see the glint of the outer case of the drive shaft bearing on the bottom, inside the hole, and part of the rubber seal above it. The bearing and seal are press fit into the housing and they completely block the passage of venting air and or oil. This is my problem that I want to fix. I put my compressed air gun in the hole and even at 150 psi no air got thru there! sigh. The other, small "weep hole" just allows incidental seawater to drain out of the interior of the drive leg, it is not part of the water pump/cooling water system.

The preview function on this forum does not display inserted pictures so I don't know if they will actually show up. So I will will add them here as attachments and you can figure it out what they are from my text above. If allowed I'll edit out any redundancy afterwards

I'll repeat: If anyone can tell me how to correct this problem (perhaps with a new bearing with a small [vent] hole in the outer case or maybe there is a hole but its just not aligned properly), please let me know. Ideally I’d like to find a Mercury mechanic of that era who came across this problem, circa 1996 or a little later. Surely many of these engines were serviced by shops and someone would have found the same problem as mine.

Thank you for any help on why this vent is not working.

mariner36bob 05-01-2016 18:38

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
You've been very polite in not telling me to go f* myself.
My replies were made with the utmost sincerity and good intent.
However your photographs are very helpful in showing I have no idea what I'm talking about in regards to this outboard.
That's a head scratcher for sure. And I wish you the best of luck with it.

ejlindahl 05-01-2016 19:02

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Hi M36Bob,

You made me laugh. I hadn't take your comments that way at all. Like I said you had a couple good ideas. But I gotta figure this out. I'll take it to a local shop and see if they'll advise me. I'm happy to cough up 15 mins of shop time for a good lead or solution from them.

Blue Crab 06-01-2016 05:43

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
My 3.3 awaits the answer. Thanks.

HappyMdRSailor 06-01-2016 06:02

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude

Originally Posted by Blue Crab (Post 2007780)
My 3.3 awaits the answer. Thanks.

I would continue with recent methodology.... install inverted and let the oil drain from the fill hole to the vent...

tinkrman69 06-01-2016 08:20

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
I'm not sure I get what the problem is. Is the bearing over heating and seizing up? lack of lubrication? in "drive leg" are you referring to the lower unit? there are two plugs in the lower unit. the one lowest on the unit is to drain the gear oil and the point to fill it from the upper plug is to vent during the filling process. it is also to indicate when it is full. filling past this point will cause excess pressure and cause the seals to shaft and output shaft seals... If you aren't having problems with the bearing and you can't fill the lower unit with oil look for the second plug. there will be a slotted screw head just below the gear housing section of the lower unit and one just above the cavitation plate on the lower unit. remove both, drain, press nozzle of gear lube tube in the lower hole...having the plug handy to reinsert when the nozzle is removed,,,and fill till the oil comes out the top hole.

The drive shaft is not "pressed" into the upper bearing. if it is seized into there bearing I would look to the seal. if it goes water will leak down to the bearing and corrode. I've seen it happen in such a small amount that it wasn't evident in the oil. in whack case pull it and replace the bearing and seal. Be carful in pulling the drive shaft. some shafts are just splined into the helix gear in the lower unit and can can just be pulled out if you move the lower unit or rotate the output shaft while its out the gear will become misaligned. some shafts have a 'C' clip on the end of the shaft to retain the gear. in that case you have to pull the put put shaft and bearing carrier to access the bottom end of the drive shaft. Lower units are pretty simple and haven't changed much since Evinrude made the first one back in Garry Coopers time. I don't know your age, but he was a famous actor back then and was the face for the Evinrude adds.

Keep it simple man and don't look for problems that aren't there.

ejlindahl 06-01-2016 17:09

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude

Thank you (and others) for the replies.

I understand that there is an upper vent hole and lower fill hole in the lower unit (which I sometimes refer to as the drive leg).

The problem is that venting air (or excess oil) can not get thru to the upper vent hole. This vent hole is blocked/plugged by the upper drive shaft bearing and its rubber seal.

If you look at the last picture in my post #6 you will see 2 holes in the water pump housing. This housing also houses the upper driveshaft bearing and its seal. The larger of the 2 holes lines up with the upper vent hole in the lower unit. (Look at the pictures in post #6 and you can see it lining up). If you click on the last picture to show more detail you will see the glint of shiny metal that is the outer case of the bearing that is pressed into the housing. You will also see a bit of the black rubber seal in the hole. That seal and bearing case is what is blocking the passage of venting air (or excess oil) when I attempt to fill the lower unit from the bottom hole. I put 150 psi compressed air into that hole and no air went through it! I am now of the mind that there is some sort of perforated spacer missing between the seal and bearing. Or the seal could be installed improperly and the spacer is above the seal…. A correctly installed spacer would allow venting air (or oil )to rise up thru the ball bearing and out the hole in the housing and then on through to the upper vent screw hole.

The schematic in my original post shows this housing as #17. Click the word “link” in the sentence just above the schematic to get a better look. There is an expanded view of the seal (#16) just to the left of the seal on the schematic. This expanded view has poor resolution but it has an odd shaped squiggly thing on the left side of the view. Its possible that this squiggle is a spacer that will allow air and oil to flow thru the vent hole. I have seen no mention of a spacer in the Clyner or Seloc books nor in this factory exploded view nor anything else I’ve read about this motor but there must be one to allow the lower unit to vent.

If there was a spacer then you would not see part of the black rubber seal through the hole in the housing in post #6. Instead you would see the spacer which would allow vent air and oil to pass through.

So, does anyone know if this seal has an integral, squiggly looking, perforated spacer? I’ll ask to see the seal at my local Merc shop but I’m sure they don’t stock this part.

Read back through my posts. I think I have explained the problem pretty well.

Thanks for any advice

PS for Tinkerman69: From the reading I’ve done I think you are right; the drive shaft is probably not pressed into the bearing. However when I pried the housing (#17) out of the lower unit the drive shaft came with it so it is at least a very tight fit. At a minimum I will have to do some light hammering with a wood block to get the drive shaft out of the bearing. There is absolutely no sign of corrosion anywhere. The ball bearings, races and gears in all areas of the interior of the lower unit are bright shiny metal.

When the shaft came out of the lower unit the helix/pinion gear dropped down into the hub cavity. It does not have a “C” clip. So I will have to open that up. The books describe holding the pinion gear in place while I lower the shaft spline on to it. I’m not sure if I have to do this with the lower unit upside down or right side as I will have to let go the pinion gear to reinstall the prop shaft. (I don’t know if the pinion gear is below the prop shaft or above). I’ll figure that out when the time comes.

I’m 63 but didn’t know Cooper stumped for Evinrude. He must have been a yachtsman like other Hollywood types.

I appreciate yours and everyones efforts to help me solve this riddle.


ggray 06-01-2016 20:49

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
I played a short while with this issue tonight, and not really sure what is going on.

I wondered if in the past, the end of the spout on the quart bottle was cut square and just the right length that it was mashed up against something in the final drive that prevented oil from entering, but that's not the case. The spout I used tonight had an angle cut on the end. It accepted oil at first...I could hear snapping and popping of air bubbles in the oil. Until the air passed, and then it was like hitting a brick wall, wouldn't accept any more oil.

I have seen the lower unit removed from only 3 different motors (so no expert) and it's been a while. But I don't recall seeing anything like what your pictures and the diagram show. As has been posted above, I thought the final drive was a separate, sealed unit and oil in it didn't travel further up.

(As an aside, about 30 years ago someone stole a Mercury off my boat on a mooring. Then about 15 years ago, the guy rebuilding our dock said he had found a Mercury in the mud. The guy who stole my motor had dropped it and didn't even try to recover it! The mud had protected a lot of the motor, but it did have some barnacles. I asked a mechanic if anything would be salvable, and he said likely only the lower unit as it was TOTALLY SEALED. I was able to sell it.)

Anyway, I turned the 3.3 upside down, and baby fed oil in the now top hole, a bit at the time until I think it's full. I've turned it right side up, and tomorrow, after the oil settles, hopefully, I'll open the top hole and let it drain for a day.

We still haven't answered the question.

ejlindahl 06-01-2016 23:20

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude

The gear oil continues 5 or 6 inches up a ~1.5" round drive shaft chamber in the lower unit to the level of the upper drive shaft bearing. It lubricates this upper bearing so its important to completely fill the chamber.

There is a seal just above the bearing and keeps water from the exhaust housing above the lower unit out of the contiguous, oil filled, prop shaft and drive shaft chambers. The top vent hole that should be open when you fill the chambers from the lower fill hole is at the level of this top driveshaft bearing and that is why its important to keep filling oil in from below til it dribbles out of the upper vent hole. Unfortunately the bearing and seal block my top vent and thus, even if I could get oil in the unit, it is impossible to know if I've filled the it to the correct level. I'm doing further investigation tomorrow.

ggray 07-01-2016 08:29

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
This seems a strange set up. Again, I've little experience working on lower units, But I haven't seen one like this. How does that top bearing get lubricated if it is that much higher than the oil level? Splash?

I'll soon be removing the lower unit on a 30hp Tohatsu to see how it may be different (need to install a new impellor). I want to see if there are any shaft bearings installed like what your diagram shows.

I appreciate your informing us of what you are going through. I've got too many things (boats) calling for my attention to get myself heavily involved in a motor that I don't need at the moment.

But I may take a moment to determine the gear ratio of the lower unit, and hope I can find the ratio of other Tohatsu clones. I still have a nagging suspicion that there is a difference.

ejlindahl 08-01-2016 20:30

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
1 Attachment(s)

The top of the bearing is at the level of the bottom of the vent hole. So, normally, if you fill the lower unit until oil is dribbling out of the top vent hole then this bearing is totally submerged in the gear oil. The rubber seal is just above the bearing and may get a little splash of lube as well.

However, in my lower unit, as I've explained in my other posts, the seal and bearing are sitting tightly against each other and thus completely block the vent hole.

I took the parts pictured in my previous post to the local Merc shop. The mechanic didn’t remember the intimate details of this model. Doubtless he’s worked on many of them over the years, though. He thought that the rubber seal had slipped down the driveshaft an 1/8" or so from its proper seated position and that is why it sits tightly against the bearing and thus blocks the vent passage. I think the seal was never pressed into its seat all the way to the stop and when the bearing was pressed in behind it, it snugged up tight against seal resulting in the blockage.

Now I have to decide if I want to try and get the drive shaft off of the bearing and then get a special (inside) puller to remove the bearing and then dig out the seal to see if the mechanic’s theory is correct. The other option is to throw it all back together with a new impeller and fill it as much as possible with warmed gear oil from the prop shaft end and just run with it for a season or forever….. I’ll let you know what I do, though I am leaning away from any more disassembly.

PS: In the middle paragraph of my post #12 I mentioned a mysterious squiggly figure, #16 in the exploded diagram of the assembly, (look at the first post to see the diagram). The mechanic said that is a section view of the seal. He explained that these ring-like seals are of the cup variety. I have seen similar seals in wheel bearings so the light immediately went off in my head as to what this seal looks like. This type of seal has a metal channel that forms a cup shape. Imagine you cut a slice out of an o-ring lay it on the table……you’d have just a small round disc. A similar slice out of this ring-like bearing is like an upside down “u”. The “u’” is steel and the inside and part of the outside of it is lined with rubber to form the seal against the spinning drive shaft. The squiggle in the diagram is this slice (section view) of this seal. The taller white part to the right of the squiggle represents the vertical drive shaft. The mechanic explained that the diagram simply is showing that the cup or “u” of the seal must face down when you install it in the water pump housing. I guess thats what being a pro means, he’s been there, seen that……… I’ll try and post a cut-away image of a similar seal from the internet but just google wheel bearing seal and you will see many or maybe this link will work.

PS#2. Ggray, good idea to see what the gear ratio is. Seems like many are interested in this. I gotta remember to do that as it should be easy with the lower unit removed.

Man this is turning into an epic for a small $250 motor, but it runs well and I love everything else about it.


tinkrman69 08-01-2016 22:34

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
if you fill the oil till it comes out of the hole at the top of the lower the pump'll blow the seals. lower units are designed to be filled to just above the pinion gear on the drive shaft, hence the upper screw on the housing. forget the assumed problem and put it back together and go boating.

as the shaft and gears turn the oil is forced uo that shaft log and saturate that bearing. that seal your are talking is a double seal sealing the water from one side and the oil on the other. if you are seeing a "squiggly thing in a drawing it's probably the spring that is in the inner side of the seal. also I have never seen a bearing pressed onto the drive shaft and any lower unit. no mater what size. even on High performance/racing outboards.

Sounds like your just looking for a problem. if it aint broke don't fix it.

ejlindahl 09-01-2016 00:34

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
2 Attachment(s)

Thank you for taking the time to help me out. Below is a thumbnail of the Merc Shop Manual. Although I understand about blowing a seal, I am inclined to follow its directions regarding how much oil to put in the lower unit.

The problem I have is real and it is that the oil level hole, (vent hole), you see in the attached thumbnail is plugged by a presumably improperly positioned upper drive shaft bearing and seal. This means I can not fill the lower unit in the normal way nor can I tell when it reaches the proper level. The need for special tools and the extra hassle to remove the bearing and seal (to truly find out and hopefully correct the problem) is a big deterrent. Its probably been this way for 20 years.

I’m pretty much to the point of doing as you suggest: Put it back together, Tipping it upside down and filling it with around 2.5 ounces of gear lube and call it good. (The manual says to put 3 fluid ounces in the non shifting lower unit and for the shiftable engine it says 2.5.)

Regarding the bearing being pressed onto the drive shaft: The lower bearing is obviously not because the drive shaft just slid out of it. The second attached thumbnail picture shows the upper bearing being hammered down onto the drive shaft. Not sure if this means “pressed on” but it sure is stuck on mine.

Thanks all for the help I think I have my game plan and it does not include fixing the problem,…. maybe later.

ejlindahl 09-01-2016 10:04

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Can't edit the above post but when I said my solution/gameplan is to tip the lower unit upside down and fill it with 2.5 oz of gear oil I meant filling it with warmed oil in a large syringe thru the lower fill hole leaving space around the tip of the syringe so that air can vent out the hole. The normal way is as described in the thumbnail picture in my previous post.

Regarding someones question about the gear ratio. The shop manual says the 2.2 hp is 1.85:1 and the 2.5, 3.0 & 3.3 hp is 2.18:1.

Unless I disassemble the whole mess in the future to find the problem I think we're pretty much done with this issue. Hope it helps others.

donradcliffe 09-01-2016 11:12

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Rather than trying to dribble warmed oil in through the lower plug, put the oil in a squeeze bottle with a tube or straw on the end. Invert the engine and get the tube down as far as you can and then squeeze.

I agree that the shaft bearing is out of place, but as long as it isn't rubbing on anything I'd fill with oil and use the motor.

tinkrman69 10-01-2016 08:35

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Hi ejlindahl

It appears I haven't worked on this particular model. But with the blow up it looks quite simple.

is the lower bearing #2 seated in the bottom of the shaft log? is the spacer No. 11 installed alt he way and is it clean with no burrs? do you have the drift tool to set the upper bearing? it will also put the bearing in the proper place on the shatf, so it will rest on the spacer and not allow the shaft do set too deep into the pinion gear No 10 at the bottom. if you don't have the tool then install the lower bearing into the housing and install the shaft with the spacer and and pinion gear in place,making sure the shaft doesn't stick too far through the gear, it should just be visible from the gearbox side. measure or mark the shaft at the top of the spacer. then remove them and use a pipe bigger than the shaft and small enough to rest on the inner race of the bearing No 13 and tap it onto the shaft using lubricant on the shaft, till you've reached the mark on the appropriate side of the inner race of the bearing. place it in the housing and align the gear at the bottom, put the seal No. 16 in the pump housing No 17 making sure to seat it all the way in and that it is in the right direction. you can use the old seal to drive it in. install the housing, install the water punp and fill the lower unit with oil in the upright position. sense the need for a special tool to set the bearing there probably won't be a shoulder on the shaft to seat the bearing. this being said placement of the bearing during assembly is critical. the method above should get you where you need to be. during the measurement stage in the beginning make sure the shaft doesn't stick past the gear. it shouldn't stick past more than 1/32 of an inch. you should see wear marks on the splines to indicate where the pinion rides.

ejlindahl 10-01-2016 10:46

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Hi Tinkrman,

Thanks for all the details. You've mentioned some nuggets in there that are not covered in the shop manual. Maybe I'll get around to fixing it right as you've described but since its been this way for 20 years I am just going to put it all back together and fill with the specified amount of gear oil as described in one of my previous posts. It's just too much hassle to figure out how to get my hands on the special gear puller for bearing #13 and everything else to do the job. Remember there was no problem with this motor until I went to drain and install new gear oil then I thought I'd post to see if other people had the same problem and what was the fix

In case you were wondering: There is no shoulder in the shaft log for the lower bearing #2 to be seated against. The shop manual says to hammer it down til about 1/8" above the bore of the prop shaft, then it says to test for drift with the pinion on the shaft by rocking the drive shaft back and forth while listing for the click of the gears contacting. Kind of hoky. If the drift is not right you have to pound the bearing thru the bottom of the log and try again. It must be seated correctly as it has worked just fine and I have not touched or measured it, ( I am at least the 3rd owner). The spacer #11 is just a sleeve and loosely fits on the drive shaft and the diameter of the bore it is in is ~1/4" larger than the spacer itself. So unless the upper bearing and lower bearing are set just right and pinch the spacer tightly, (no way to know that) it really looks like it just spins around in the bore and the shaft spins around inside the spacer. I imagine its all whirling around in there, (but maybe not if it is pinched just right by the upper and lower bearings). There are no burrs or wear marks that would indicate that this is anything but as designed.

Hears the shop manual I'm using that should shed more light on this:

Thank you for taking the time to help out. I have a plan of action now. Fair winds.

tinkrman69 10-01-2016 19:37

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude

no special puller needed for bearing 13. get a pice of pipe the same size or close as the spacer that can fit over the shaft and is as long as the shaft or longer and tap it down the shaft. make your measurements as described before and reinstall it to the specified location on the shaft. simple!

tinkrman69 10-01-2016 19:42

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
most special tools aren't as necessary as the manufacturer would have you believe. altho they may make a job a little easier, it's also a source of revenue, for them.

HappyMdRSailor 15-01-2016 05:17

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude

Originally Posted by ejlindahl (Post 2010401)

Man this is turning into an epic for a small $250 motor, but it runs well and I love everything else about it.


No kidding huh???

I think it would be eye opening if we actually calculated the amount of time spent thinking on projects like this...

Of course it doesn't count if you are relaxing with a beverage simultaneously...

ggray 29-01-2016 09:13

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Have you gotten everything back to your satisfaction?

I may try to find the time to replace the impeller--after all, it is about 25 years old-- so maybe I'll have a better understanding of what you found inside.

I'm remembering something else about this motor. The tendency to snap shear pins when shifting into gear unless the motor is at a very, very low rpm. Lower than you would expect.

Have you had this problem? Many times, after warming up in neutral, I would shut off, put in gear, and then start again.

donradcliffe 29-01-2016 10:28

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
I had two of the Tohatsu 3.5 engines. The first one (bought in 1994) had no gear shift and went through shear pins until I switched to homemade SS pins. The second one was bought in 2008 with a F-N shifter, and still had the original shear pin when I sold it last year. However, the shift lever had to be replaced when it got caked with salt and had to be destructively removed. These motors were used over 200 days a year as my primary dinghy power source.

I never did figure out why the newer motor didn't break shear pins.

ejlindahl 30-01-2016 16:09

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
The saga continues:

Strange coincidence; I'd put this aside for the last 20 days (took awhile to get the parts), and only just this morning decided to put it all back together and lo and behold there are a couple new posts from yesterday. Here’s some feedback.

(The part reference numbers below refer to the thumbnail in my original post. They are different than what some parts sellers use! i.e. Iboats bearing part reference number may be 13 but Mercury Marine may have it as 11. Same picture, different number. So if you are following this closely you should use my thumbnail in the original post.)

On the 10th you described how to get the top bearing, #13, off the drive shaft. That is not the problem. My issue is that the bearing is stuck in the carrier portion of the water pump base, #17. I think (and the shop manual says) that I will likely destroy the bearing and or seal when this bearing and seal are removed. The manual says this even with their assumption that one uses the special tools.

Once out of the carrier, I'm sure I would have gotten the bearing off the shaft in the manner you describe. However, upon a second look at the shop manual with fresh eyes, and seeing how the sketches of the procedures are portrayed I now realize that they expect that the bearing just comes/pulls/drops out of the carrier easily with out having to hammer on it. No way on mine! On my unit I think corrosion has siezed the two together where the bearing is seated into the female bearing carrier area of the water pump base, (#17). The bearing is stainless steel and pump base is aluminum. This was the same problem for me getting the water pump base out of the lower unit casting. Took me several hours of carefully warming, wd40, tapping, cajoling, cussing to get it out, and yes there was just a small amount of that white aluminum oxide & corrosion in that interface too. (you don't need to do this to change the impeller or gear oil. I just wanted to see and fix why my gear oil vent hole was plugged.)

One cannot hammer the bearing, #13, out of the carrier with a correct size socket (or what ever mandrel you can devise). The diameter of the hole in the water pump base that the drive shaft goes through is only a few 64ths" larger than the actual shaft diameter. There is no visible lip of the bearing to hammer on. It needs a special "expanding shaft" puller from below to pull (or slide hammer) it out. You might be able to hammer it out with a screw driver at an angle thru the drive shaft hole but you will definitely ruin the bearing and seal doing that, (and run the risk of breaking the $85 cast aluminum pump base). Ages ago I removed a few bad wheel bearings and seals in this manner so I have an idea how it goes.


I found changing the impeller pretty obvious. The big caveat is to not completely unscrew the shift link clamp (and lose it) in the little hole in the exhaust housing. And though I haven’t got there yet I read that its a pain to get the shift linkage, drive shaft and water pump tube lined up to reinstall. A trick was to use some small wire to hold stuff as you slid it together. I found some threads or posts that discussed this, sorry no link though. If you need it here is a shop manual I found online. Boatinfo - Mercury Service Manual for 2,2-2,5-3,0-3,3 hp I don’t think you can download it or print from it but I have it on my laptop next to me in my shop. And I have printed some pages as screen shots but the illustrations and text are hardly useful. Maybe someone more techy can do this but that ain’t me

My impeller was really shot, just stub arms where the vanes used to be, and there was a thin layer of black rubber goo smeared inside the stainless steel cup insert that the impeller spins in. I took all the parts to the merc mechanic to answer a few questions, (he was very nice). He commented that when impellers look that burned up it is likely that the stainless steel cup insert (#23) that the impeller spins in has also, itself, spun inside the upper water pump housing (#24). There is a tang protruding from the bottom of this cup insert and it keys into the upper water pump housing. The tang normally prevents the insert from spinning inside the housing. But the friction of the melting impeller against the insert cup heats up the cup and housing and eventually the tang starts plowing its way thru that area of the melting/gooey plastic housing thus allowing the insert cup to spin, or at least rotate out of alignment. This creates a misalignment of the impeller/cup insert/housing assembly with the drive shaft. They are mounted a bit out of line in that the hole for the drive shaft through the cup insert is not exactly in the center of the cup itself. This is what makes the pump pump; the impeller kind of wobbles around in there in an eccentric motion as it spins with the drive shaft. (remember this is a new used motor to me, I never run my motors dry) So if they are out of alignment then it won’t pump as well and may create excessive heat and wear in a newly replaced impeller. It may also crack the plastic housing itself or put a side ways pressure on the drive shaft. It looks like mine might be ok, the mechanic said when they look like mine he not only installs a new impeller but replaces the whole pump housing. Of course we all understand why; a half hour of his labor is more than the cost of a complete rebuild kit and pump housing. Me, my labor is free and I like “taking apart stuff” and parts cost retail $$$ to me. I will use it as is and monitor the water pump flow. Having done this disassemble once I know i can do it again in less than an hour.

Shear Pin:

I heard/read somewhere of a guy that had an aluminum prop and was always breaking shear pins. A plastic prop solved the problem. I think he said there was an improvement in performance as the prop blades flexed to give more bite out of the hole and then as top end was reached it seemed to go faster, but don’t hold me to this. My prop is plastic but i’ve only run it 15 minutes before deciding to do some preventative maintenance and change the gear oil,,……. leading to this thread. Another guy with the same problem wrote, (maybe it was Donradcliffe) that he made SS ones to solve his shear pin breakage. I have a plastic prop and brass pins but just in case I made some extras out of same diameter brass bolts I had lying around. I’d be a bit nervous about subbing ss for brass pins. They are there for a reason…… but if it works……

I’ve spent so much time writing that I don’t know if I can finish the reassembly today. But will report back when I do.


ggray 30-01-2016 17:39

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Yes, you have done a lot of writing!
Actually, I have the plastic prop, so that won't be a solution for me!
I'll just be careful.

karl m 24-11-2016 18:07

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Hi Ejlindahl,
Am joining this conversation very late, but am having the exact same problem as you. I noticed this thread as I was browsing for info.
Have you solved your problem, given up or other?

ejlindahl 27-11-2016 12:03

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Hi Karl,

I reread the whole thread. If you did too you will see that I think I know what the problem is but I found a way around it and did not go to the trouble and cost of further dissassembly to fix it right.

Toward the end of the tread I mentioned that the problem most likely is that the upper drive shaft seal was not fully seated all the way up in its seat in the water pump base (housing). Thus it blocked the vent hole that allows air to escape as you fill the lower unit from lower fill hole. 150 psi compressed air could not blow thru this vent because the misaligned seal blocked it.

I did not want to go thru the hassel and cost of dissassembling the bearing and seal (there was no indication that they were failing anyway). So I just buttoned it all back up, layed it down and filled the oil from the opened gear case. (I had to get in there anyway to reinstall the pinion gear on the drive shaft). I did carefully measure the 2.5 oz of oil that the repair manual prescribed, wife had an empty 2.5 oz squeeze bottle with a nozzle that I calibrated from a measuring cup. I spend a good 1/2 hour to allow the chamber to burp any air out by; spinning the shaft, shaking the whole unit, lifting it up and down 10 or 20 degrees as I allowed time to for the oil to find its way into the neather regions of the chamber. As I recall the chamber (gear case) was completely topped off with the 2.5 oz so I feel good about it being filled to the proper level. I put the cover on and have run it only 1or 2 hours but it seems fine.

If your situation is similar that is what I would recommend. I imagine that if you were to have a shop do the fix properly the cost would exceed the cost of a new used motor.

karl m 28-11-2016 02:09

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Hi Ejindahl,
Thanks for coming back.
It seems like a fair coincidence that 2 motors have identical problems.
Yes the vent on my 3.3 is also totally blocked, can't pump air in, with motor on it's side I poured methylated spirits into the vent hole and let it sit, eventually over a few hours the metho did disappear but I suspect that was mostly evaporation.
I guess I will just do as you have and fill via gravity with the motor inverted and hope for the best.


ejlindahl 29-11-2016 10:53

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Be advised I did not fill it from the bottom "fill" hole. I took off the prop & housing cap the holds the prop shart into the hub and filled it from there. It seemed faster than waiting for air to burb out of the small fill hole while slowly dribbling in the oil thru the fill hole. I did not replace the oring seal, I just put it back on.

As I mentioned, when I took my problem to the mechanic he suggested that the rubber seal had slipped down the drive shaft and that is what is blocking the vent hole. Maybe he'd seen that before. I do think that is the problem as I can see the black rubber seal, as well as feel it with a probe. Unfortunately it is quite involved to get at and replace. And maybe it will just do it again..... Maybe its a problem with that year.... a lemon. Mine is a 1996, is that what you have?

Good luck

karl m 02-12-2016 01:55

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Hi Ejlindahl'
Yes I am on top of all that.
I have already refilled once through bottom hole, using a syringe, worked fine.
I am emptying and starting again as I have decided to replace the prop shaft oil seal and bearing[ the seal required a second mortgage to pay for it]
My motor is a 2003 model, so the problem does not seem to be manufacturing year related.
Will try to keep you posted on results [interest only, I guess]

Steamboats52 30-05-2017 15:09

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
I have only just come across this thread after experiencing exactly the same problem with my 8 year old Tohatsu 3.5 (the same assembly). I also had a few bits of gunge in the top 'overflow' filler but soft and seemingly insufficient to cause the complete pressure seal that prevented me from filling through the lower filler hole (using a pump).
I came to the same conclusion that the pragmatic solution was to remove the end of the gearcase behind the prop and fill the whole thing that way. I had just removed and refitted this to replace the prop shaft oil seal. The engine has been running smoothly for a while now.
With a few of us experiencing the same problem, this does seem to point to a design/assembly weakness. I would be interested if anyone does find a simple way of clearing the apparent blockage.

Rey3475 03-06-2017 10:47

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
I have the same problem.......Nissan 3.5 Hp Long Shaft. This seems to be an on-going issue with these engines. Like you other guys, I'm just going to fill the gear lube as much as possible, and use the engine. By the way, I have another Nissan 3.5 short shaft that fills as it should, so the problem isn't universal on these engines. Good Luck !


Pontoon Joe 22-10-2018 14:45

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
I have had the same problem, unscrew the pump from the container of oil you are trying to pump into your lower unit. It may be full of oil already when on the up stroke. Drain the oil out of the pump unit and try again it will not pump oil into the lower unit if the pump can't suck the oil into the tube because it is already filled with oil then on the down stroke it locks up and will not pump the oil into your lower unit. It is not the lower unit that has the problem, it is the pump.

ggray 22-10-2018 17:09

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Well, I tried filling my Merc 3.3 with a squeeze bottle, and no go.

I think it's an issue in the lower unit.

I "filled" mine slowly, with the motor inverted.

ejlindahl 25-10-2018 08:06

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Pontoon Joe,

It sounds like the problem you have is with the oil container pump. This thread is about the vent hole on the top of the lower unit (drive leg) being plugged by the top drive shaft seal and bearing of the lower unit. This may only be a problem for the long shaft version of the motor. The work around is described upthread.

ggray 25-10-2018 09:37

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Mine is a short shaft, and has this problem.

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