2GM20 Engine Overheating

[MikeHoncho]

If it sat for 2 years with moisture in the cylinders you may want to take a better look at the cylinder walls and rings. Also verify that the block deck is flat.

[Columbia65guy]

I was planning to take a straight edge next and look for major distortion before reassembling. Any chance you can point me to any videos or write-ups on checking the cylinders/sleeves? I'm not sure what to look for or how to test anything

[MikeHoncho]

I’m sure you could find a video on YouTube but it will have obvious corrosion on the cylinder walls that are above the piston( dependent on the stroke). I’d take a trip to a machine shop and ask them questions on checking for flatness. I think you’ll be surprised how flat it really needs to be and a straight edge isn’t going to prove much. Assuming there is a a obvious burn through area on the head gasket, that area would be my guess if the problem area unless the head bolts where not torqued properly. Did you check the torque of them prior to disassembly? If the head checks out to be ok then it was either a assembly error or the block is warped and will need to be machined. If this is the case then why? Check your coolant system. Might be worth contacting the previous builder just to see what they have to say but don’t use them again. Don’t put it back together without a good idea what caused the issue.

[Compass790]

You should get a copy of the workshop manual, there may be one in the CF library. If it has no visible rust on the bores it should be ok. Was it a fast starter before? If it started instantly compression should be ok, ergo bores cant be be too worn. You are in fresh water so its not as bad a scenario.
You can do a reasonable job checking for block/head flatness with a decent straight edge & feeler gauges. As I said in previous post 518 can deal with up to 0.25mm gap.

[Columbia65guy]

Hi all, quick update:

-I replaced the head gasket and as I torqued up the bolts I snapped one of the smaller bolts (#7/8) in the block. I've been able to get it out and source new ones. I have installed yet another gasket and re-torqued the bolts again according to the manual. As I'm torquing the smaller bolts again, I got about 180deg of rotation while the torque remained at a steady ~50ftlbs. I'm not sure if this is bolt stretch or it is about to yeild the bolt again. Any guidance or advice would be greatly appreciate!

[MikeHoncho]

you are using all new bolts? Typically new bolts/studs are used but I'm not certain with these motors. I posted a shop manual in sailboatowners.com. do a search of my name and it should pop up if you don't have a manual yet.

I'm not experienced with replacing head gaskets on these motors so I won't give guesses but 50 ft/lbs isn't a lot. Have you checked your torque wrench for calibration?

[skipperpete]

Originally Posted by Columbia65guy

Hi all, quick update:



-I replaced the head gasket and as I torqued up the bolts I snapped one of the smaller bolts (#7/8) in the block. I've been able to get it out and source new ones. I have installed yet another gasket and re-torqued the bolts again according to the manual. As I'm torquing the smaller bolts again, I got about 180deg of rotation while the torque remained at a steady ~50ftlbs. I'm not sure if this is bolt stretch or it is about to yeild the bolt again. Any guidance or advice would be greatly appreciate!

The small diameter bolts only have 21.7 ftlbs torque, it’s a little obscure in the manual but it’s the 1GM that has 55 ftlbs... but on all the bolts, no small ones at all.
Pete.

[Bill Seal]

When tightening bolts, if the resistance stops increasing, STOP!!!
Either the threads are stripping or the bolt is breaking. Re-check what's going on, nothing good can happen if you keeping twisting it.

[LittleWing77]

Originally Posted by skipperpete

The small diameter bolts only have 21.7 ftlbs torque, it’s a little obscure in the manual but it’s the 1GM that has 55 ftlbs... but on all the bolts, no small ones at all.
Pete.

Pete's got it!

Yanmar 1GM/2GM/3GM Service Manual
https://www.downloadboatmanuals.com/motors/yanmar/

If you look at p. 2-16R, the first set of torque values are for the 1GM. The second set are for the 2GM indicating 86.6 foot-lbs for the M12s (big bolts) and 21.7 foot-lbs.for the M8s.

Good luck!
LittleWing77