Yanmar SB12 Using Oil

[Compass790]

I guess you didnt do leakdown test as A64 suggested? Head repair place must have crack testing gear even if it's only dye penetrant.
Good to hear yr maintenance schedule was on the ball except for tappet setting. Clearly the 300 hrs checking recommended in the manual is a bit OTT if your v/v seats survived fine.

[a64pilot]

Machine shop will I’m sure do a mag particle check.
All it takes is powdered Iron and a big horseshoe electromagnet.

[Wotname]

Originally Posted by Homer Shannon

The mystery of the oil consumption deepens.

................


but I have no reason to believe that I will have done anything that would reduce the oil consumption. What next?

OK, I didn't see any mention of liner (bore) diameter measurements.
From the YSE12 manual, max wear limit for the liner is +0.34mm or if you have a new ring, a ring gap of 1mm.

While the YSE12 and SB 12 have the same bore, the liner part numbers are different but I expect the wear limits are the same (given piston & rings part numbers are the same).

I'll go out on a limb here and suggest the increasing oil consumption is a result of increasing liner and ring wear but you have caught it before excessive bore glazing has occurred. If you bore hone marks have disappeared (or almost disappeared), this would support my assumption.

Perhaps box 3 is relevant from YSE manual (although liner part numbers are different so may not apply)-

[model 10]

[QUOTE=Wotname;
They should hold alcohol for 24 hours.[/QUOTE]

In the real world, you measure in seconds. The proper test is with Stoddard solvent. "Wetness should not appear before about 25 seconds". If you want to cheat, use water, it takes longer to seep out. If you want to show a leak, use gas, it seeps out faster.

[Wotname]

Originally Posted by Homer Shannon

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.

.........

I'll go out on another limb here

The SB12 is raw water cooled so runs cold - say around 120F, maybe the 15W40 is too thin at this temperature. When you ran straight 30, the oil was thicker than the 15W40 at 120F and given the existing engine wear, oil consumption increased when you changed to the multigrade.

Note, this is only an assumption .

[Compass790]

Originally Posted by Wotname

I'll go out on another limb here

The SB12 is raw water cooled so runs cold - say around 120F, maybe the 15W40 is too thin at this temperature. When you ran straight 30, the oil was thicker than the 15W40 at 120F and given the existing engine wear, oil consumption increased when you changed to the multigrade.

Note, this is only an assumption .

I think ya went too far out on a limb Wotname. If you look up 30 monograde viscosity specs it will be around 95cst at 40oC ( less than 120oF) whereas 15w-40 will be about 120cst. It's a complete myth that monograde oils are thicker at raw water cooled engines

[Compass790]

Originally Posted by Wotname

I'll go out on another limb here

The SB12 is raw water cooled so runs cold - say around 120F, maybe the 15W40 is too thin at this temperature. When you ran straight 30, the oil was thicker than the 15W40 at 120F and given the existing engine wear, oil consumption increased when you changed to the multigrade.

Note, this is only an assumption .

I think ya went too far out on a limb Wotname. If you look up 30 monograde viscosity specs it will be around 95cst at 40oC ( less than 120oF) whereas 15w-40 will be about 120cst. Gulf western in yoour homeland publishes spec sheets for their oils shud you not take my word for it.It's a complete myth that monograde oils are thicker at raw water cooled engines operating temp. Our operating temp at oil filter is 59oC BUT & this a big but, that in no way reflects what the oil temp is inside a crank main bearing or at the piston ring face. As you can well imagine they are waay higher. You can read real world temp measurements if you do some online research.
Sorry first short post got snt by mistake

[Wotname]

Originally Posted by Compass790

I think ya went too far out on a limb Wotname. If you look up 30 monograde viscosity specs it will be around 95cst at 40oC ( less than 120oF) whereas 15w-40 will be about 120cst. Gulf western in yoour homeland publishes spec sheets for their oils shud you not take my word for it.It's a complete myth that monograde oils are thicker at raw water cooled engines operating temp. Our operating temp at oil filter is 59oC BUT & this a big but, that in no way reflects what the oil temp is inside a crank main bearing or at the piston ring face. As you can well imagine they are waay higher. You can read real world temp measurements if you do some online research.
Sorry first short post got snt by mistake

I'm always happy to have my assumptions corrected and I must do some reading if only to find out how multigrade oils actually perform at various temperatures. I have always accepted the concept they are thin cold and don't get thinner when hot. Off topic but I must read up on he differences between say15/40 and 15W/40.

Thanks for heads up.

But ya don't need to tell me twice

[Wotname]

Originally Posted by Ecos

In the real world, you measure in seconds. The proper test is with Stoddard solvent. "Wetness should not appear before about 25 seconds". If you want to cheat, use water, it takes longer to seep out. If you want to show a leak, use gas, it seeps out faster.

to your greater engine experience, I pretty much a self taught DIYer when it comes to engines.

But crikey, 25 seconds ain't long, is this just a workshop number where it is close enough to comply. The thing is, I don't mind spending hours to get things better but no way could some one pay for that extra time taken. Of course, trying to gets stuff better doesn't alway show up any better result in the real world

Recently I spent several hours trying to chase down a very minor gas regulator leak, no amount of leak detection techniques showed up any leak but you could get a faint whiff of gas every so often. A gas fitter would not have wasted the time, he would have changed out the regulator right away. The leak disappeared after I changed the regulator .

Back to the valve seat, if it doesn't leak after say 10 minutes or so, my limited experience tells me it doesn't really leak at all but rightly or wrongly, I leave it at least overnight.

[Homer Shannon]

I'll get back on the bore diameter. I measured it with a caliper - not the right way, but I don't have an inside micrometer - and I'm getting a bore of 84.85mm. Spec is 85mm with a tolerance of +0.26. If the reading is accurate, the bore is good. I'm going to try some readings with a feeler gauge and see if they confirm this reading at different depths in the bore.

Ring gap was 1.10mm, which is pretty close to spec based on Wotname's YSE specs. I saw YSE specs at 0.40, but perhaps I had the wrong document. In any event it was not an SB12 specification.

Based on everything I can measure with good confidence, this engine is not worn out at all. Perhaps the issue does, somehow, stem from the head and gasket. I suspect we won't know for sure until sometime next summer.

[Wotname]

I don't have an inside micrometer so my quick and dirty method for bore diameter is to stick the piston in (no rings) and jam a feeler gauge down the side. Measure the piston diameter and add the feeler gauge. Sort of works

[Homer Shannon]

That's the plan!

[Wotname]

FWIW.

While the YSE manual gives guidance on measuring bore diameter, it doesn't give as much information for measuring ring gap. It states to measure gap with the ring in the liner skirt but doesn't say if the liner should be new or if worn within limits is OK. Clearly one will get different readings depending on liner wear. Elsewhere I have read to measure the gap with the ring near the bottom of the liner and presumably there is little wear at that point.

Being pedantic, I would only do it with a liner without any wear .

[Homer Shannon]

So, I realized that my last statement about the bore diameter was totally bogus. The PISTON, measured at the 15mm point on the skirt, is 84.85mm - max spec is -0.26mm. I'm good by 0.11mm.

Using a pointy feeler gauge, today I checked the bore, measuring the gap between the bore and the upside down piston at the skirt. The fattest gauge I could get in there, and it was pretty snug when I pulled the piston up onto it a bit, was 0.004" or about 0.10mm. Since the piston skirt is 84.85mm, I can conclude that the cylinder must be very near its full spec of 85.00mm. The tolerance on the cylinder is +0.34m and I'm confident that I'm with that spec.

Reasoning would say that I should have been able to get at least a 0.15mm gauge into that bore - I couldn't. I would agree that this isn't the most accurate way to measure the bore, but I think it is sufficient to demonstrate that the bore is good.

[Compass790]

actually yr rings are closer to worn out than to good as good is 0.4mm & worn out is 1.5mm but agree it's not likely to explain your high oil consumption. Our engine had a lot worse clearances in the cylinder than yours & still used less oil than yours ( but still lots ) E.g. our piston was 1.2mm undersize & bore had a noticeable lip.
Only leaves crack in head, restricted air intake or v/v guides & as said previously hardly likely to be valve guides as yr bore is only 1/2 worn