Your Opinion Please - SB12 Yanmar - SailNet Community
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post #1 of 11 Old 12-25-2013 Thread Starter
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Your Opinion Please - SB12 Yanmar

I'm not sure what direction to go from here being new to diesel marine engines. Your advice is very much appreciated.

Ok, this is a Yanmar SB12 that locked-up about four years ago from two owners ago (don't know details). I pulled the head to check-out the piston and cylinder (first photo) and they don't look so great. I went with the Coca Cola method and let that sit for a day and tried to manually crank and even with my body weight on the bar, she wouldn't break free. Then tried wood on the piston and tapped with a hammer - nothing. I drained the oil to see if I could learn anything from that and it looked pretty clean and was definitely free of water.

I don't want to give up on the engine because I enjoy learning and have the time but want to know if I should start parting it out.

In the second picture, I cannot get "1" to spin or break free either. Any suggestions? What is the purpose of "2?"

Thanks everyone.
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post #2 of 11 Old 12-25-2013
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Re: Your Opinion Please - SB12 Yanmar

Coca-Cola?????
Try a good penetrating oil and let it sit for a few days adding more as it soaks in. Then "gently" tap on the pistons. Once free, slowly and carefully work the crank back and forth lubricating the pistons at the same time.
I don't know much about this particular model of Yanmar but I can tell you that they make a great product. Number "2" is probably a fuel related camshaft or pump driveshaft. Other members may know more.
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Re: Your Opinion Please - SB12 Yanmar

It would be nice if you could break it free but I bet she'll need new rings and a good honing at the very least. That means opening the pan and pulling the piston.I usually put the whole critter on a bench but it can be done where is. Not rocket sci and a great learning experience.(too much wham bamm can damage the ring lands and that means a new piston, not cheap)
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post #4 of 11 Old 12-25-2013
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Re: Your Opinion Please - SB12 Yanmar

This is what "kroil" was meant for. Tke the Kroil five day challenge-fill the cylinder to the top with kroil on the morning of day one, top up on the morning of day two, repeat on day 3 , etc. let it soak, let it soak, let it soak. on day 5, have at it with a 1 pound hammer and a chunk of hardwood. If it doesn't break free within 15 mintues of pounding, step up to a 2 or 3 lb hammer. watch your fingers. If at this point it still doesn't want to break loose, Capt len is on the mark- time to pull the engine, bolt it to a stand and turn the engine over on the engine stand, drop the pan and check the crank case. If the crank isn't a corroded lump of crud, there's hope. Unbolt the connecting rod, and try to send the piston north instead of south.

It's 5 o'clock somewhere:


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Re: Your Opinion Please - SB12 Yanmar

Thank you. Wheel "1" will not rotate in either direction either. Is that relevant?
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Re: Your Opinion Please - SB12 Yanmar

yep. it is all connected. if the piston is jammed, up the crank is jammed up and if the crank is jammed up, everything else is jammed up.

It's 5 o'clock somewhere:


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post #7 of 11 Old 12-27-2013
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Re: Your Opinion Please - SB12 Yanmar

After four years, breaking this thing free is not going to be easy. Not very heavy since its only a one cylinder. Pulling it and taking it to a shop may be the best option.
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post #8 of 11 Old 12-27-2013
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Re: Your Opinion Please - SB12 Yanmar

If you remove the pan, unbolt the rod cap, drive out rod piston and liner in one operation! Throw these out as they need to be replaced. Check to see if crank is free. Don't fix it if it's not broke......Dale

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post #9 of 11 Old 12-28-2013 Thread Starter
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Re: Your Opinion Please - SB12 Yanmar

I pulled the face portion off and now I'm confused what I should do to isolate bent rod vs seized piston (no luck with freeing using Mystery Oil).

What do you think I should do next?
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Re: Your Opinion Please - SB12 Yanmar

You are on wrong side of motor! Pull pan side , disconnect cap bolts on rod, cover threads with hose to protect crank! Use wood block to drive rod, piston,and liner out head side. then see if crank is free if so replace parts you need after cylinder is out you may be able to save rod! I own a yanmar Ysm8 same motor type easy to work on!…Dale

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