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Thread: Yanmar SB12 Using Oil

  1. #36
    Join Date
    Oct 2006
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    Michigan
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    34

    Default Re: Yanmar SB12 Using Oil

    Wouldn't a new head gasket be cheap enough and slap it back together? If that dosen't work pull the engine and do it right. John

  2. #37
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    Default Re: Yanmar SB12 Using Oil

    Quote Originally Posted by CJM View Post
    Wouldn't a new head gasket be cheap enough and slap it back together? If that doesn't work pull the engine and do it right.
    I tend to agree. While the head is off remove the valves and inspect the guides, and lap the valves.
    Without removing the engine from the boat I would avoid removing the liner.

    Re the valve stem seals, I received a newsletter from RockAuto which has a small article about them. It may be possible to retrofit some to fit your engine. https://www.rockauto.com/Newsletter/
    There is nothing quite as permanent as a good temporary repair.

  3. #38
    Join Date
    May 2002
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    Deer Isle, Maine
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    1,523

    Default Re: Yanmar SB12 Using Oil

    Quote Originally Posted by Cinderella View Post
    ...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.
    I've used these with success before...

    https://www.youtube.com/watch?v=fjlnqlWDbfo

  4. #39
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    Oct 2017
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    windham , nh
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    27

    Default Re: Yanmar SB12 Using Oil

    I used the bad weather today to clean up the head and a few other parts. When cleaning the head I noticed dent marks right by the injector precombustion chamber. Associated with this is a "lumpy" area by the adjoining water jacket port that looks like a possible crack. I went and got the head gasket from the boat and you can see that these marks match marks on the head gasket in the same area. The head gasket looks to be in poor shape in this area, though I don't know how much of the damage came from the life of the engine and how much came from disassembly.

    Should I get the head machined to remove this damaged area and get a perfectly flat surface again? I don't think this engine has ever been apart before, so it's a bit of a mystery as to how these imperfections were created. If I get the head machined, how do I remove the precombustion chamber? This is shown in the parts diagram, but there is no mention of it in the workshop manual. I see in the parts list that there is a locating pin associated with the chamber, but no mention as to where it goes or how it is installed or removed. (I think I could take about 0.020" off the head without impacting the precombustion chamber, maybe this would be enough.)

    I also removed the valves, which seem ok. Some crud and build up on them, but otherwise they look good (to my untrained eye). I will give them a light lapping on re-assembly. The valves are very tight in the guides with no lateral play at all. I'm assuming the guides do not need replacement.

    Photos are posted at (Note, the photos are full 16MP resolution, they will take a moment to load.) https://1drv.ms/f/s!AiR9KzpBmPls1jA4sX7lMEUkrMwM
    Last edited by Cinderella; 10-11-2018 at 04:16 PM.

  5. #40
    Join Date
    Apr 2012
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    228

    Default Re: Yanmar SB12 Using Oil

    Not familiar with the engine at all, but sometimes rust throughon a thin casting will show up as bumps on the opposite side from the rust. If the cross section under the rough area is thick, that is not the problem and the head needs to be machined.

  6. #41
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    Jun 2003
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    Default Re: Yanmar SB12 Using Oil

    Have the head resurfaced.
    This begs the question, how is the fire ring surface on the liner?

  7. #42
    Join Date
    Oct 2017
    Location
    windham , nh
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    27

    Default Re: Yanmar SB12 Using Oil

    I think the fire ring surface in the liner is fine. The bottom of the head gasket is OK. The photos are of the head and gasket flipped over to show the gasket where it joined the head. I'm planning to get the head machined, but now I have to figure out how to remove the precombustion chamber. Not a word about it in the workshop manual, very little online. The parts list show a "pin" that holds the thing in somehow but nothing more than a part number for a description or what it does. Just the next mystery I have to figure out!

  8. #43
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    Jun 2003
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    Default Re: Yanmar SB12 Using Oil

    I would leave the precombustion chamber in place, and surface it as one unit. (Typically this would be done on a big deal belt sander, specifically for cyl heads)

  9. #44
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    Oct 2017
    Location
    windham , nh
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    Default Re: Yanmar SB12 Using Oil

    Canoeyawl; that's what I was hoping to do. I think there is about 0.020 available before the milling would hit the precombustion chamber, it sits in just a bit. I'm not even sure I need that much to clean up the divots - I don't have a good way to measure them. I'm going to stop at an automotive machine shop nearby tomorrow and have a word with them. They might have some good ideas or advice.

  10. #45
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    Default Re: Yanmar SB12 Using Oil

    I have seen plenty of precombustion chambers surfaced along with the head. I'm not sure on that engine, but some engines they may even be slightly proud of the head surface, maybe .001-.003"
    (It is pretty common to just grind them in situ with the head. Worst case scenario is that they are loose, watch for movement while you surface it)

    edit;
    I found an image now, the pin just orients the chamber and that should all come out with the injector. The copper seals are Yanmar parts. There must be a spec for thickness of the head?
    (I still might be tempted to surface it in situ and clean it after)

    IMG_4291.jpg

    edit;
    I see you check the head clearance using a piece of lead wire. Only .010 tolerance.

    IMG_4294.jpg
    This above is from Google books, somewhere here I have the manual, I'll try to dig it out...

    OK, found the book!

    Liner protrusion is .003-.006 (pretty standard...)

    It shows a simple puller to remove the liner, you will need a "plug" to seat at the bottom of liner. (Usually I make one up in the lathe and have quite a collection, but I have made these "one off" on the job with threaded rod, a length of flat bar a hacksaw, drill and angle grinder.
    Last edited by Canoeyawl; 10-11-2018 at 10:40 PM.

  11. #46
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    Oct 2017
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    windham , nh
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    Default Re: Yanmar SB12 Using Oil

    Canoeyawl: We must be on the same wavelength. I found that same book last night on Google Books - it is much more thorough than the Yanmar manual I have a copy of. I think I am going to try and get the precombustion chamber out. At very least, I should be cleaning the injector pin, so I'll need to get in there anyway.

    What do you know about the liner protrusion? I saw the specs on the protrusion in the section about installing a new liner, but nothing else. I'd say my liner, right now, is flush with the block deck. Is this a major issue?

    Thanks for your research - you've gone above and beyond!

  12. #47
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    Default Re: Yanmar SB12 Using Oil

    Well, .003" (the minimum) isn't much (about the thickness of a human hair) but the head gasket seal relies on that.

    So yes, it is important, and you need to measure it accurately. You could use a straight edge and feeler gages. The steel ruler from a good combination square should work.

    (I use a depth micrometer or straightedge depending on application. Before assembly I measure the thickness of the lip and the depth of the counterbore and subtract the difference.) It surprises me there isn't "any" protrusion, that is usually caused by the liner "bouncing" in the bore. A loose bolts, severe overload etc can cause that.
    Have you checked the other tolerances, connecting rod, bore to piston end etc? You can check the bore to piston with feeler gauges also, and use plastigauge to check the rod bearing.

    (note* This is not an uncommon problem and there are ways to fix the liner protrusion, but the simplest is to check the thickness of the "lip" against a new liner. If not and the counterbore is good, the top of the block could be "decked" and added shims are not uncommon)

  13. #48
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    Oct 2017
    Location
    windham , nh
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    Default Re: Yanmar SB12 Using Oil

    The mystery of the oil consumption deepens.

    I took the piston out today. It was not overly difficult though I can see that reassembly will be a hassle. That was the good news. The bad news is that nothing is wrong. It was immediately obvious that there were no broken rings or significant wear that would cause high oil consumption.

    All of the parts were clean and well oiled. There is negligible carbon build up anywhere other than on the top of the piston. Even that is not too bad. The wrist pin is nice and tight and the rod bearing looks fine.

    I spent the better part of the afternoon going over all the parts, checking them with a micrometer and a caliper to determine their wear state. Nothing is beyond spec requiring replacement. Most notably, the oiler ring had less wear on it than any of the other rings and it is perfectly clean. But all of the rings are within tolerance for breadth and thickness. The piston diameter is OK and all the ring landings are within spec - except the compression ring landing. That is at maximum spec (+0.15mm) but not over spec. I’m not sure about the ring gaps. I measured the gap on the compression and oiler rings and it is about 1.10mm. I don’t have a spec for this but this seems reasonable (and shouldn’t be out of spec unless there was a significant loss in the ring thickness and or an increase in the bore diameter).

    I also checked the valves. The valve shaft diameters are well within maximum tolerance and the spring heights are within limit.

    So, now I’m in a bit of a quandary. I have a list of parts I’ll be replacing, including the rings and the rod bearings, I’ll hone the cylinder and I’m getting the head worked on to fix the damage near the precombustion chamber. I also plan to pull out the injector and clean the injection pin. I have no doubt that my work is going to prevent a head gasket failure that was probably going to happen next season and the engine may run a bit better, but I have no reason to believe that I will have done anything that would reduce the oil consumption. What next?
    Last edited by Cinderella; 10-12-2018 at 03:32 PM.

  14. #49
    Join Date
    Apr 2005
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    Hills of Vermont, USA
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    25,509

    Default Re: Yanmar SB12 Using Oil

    I suspect either that bad spot on the head or a crack in the head - or both possibly.

    The 8HP I mentioned above had a really difficult to see crack (between the valve seats IIRC) & that caused lots of oil consumption. In my case I replaced the head (& gaskets of course) & no more blue smoke at all.
    "If it ain't broke, you're not trying." - Red Green

  15. #50
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    Default Re: Yanmar SB12 Using Oil

    Have the shop magnaflux it before they do any work.

  16. #51
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    Jan 2003
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    Default Re: Yanmar SB12 Using Oil

    Interesting that the oil ring shows less wear. Maybe it hasnt bedded in properly.
    It seems that you are on the right path with your repairs.
    There is nothing quite as permanent as a good temporary repair.

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