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Old 17-09-2018, 08:56   #1
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Yanmar SB12 Using Oil

I have a Yanmar SB12, 12hp, single-cylinder, diesel motor in my Bristol 29.9 sailboat. I have owed this boat for twenty of its forty years and I've put about 1,100 hours on the motor. I have no idea what the previous two owners contributed, but the boat was always in New England and I'd suspect their usage was similar. If so, this motor probably has less than 3,000 hours on it.

Since I've owned the boat, the motor has always used a fair about of oil. Years ago, I calculated the use at about a cup per 10 hours of operation. This year we noticed a significant increase in the oil usage and I'm now calculating it to be around 3oz of oil per hour of operation - a significant and concerning increase.

The motor still starts and runs well. It does create a bit of blue smoke that is quite noticeable at idle, especially in calm, moist conditions. Underway on a dry day the smoke is nearly invisible.

I've talked with a number of local boat owners and they seem be in agreement that my issue is piston rings, valve guides, valve seals or possibly the wrong oil. (I'm now using 15-40 oil, in the past I'd been using straight 30 weight.) I don't have a compression gauge, though clearly the compression is not really bad or the motor would not start reliably.

My question is: What is the best way to determine which issue is actually causing the oil consumption?

Taking the head off this motor and getting it rebuilt should not be difficult or expensive - it's a one cylinder motor with good access. getting the piston out would be rather more difficult. IF I can get the oil pan off the motor while it is in the boat, it may not be that difficult to remove the rod bolts and pull the piston out of the top of the motor. (I'll check this out this fall while I'm doing its annual oil change.) In either of these cases, a partial rebuild would be pretty straight forward.

I'd be interested in comments before I get too deep into this.
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Old 17-09-2018, 10:56   #2
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Yanmar SB12 Using Oil

You do a leak down test.
Itís how compression tests are done on aircraft engines, compressed air is blown into the cylinder with the piston at TDC compression, then you measure air leakage, but more importantly, you listen to see where it leaks from. Intake means intake valve, same for exhaust, air hissing from the valve cover means rings.

In your case Iíd bet money its a little bit of all the above. Iíd go back to straight 30 or 40 weight oil and not worry about it. As long as it doesnít smoke excessively, cranks easily and makes good power, there isnít anything wrong with it.
In fact it was decades ago, but there was a paper published by maybe Rolls Royce aircraft branch that had shown that once an engine begins to consume oil, the upper cylinder wear rate drops quite a bit, it seems this excess oil lubricates piston rings and cylinder walls.

Our old low power low stressed non turbo charged Diesels just donít need a multi vis oil and as an engine wears, it will benefit from a thicker oil.
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Old 17-09-2018, 11:10   #3
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Re: Yanmar SB12 Using Oil

To clarify a little, a multi vis oil of course is thin when itís cold and thickens up when hot.
We just donít get hot enough to thicken it up. A modern automobile not being run hard has an oil temp in the low 200ís, our motors oil temps are way down from that, measure the oil filter at cruise, you may be surprised, I was.
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Old 17-09-2018, 11:26   #4
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Re: Yanmar SB12 Using Oil

If no leaks, no oil in the coolant, the engine is burning oil during combustion. If so there may be a slight sheen in your exhaust water. If you can afford the oil use, and there isn't a noticeable sheen, you should be able to run for many more years. ditto thicker oil. STP in the old days.

The cause is most likely the rings. A new cylinder has cross hatching, small scratches purposely made during final honing. Those scratches allow tiny amounts of oil to travel with the rings and lube the top of the cylinder. As an engine wears in old age, the cylinder is worn and those scratches no longer carry oil, the cylinder and rings wear faster and eventually oil gets by the rings.
A cheap fix is to pull the piston, rehone the cylinder, restoring the cross hatching, and new rings. It lasts around 1-2000 hours. Otherwise an overhaul. I have repaired or overhauled dozens of diesels. Rarely have I found an valve guide worn enough to pass oil. But since the head would be off, I'd get it rebuilt. Since the pan is off, new rod & main bearings, seals, too. Sometimes you can disconnect the engine as necessary and jack up one end to access the pan. Block or restrain so it can't fall in a wake.

Pic is cross hatching in Detroit sleeves.
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Old 17-09-2018, 14:55   #5
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Re: Yanmar SB12 Using Oil

a64piot - your comments ring true. This engine really never does get very hot. Good and warm, yes. Broiling hot like a normal car engine - never. I can afford a lot of oil for the cost of a rebuild, or a new motor. Sadly, there isn't enough time in this season to switch back to straight weight oil and see if it really makes a difference. I guess this issue is going to have to wait another year and I'll see how it does next season.

What equipment is required for a leak down test? Is this commonly rented at tool rental joints?
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Old 17-09-2018, 15:14   #6
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Yanmar SB12 Using Oil

You can get a leak down tester for less than $50. Iím sure they can be rented too, you will need a compressor, doesnít have to be a huge one but a bicycle pump wonít work.
The trick is going to be how to connect it to the engine, usually itís remove a spark plug, we have injectors and your likely going to have to fab something up, but you have to do that for a regular compression test too.
I havenít read this article, but bet itís a good guide.
Just google engine leak down tester and youíll get hits.
https://mobiloil.com/en/article/car-...-leakdown-test
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Old 17-09-2018, 16:17   #7
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Re: Yanmar SB12 Using Oil

I'm all for doing a proper analysis of any defect for pulling stuff apart but in the OP's case it might well be of academic value only. Regardless of what is causing the oil burn, the head has to come off and given the history (hours), one will be checking the valves and guides as a matter of course. The bore can be checked for wear (diameter and cross hatching) at the same time.

How much blowby is there currently, this will give some indication of the condition of the bore/piston/rings? I don't recall if the SB crankcase is vented directly or via the air intake but it is easy to work out.

Some dot points:

The engine is never going to get hot enough to justify a multigrade and the OP probably does't use the engine when it's really cold. Being RWC, the normal operating temperature of this engine will barely be 50C.

The engine has wet liner and I'm almost certain there are after market liners, pistons, rings and valves available from Asia at well below Yanmar prices. There is for the YSB and many parts are interchangeable between YSB and SB.

Before planing any major work, price up the various parts; big end bearing shells, main bearings etc. The Yanmar prices might shock you, especially the main bearings. In Oz, one YSE main bearing is ~$60 and the other is ~$500

If the prices are shocking, just do the minimum work necessary to keep the engine alive and live with some oil consumption. I'm sure the with the head done and if necessary, a liner and piston, the oil consumption will be back to normal.

Finally, what is the oil pressure like? These engines are know for oil pump issues and if you haven't fitted an oil pressure gauge, please do so this winter. No point is spending big on an engine unless you know the oil pump is good (or going to fix it).
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Old 17-09-2018, 16:53   #8
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Re: Yanmar SB12 Using Oil

We had the same engine in our 1977 29.9, it was a solid runner, noisy as hell and shook everything in the boat when running �� we opted to replace it with a beta 16, it was a decision made because we are headed south and wanted a quite engine while going down the ICW, hope the sevicing of your SB goes off without a hitch.

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Old 17-09-2018, 19:00   #9
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Re: Yanmar SB12 Using Oil

Yes, I think this would be a good idea and I have a good compressor. Not sure where you would connect the hose - injector or decompression valve possibly?
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Old 17-09-2018, 19:04   #10
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Re: Yanmar SB12 Using Oil

Despite many people, like yourselves, saying that the SB12 is noisy and has too much vibration, I don't find it so bad. I think it's smoother than a 2GM, though it's vastly more noisy and shaky than a Kubota 4 cylinder. We normally cruise it at about 2700 RPM. If we can get a bit of sail assist (we always try) bringing it back to 2500 RPM really smooths it out. Occasionally we've been able to motor-sail with the engine at just 2200 RPM and you hardly know it is there.
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Old 17-09-2018, 19:28   #11
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Re: Yanmar SB12 Using Oil

Quote:
Originally Posted by Homer Shannon View Post
Yes, I think this would be a good idea and I have a good compressor. Not sure where you would connect the hose - injector or decompression valve possibly?


Only de-compressors Iím used to are mechanisms that hold the exhaust valve slightly open.
Only way I have ever seen a Diesel compression tested and or leakdown tested was someone had taken an old injector and modified it to fit an airline quick disconnect fitting and replaced the injector.
Gasoline engine you replace the spark plug of course.

Just me, but if it has good oil pressure and cranks easily etc. I wouldnít do anything but run it.
Maybe start gathering parts for an eventual overhaul.
If I pull one apart, itís getting a new cylinder if itís a wet sleeve, usually the piston is OK, but often people replace them as a matter of course, then the head goes off to a machine shop, and you mic the crank, itís usually fine, but you polish it or have it polished and fit new bearings.
Whenever possible I always replace the oil pump, even if it looks fine, sometimes that isnít possible.
Assuming you overhaul correctly, itís at least as good as new, which means it ought to last decades.

If itís decades old and low hours, itís entirely likely that all it has is polished bores, a decent honing and new rings is all it needs.

Bores get polished a tiny bit every time an engine is stored, a light film of rust forms when it sits over Winter and is cleaned off in the spring very quickly when the engine is run, do that for a couple, of decades, and eventually all the cross hatching is worn away and you have a polished bore, not worn out, just cross hatching cleaned off.
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Old 17-09-2018, 20:23   #12
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Re: Yanmar SB12 Using Oil

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Yes, I think this would be a good idea and I have a good compressor. Not sure where you would connect the hose - injector or decompression valve possibly?
The only place is the injector. As a64pilot says, the decompression lever simply holds the exhaust valve slightly open on this engine (and all others that I have seen).
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Old 17-09-2018, 20:38   #13
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Re: Yanmar SB12 Using Oil

@OP, I do have a complete parts breakdown for your engine but not a service manual. PM me if you want a copy of the parts breakdown but be aware it is 10.6M.
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Old 18-09-2018, 14:26   #14
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Re: Yanmar SB12 Using Oil

Wotname: Thanks for the offer, but I have the complete parts list in PDF and the workshop manual in PDF and hard copy. I've been scavenging documentation for this machine for years and am pretty well supplied, now.

Ha! Thank goodness for the internet. Decades ago, before the internet, I had an Albin motor and needed a head gasket. I wound up making one because I couldn't find one for sale - and it worked. Somewhat after the time I sold that boat and motor, I was able to find Albin parts on the web and the head gasket was available from a supplier - at a reasonable price, too.
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Old 20-09-2018, 06:09   #15
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Re: Yanmar SB12 Using Oil

I started putting together a list of the parts I would need if I go ahead and rebuild my engine's head and put in new rings. In this process I have noticed that no valve guide seals are shown or listed in the parts diagram. (Item 33/34 in the attached diagram is a retainer.) https://1drv.ms/u/s!AiR9KzpBmPls1hfnrR3o8rBWZFZi

Is it possible that this design does not have valve guide seals, and if so, wouldn't wear on the valve guides themselves, therefore, create more oil consumption than usual?

Am I right in my conclusion about the seals not being used?
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