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Thread: Yanmar SB12 Using Oil

  1. #1
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    Default 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.

  2. #2
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    Default Re: Yanmar SB12 Using Oil

    I rebuilt the 10 hp version of that engine. It was easy. I would recommend pulling the engine and doing it on the bench. A 4x4 across the companionway and a come-along winch should make it easy. Two people can lift it from there I expect.

    Rings, hone the cylinder, lap the valves. Done.

    Or just put a bottle of STP in and see if it tightens up.

  3. #3
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    Default Re: Yanmar SB12 Using Oil

    Considering the age , would hard valve stem seals be a cause?

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    Default Re: Yanmar SB12 Using Oil

    I had one, it's a great engine but noisy. Mine burned up it's head gasket after clogging the cooling passages in the head with rust/salt. It still ran, though it got very hot and puffed out from under the head. It wasn't hard to remove the head, but the engine boil out guy told me that it wanted chemicals no longer permitted. I took it out and sailed without power. I also found a new old stock crate engine. I never put it in because I enjoyed sailing without. That engine went to the midwest with the boat. I never heard of either again, I assume the boat died, but the engine may be out there still.

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    Default Re: Yanmar SB12 Using Oil

    I agree with J Madison,Try a bottle of anti smoke snakeoil first.
    If you decide to re-ring I would include at the least valve seals as well. A simple test for valve guides is to put a finger over the end of the guide and pull the valve out. If there is a 'pop', then new valve seals will be enough.
    There is nothing quite as permanent as a good temporary repair.

  6. #6
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    Default Re: Yanmar SB12 Using Oil

    Some good thoughts here, thanks. J. Madison, can you tell me: When you pulled the engine out of the boat, did you pull it with the transmission or remove it from the tranny? I had to move the whole assembly two years ago to fix issues with the motor mounts, and its a pretty beefy machine. If you could separate the motor from the tranny easily, the motor alone might be light enough to move out and back without much difficulty , but I'm not familiar with the transmission/engine separation procedure. And, if you can leave the tranny in place you wouldn't have to mess with the prop shaft or transmission linkage, making the process that much easier.
    Last edited by Cinderella; 09-17-2018 at 08:49 PM.

  7. #7
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    Default Re: Yanmar SB12 Using Oil

    3,000 hours can be enough to wear out an engine if it either gets used hard or is not taken care of. I just replaced a 2900 hour motor in a Kubota tractor that was absolutely shot. In my case, a good used motor was the cheapest route - but it may not be in your case.

    I had an 8HP Yanmar (forget the model) that burned a lot of oil & it was all in the head: a crack + shot valve guides. It may make sense for you to pull the head off, as that's fairly simple. Then check the cylinder for wear & if it's bad, put the head back on (but not all hooked up - just do the head bolts) so you can pull the engine. If the cylinder looks OK, take the head to a machine shop to get it checked for cracks & also valve guide/seal condition.
    "If it ain't broke, you're not trying." - Red Green

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    Default Re: Yanmar SB12 Using Oil

    It may have a carboned fouled oil control ring. As well as what Garret mentioned.

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    Default Re: Yanmar SB12 Using Oil

    Quote Originally Posted by Cinderella View Post
    Some good thoughts here, thanks. J. Madison, can you tell me: When you pulled the engine out of the boat, did you pull it with the transmission or remove it from the tranny? I had to move the whole assembly two years ago to fix issues with the motor mounts, and its a pretty beefy machine. If you could separate the motor from the tranny easily, the motor alone might be light enough to move out and back without much difficulty , but I'm not familiar with the transmission/engine separation procedure. And, if you can leave the tranny in place you wouldn't have to mess with the prop shaft or transmission linkage, making the process that much easier.
    Mine had the transmission attached, and I never had it apart sorry. It would be easier to remove without it attached.

  10. #10
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    Default Re: Yanmar SB12 Using Oil

    When mine came out, I unbolted it and had the yard hoist it out with a boom crane.

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    Default Re: Yanmar SB12 Using Oil

    Alas, the yard my boat will be in is my yard and I don't have a boom crane.

  12. #12
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    Default Re: Yanmar SB12 Using Oil

    You have a mast and boom, you have a crane...

  13. #13
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    Default Re: Yanmar SB12 Using Oil

    Perhaps a block and tackle from a big tree? Cross beam in a neighbor's barn? When I bought the crate engine I moved it by car after removing a seat. I slid it up a couple of timbers into the car, and hoisted it out using an overhead beam in my garage. An A-frame of 4x4s should be sufficient. You'll work it out.

  14. #14
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    Default Re: Yanmar SB12 Using Oil

    When the boat is in the yard, the mast and boom are down, so I don't have a "crane to use".

    I went to the boat today and I changed the oil to straight 30 weight Rotella. Hopefully, I will get enough hours on the engine before the boat is hauled in early October to see if this actually makes any difference. From the amount of blue smoke and the smell of the exhaust in a half-hour excursion, I'm not expecting a big change.

    While I was there I took a good look at the oil pan and measured the distance from the top of the block to bottom of the cockpit sole. I'm pretty certain I can get the oil pan off without moving (let alone removing) the motor and there is room to work underneath. There is 15" between the top of the block and the cockpit sole. I think it may be possible to remove the piston and hone the bore without removing the motor.

    After the boat is home for the winter, I think I will pull the head. Rebuilding the head should not be difficult or too expensive if I send it out. I can check the bore and consider what I want to do with the piston and rings from there. BTW, I reviewed my logs and I've put 1,100 hours on this engine in 19 years. That's about 60 hours a year. If the previous owners used the boat a similar amount - and I have reasons to suspect they did - then this forty year old motor has somewhere around 2,400 hours on it. I know this motor has always been well maintained, so it should be in pretty good shape.

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    Default Re: Yanmar SB12 Using Oil

    While you could probably get away with honing the cylinder, no way could it be bored to oversize in situ. Another issue is that honing puts a lot of oil & grit into the cylinder & all over the crankshaft & block. Getting it all clean will be tough - and the crank and all the rest of the bottom of the engine need to be very, very clean before you bolt the oilpan back on.
    "If it ain't broke, you're not trying." - Red Green

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    Default Re: Yanmar SB12 Using Oil

    Quote Originally Posted by Garret View Post
    While you could probably get away with honing the cylinder, no way could it be bored to oversize in situ. Another issue is that honing puts a lot of oil & grit into the cylinder & all over the crankshaft & block. Getting it all clean will be tough - and the crank and all the rest of the bottom of the engine need to be very, very clean before you bolt the oilpan back on.
    Yes, you are right on all points. If I need an over bore, there is no way I can do this in place. I'm assuming I just need rings. If I find out that I'm wrong, clearly I'll have to get the block off the boat and to a machine shop. As for keeping the crank clean, I don't see why I couldn't cover the lower end with rags and other material to keep the debris from the honing off the crankshaft and bearings. This will require some care and I am only at the speculation point until I get a look inside.

  17. #17
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    Default Re: Yanmar SB12 Using Oil

    I started making a list of parts I will be needing to do the work I am expecting to do and discovered something that may be relevant to this discussion. The SB12 factory parts list does not list any valve guide seals. This was confirmed by looking at the Wooden Boatshop Quick Reference for Yanmar, http://j30.us/files/Yanmar-parts-Qui...rence-2007.pdf. This parts list lists valves, guide and seals for most Yanmar motors. For the YSB, YSM, YSE, 8 and 12 motors, valves and guides are listed but no seals are listed.

    If the motor does not have valve guide seals, it would be normal for the motor to use a bit of oil and probably smoke a bit on start up - which my motor has always done. As the guide wears, the oil consumption would increase, and probably more so than with a motor that has a seal.

    I won't have any answers until the boat is home and the head is off, but I'm wondering if worn valve guides may be my whole problem. I guess I'll know in a month or so.

  18. #18
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    Default Re: Yanmar SB12 Using Oil

    Doesn't that engine have a replaceable liner?

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    Default Re: Yanmar SB12 Using Oil

    Before any dismantling, consider doing a compression and leakdown test. If the compression is good, then rings, valve seating and cylinder condition are probably good. If valve seals have failed, these can be replace without major dismantling of the engine, in most cases.

  20. #20
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    Default Re: Yanmar SB12 Using Oil

    I pulled the head off of this motor this morning. It was a lot easier than anticipated. Everything looks pretty normal, but I am concerned that one of the three head bolts (studs, actually) had a lot of oil on it and there was a lot of oil in this area on the head gasket. I think most of this oil got on the head gasket during disassembly, but in the attached photo of the head, it does look like some of that oil may have been finding its way into the combustion chamber. It is possible that my oil consumption issue is due to a failing head gasket? Seems a bit far fetched to me as I don't know why oil, that is not under pressure, would be getting sucked into the combustion chamber. Still, I'd like to hear from more experienced mechanics before moving forward.

    I have also confirmed that I can remove the piston rod cap through the side panel. It is not necessary to remove the motor to get the piston out. I'm not going to take that next step, however, until I hear from others about what I have found so far. SB12-4.jpgSB12-2.jpgSB12-1-2.jpg

  21. #21
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    Default Re: Yanmar SB12 Using Oil

    On the intake stroke, there is negative pressure in the cylinder & oil is @ 40PSI or some such. A bad head gasket will certainly allow oil into the cylinder.

    Can you turn the engine over to put the piston at the bottom & take a pic (preferably a closeup of the side)? That way we can see what wear there is on the cylinder wall. A ridge at the very top is normal - it's below that that I'm wondering about.

    ETA: You will want to get the head scrupulously clean & carefully check it for any cracks - with a magnifying glass - or maybe even better, have it magnafluxed - but it'll have to be cleaned for that as well.
    "If it ain't broke, you're not trying." - Red Green

  22. #22
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    Default Re: Yanmar SB12 Using Oil

    This probably isn't good enough, but it all I took today. Shooting into a cylinder is not an easy photographic thing to do! https://1drv.ms/u/s!AiR9KzpBmPls1idSUZdk-X8FQNY9

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    Default Re: Yanmar SB12 Using Oil

    As for the 40psi, the oil in the head gallery, where the head bolts are, is not under pressure. It would just be leaking around the nut and the spacer below it. I can't imagine much would work its way down to where the gasket is.

  24. #24
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    Default Re: Yanmar SB12 Using Oil

    Sure looks like a leaking head gasket.

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    Default Re: Yanmar SB12 Using Oil

    Any explanation why one bolt, out of the four, is all covered with oil? If it is just a head gasket, it would save me a lot of work to put in a new gasket and bolt it back together!

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    Default Re: Yanmar SB12 Using Oil

    Quote Originally Posted by Cinderella View Post
    Any explanation why one bolt, out of the four, is all covered with oil? If it is just a head gasket, it would save me a lot of work to put in a new gasket and bolt it back together!
    The strange thing to me is the piston and valves don't have a build up like they have been burning 3 oz hr. You say it runs clean when warm. That's a heap of oil and they look pretty normal. Any signs of oil in the exhaust? Since you have the head off and rod exposed I'd go ahead and pull the piston and at least check for a broken ring and cyl condition. Its a good time to replace bearings and rings. Its very easy and you certainly CAN hone in the boat and don't have to bore unless the cyl is bad or measures out of limits. I've done it several times and I'm just a home dyi guy who wrenches my own stuff. Also, a friend of mine had a very low time 8 hp Yanmar that ended up having a cracked piston. I don't remember what the symptoms were but all he did was replace the piston/rings and go cruising for a couple yrs.

  27. #27
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    Default Re: Yanmar SB12 Using Oil

    That engine has a replaceable cylinder liner.
    If you replace that and the piston, install a new rod bearing and quick valve job it will be as new.

  28. #28
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    Default Re: Yanmar SB12 Using Oil

    I think you and I are on the same wave length. Your comment about the oil build up is intriguing. What I saw this summer was a steady consumption of 3oz of oil per hour. The boat does smoke a bit, though not a cloud, and the smoke is blue and smells of oil, not diesel, which would lead you to believe it was burning oil. If the oil got into the exhaust, via the water jacket and a leak in the head gasket, would it burn as it came through the exhaust system? Seems to me that it would be too cold. However, I do have oily build up in the exhaust. If you hit the throttle hard, as in coming into a dock too fast and back down hard, the exhaust will spit out a bunch of black, carbony crud. This seems to dissipate without an oil stain, so I have always assumed it was just diesel carbon build up getting blown out. What would I expect to see if I was dumping raw oil into the exhaust?

    As for moving ahead with pulling the piston, doing a hone and replacing the rings, it seems like the right thing to do as long as I have the motor torn down this far. My concern is torquing the rod cap bolts. The opening in the side of the engine is sufficient to do the work but the opening is not aligned with a convenient place to swing a wrench in the engine bay. I think if I use a 3/8 click stop torque wrench, it might go OK. Harbor freight has these for about $20. My experience with Harbor Freight tools, however, is not good. Perhaps I can borrow a better tool or (god forbid!) spring the bucks for a good quality tool.

  29. #29
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    Default Re: Yanmar SB12 Using Oil

    I'm aware of the replaceable cylinder liner, but I think I'll try to avoid doing that. I don't think it is something you can do in the boat easily and it definitely requires tools that I don't own. Of course, depending upon what I find when the piston comes out, it may be necessary.

  30. #30
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    Default Re: Yanmar SB12 Using Oil

    I had a Lister-Petter, 12HP air cooled diesel (generator) whose manual said for the 30 ft. lb torque on the rod bolts" "If you don't have a torque wrench, that's very tight with an 8" spanner". I am not recommending skipping the torque wrench part.

    Around here, many parts stores have "free loan" tools - leave a deposit, bring it back in good shape & you get your deposit back.
    "If it ain't broke, you're not trying." - Red Green

  31. #31
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    Default Re: Yanmar SB12 Using Oil

    Yes, I'm aware that I need to find, borrow or buy a decent torque wrench. For the rod bolts I'll have to use the click-stop type as there is no way to read a gauge when the tool is upside down. My concern is having the swing room to use the tool properly. I can try out a few wrenches to determine what is the maximum size I can work with and then go get a proper tool. I'll probably need a different, larger torque wrench for the head. The rod bolts are 40 ft/lb, the head bolts are 90 ft/lb and the socket size is 22mm. I'll probably want a 3/8 wrench for the rod bolts and 1/2" wrench for the head. I think for the head I should be able to borrow one from a local parts store.

  32. #32
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    Default Re: Yanmar SB12 Using Oil

    If you install new rings without removing the "ridge" there is a risk of crashing the new top ring into the old ridge resulting in a damaged ring and piston, among other things. There are some clearances you should check while you have it apart. (The liner should pull right out, the only thing retaining it now is habit, and a stuck o-ring or two at the bottom, which I would also replace if I was that far into it. It is best to do any honing work on the bench and keep the abrasives out of the engine if possible)

  33. #33
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    Default Re: Yanmar SB12 Using Oil

    Point well taken about the ridge. I'll examine it carefully. Do you really think the cylinder liner will pull out easily with forty years of corrosion on the outside? This is a raw water cooled engine (though all the water chambers look fine). I would think that it would be very difficult to pull it through the head deck.

  34. #34
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    Default Re: Yanmar SB12 Using Oil

    As the head is already off, check it for flat.
    Creationists aren't mad - they're possessed of demons.

  35. #35
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    Default Re: Yanmar SB12 Using Oil

    More progress today. I took off the oil pan and I think this is the way I am going to have to work on the crankshaft cap bolts. Side access is difficult and could cause a stripped bolt head or bad torque reading on reassembly. Lying on my back and working over my head, I think I can reach in and do the work OK, if painfully. I have not taken the piston out yet - still a bit nervous about that step - no turning back from there. I took photos of the bore which are posted at https://1drv.ms/f/s!AiR9KzpBmPls1ir6MmvonE1GHj9f. These are pretty high res, you can zoom in on them. The ring that you see at the top is not a lip. If anything, the opening of the cylinder is slightly larger than the bore itself. You can feel something when you run your finger over this, but it is not enough to catch your nail.

    Any suggestions on how to remove the head studs? I gave it a half-hearted try with two nuts, but the one I attempted did not budge. I don't know that I have to remove them but it would make honing easier.

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