Oversized Yanmar 3GM30 Cylinder bore

[pierrrre]

Hi

I am in the process of overhauling my 3GM30 Yanmar engine. The cylinders diameter are at the max of the normal spec (2.9540 inches). There is also a bit of rust on the cylinders wall. I decided to bore the cylinders oversized and install oversized pistons/rings. My problem is that I will not get the pistons for a few more weeks. This means I can not measure the pistons size and bore the cylinders accordingly.

Yanmar sell the pistons 10 thou (.25 mm) oversized but no manual I found gives the actual size of the oversized piston or the oversized cylinder bore. Does anyone know the actual size of the oversized pistons and/or the measurement to bore those cylinders oversized?

Also, when I look in the Yanmar catalog it shows "PISTON W/RINGS OS.25". I assume that the pistons and rings are oversized but could it mean standard pistons with oversized rings? I know this is not the way to do this but , hey, I have seen worse.

Thanks

[perchance]

Piston rings .25" would be 1/4". Do you mean .25mm which would be .010" approx,

[delmarrey]

Wait until you get the pistons and take them and the rings to the machine shop with you. They will measure and bore accordingly.

Piston manufacturers do not go by a set standard in their sizes. They each vary. It best you wait!!!

[pierrrre]

Originally Posted by delmarrey

Wait until you get the pistons and take them and the rings to the machine shop with you. They will measure and bore accordingly.

While this is the plan, I just want to double check if the measurement, thus required piston/cylinder clearance, that we will find make sense. Maybe Yanmar has a trick up their sleeve that I am not aware of and once the cylinders are bored, it is too late to go back if bored too big.

PERCHANCE, the quotation mark after .25 does not mean inches. It is the closing quotation mark of "PISTON W/RINGS OS.25". .25 do refere to .25 mm or .010 inches.

[streaker1]

Hey pierrrre, just a thought, you won't be going far without the pistons anyway, a few days here or there?,why not wait wait until the pistons and rings arrive to make sure, knowing that you will be able to "size" the bore accordling, I assume you mean "boreing" that is "machining" not just "honeing' as with "stones" etc, as for the actual sizes sorry mate can't help there, have fun cheers.

[delmarrey]

The chances are the the OS pistons will be an aftermarket part. So w/o actually knowing who makes them the chances are nil of getting dim's.

A lot of automotive and aircraft parts are sub'd out to bidders and occasionally they will change suppliers for the money, or politics like with government stuff. And I imagine the tolerance in the prints are around + or - .002" (for the money).

[four winds]

That's the maintenance standard. 2.9528 - 2.9540 in. A .012 in. range.

The wear limit is 2.9567 in. An increase of .027 in.

Are you past the limit at 2.9567 in. ?

edit,.... never mind, now I think that's the limit for a serviceable block not a first rebuild.


The pistons are oval shaped for what it's worth.

Standard size measured a right angles to the pin on the lower part of the skirt..... 2.9492 - 2.9504 in.

This indicates the factory piston to wall clearance .036 in. Wow, 36 thousandths piston to cylinder clearance? I've only rebuilt gas engines of several different types. Some were .002 in. clearance.

Do diesels typically have this much clearance? i.e. .036in


I was going to say no way I would bore without the new pistons, but I bet it would work with these tolerances.

My numbers below, be very, very careful taking them to heart.

A "10 over" piston should be 2.9502 - 2.9514 in.

The bore would be 2.9538 - 2.9540 in.

Again, that's my guess on the last two lines.

[four winds]

I would wait on the pistons too.

I'm just amazed the clearances are so large.

[delmarrey]

Here is a couple pages out of a JH model. This should give you some idea of the clearances. You do have a smaller motor so the clearances will be less.


.

[four winds]

Well, there it is on the second picture.

18 to 38 thousandths clearance on those four Yanmars.

I didn't know that was possible.

Higher compression makes for more heat and expansion?

Making this clearance needed.

[pierrrre]

Originally Posted by four winds

The wear limit is 2.9567 in. An increase of .027 in.

Are you past the limit at 2.9567 in. ?

edit,.... never mind, now I think that's the limit for a serviceable block not a first rebuild.


The pistons are oval shaped for what it's worth.

Standard size measured a right angles to the pin on the lower part of the skirt..... 2.9492 - 2.9504 in.

This indicates the factory piston to wall clearance .036 in. Wow, 36 thousandths piston to cylinder clearance? I've only rebuilt gas engines of several different types. Some were .002 in. clearance.

Thanks for the math. The cylinders diameter are now 2.9540. And we (the machine shop guy and I) came up with something around .0036 inch by subtracting the standard size piston diameter from the standard size cylinder diameter. This is this number that I hope will be able to confirm with this thread. I don't want to bore (not hone) with this clearance only to find out that the clearance should be half, a quarter or a tenth of this after the fact.

Also, do not worry, there will be no boreing before I get the new oversized pistons.

[delmarrey]

Originally Posted by four winds

That's the maintenance standard. 2.9528 - 2.9540 in. A .012 in. range.

The wear limit is 2.9567 in. An increase of .027 in.

Are you past the limit at 2.9567 in. ?

edit,.... never mind, now I think that's the limit for a serviceable block not a first rebuild.


The pistons are oval shaped for what it's worth.

Standard size measured a right angles to the pin on the lower part of the skirt..... 2.9492 - 2.9504 in.

This indicates the factory piston to wall clearance .036 in. Wow, 36 thousandths piston to cylinder clearance? I've only rebuilt gas engines of several different types. Some were .002 in. clearance.

Do diesels typically have this much clearance? i.e. .036in


I was going to say no way I would bore without the new pistons, but I bet it would work with these tolerances.

My numbers below, be very, very careful taking them to heart.

A "10 over" piston should be 2.9502 - 2.9514 in.

The bore would be 2.9538 - 2.9540 in.

Again, that's my guess on the last two lines.

You need to get your numbers straight. .0000 four places.

[pierrrre]

Originally Posted by four winds

Well, there it is on the second picture.

18 to 38 thousandths clearance on those four Yanmars.

I didn't know that was possible.

Higher compression makes for more heat and expansion?

Making this clearance needed.

The picture seems to show 1.8 to 3.8 thou

[four winds]

oops,.... a .0012 range.

and a .0027 increase.

Sorry, so that is a wear limit for a standard bore. not a block

I knew something didn't look right.

The large wall clearance had my attention.

But I was wrong there, too.

My apologies. I screwed the pooch on that one. That's been happening too much lately.

So diesels have a similar wall clearance.

[four winds]

Several years ago I stopped doing math in my head, on purpose.

This is why. Need to stick to that.


pierrre, as you have measured the current parts are near the limit listed for those newer Yanmars. .0018 - .0038 seems reasonable to me. Your at .0036.

But in my 3gm manual I can find no reference to a similar spec, same as your research. That's why I was doing the bad math to try to determine it. I got .036, but it was actually .0036. Interesting that it's what yours currently measures. Do you have a ridge at the top of the bore, after removing carbon?

So the question is - Should the older 3gm be good to go at .0018 to .0038? You are still within that range now, with wear.

Disclaimer, I have the same engine, and build experience on gas engines, or I wouldn't be butting in here. I appreciate the pros taking it easy on me. I'm trying to learn and help.