Yanmar 2QM20 running issues

[Wotname]

Can you post some pictures of the bearings (mains and rods)? I would be keen to see them.

Have you discovered the reason for the low compression yet (valves or rings or sleeve or ....)?

[Islandcruiser]

Here is a Pic of the crank, it was a real PITA to get the flywheel and V-pulley off.
The liners were a bit scratched but not terribly, so I think the low compression was due to the piston not travelling the full stroke.
I would have posted more but the file sizes were all to big.

The next step in the saga is a 3GMD that a friend gave me to see if it was worth installing.

Cheers

[Compass790]

Thanks for update.
The crankpins are a bit ugly.
I'm a bit surprised that your worn out main on one end caused the massive compression drop. Fixed a single cylinder that had no big end bearing left & it' conrod was worn egg shaped & it still ran! ( very noisily) Needed a squirt of oil to start from cold tho. From admittedly unreliable memory it still had 280 psi compression.
Still you say you checked the tolerances.
Yea those flywheel tapers can be a battle. Shock & heat help.
Post back please after you get all the parts

[Islandcruiser]

Originally Posted by Compass790

Thanks for update.
The crankpins are a bit ugly.
I'm a bit surprised that your worn out main on one end caused the massive compression drop. Fixed a single cylinder that had no big end bearing left & it' conrod was worn egg shaped & it still ran! ( very noisily) Needed a squirt of oil to start from cold tho. From admittedly unreliable memory it still had 280 psi compression.
Still you say you checked the tolerances.
Yea those flywheel tapers can be a battle. Shock & heat help.
Post back please after you get all the parts

The main bearings were not the issue, it was the crank journals that the conn rod bearings ride on. 50 thou out of round, I wasnt able to find a bearing that undersized and so I will be selling the parts.

[Compass790]

Originally Posted by Islandcruiser

The main bearings were not the issue, it was the crank journals that the conn rod bearings ride on. 50 thou out of round, I wasnt able to find a bearing that undersized and so I will be selling the parts.

Oh I see it was a terminology thing. I refer to the big end journals as crankpins.
So do you mean you have given up on the rebuild & selling the engine for parts? You can get the crankpins built up, hardened & reground here but dont know about where you are.
You must of had lubrication failure.

[Wotname]

AFAIK, the biggest undersized big ends are 30 thou so I'm guessing the crankpin needs building up around 20 thou.

If there was a lube failure, I would except the mains to be shot also. I wonder why they escaped massive wear.

IIRC, only one cylinder had poor compression so were both con rod journals worn equally or only one?

[Compass790]

Originally Posted by Wotname

AFAIK, the biggest undersized big ends are 30 thou so I'm guessing the crankpin needs building up around 20 thou.

If there was a lube failure, I would except the mains to be shot also. I wonder why they escaped massive wear.

IIRC, only one cylinder had poor compression so were both con rod journals worn equally or only one?

Maybe he had blocked crankpin galleries only but in my experience the crankpin wears much faster that the mains as I think the waggling back & forward of the conrod mucks up the oil wedge on the big-end.
I think thats why the big-end bearing is different bearing metal than the mains.

[kmacdonald]

Looks like a lot of carbon buildup on the crank under one cylinder. Lots of blowby. Was it run without oil? Diesel diluted oil or water diluted oil?

[jimbunyard]

Seems I remember he said one of the bearings was in backwards. Perhaps he meant upside down, and the oil hole in the main was in the cap, and no oil could flow to the rod bearing.

What seems odd to me is that the loss of probably a maximum of 1/8" stroke would cause such a large drop in compression; I didn't do any math on it, so it's probably just counterintuitive.

[Compass790]

Originally Posted by jimbunyard

Seems I remember he said one of the bearings was in backwards. Perhaps he meant upside down, and the oil hole in the main was in the cap, and no oil could flow to the rod bearing.

What seems odd to me is that the loss of probably a maximum of 1/8" stroke would cause such a large drop in compression; I didn't do any math on it, so it's probably just counterintuitive.

No I think you are right from experience not calc. probably he had another problem causing the low compression as I had a much worse conrod & the engine had much higher compression.
They are pressed in single part mains & you'd have to be really drunk not to line up the oil hole in the bearing with the gallery but anythings possible I 'spose.
You'd have good oil pressure tho if you covered the oil gallerys with the bearings as it would be set by the bypass v/v.

[kmacdonald]

Originally Posted by jimbunyard

Seems I remember he said one of the bearings was in backwards. Perhaps he meant upside down, and the oil hole in the main was in the cap, and no oil could flow to the rod bearing.

What seems odd to me is that the loss of probably a maximum of 1/8" stroke would cause such a large drop in compression; I didn't do any math on it, so it's probably just counterintuitive.

That 1/8" would make a huge difference in compression. There is VERY little clearance between the piston and head so the 1/8" is extra clearance is huge.

[Wotname]

Originally Posted by kmacdonald

That 1/8" would make a huge difference in compression. There is VERY little clearance between the piston and head so the 1/8" is extra clearance is huge.

That is my first thought also but I not so sure that I'm am correct in this thinking.

While the piston is flat and does come right to the top of the sleeve (almost) and the head is mostly flat against the sleeve, there is a considerable volume in the recessed part of the head and especially in the pre-combustion chamber.

Clearly doing the maths is the better approach but I'm not smart enough to do that.

[kmacdonald]

Bore 8.8cm
1/8"=0.3175cm
volume= pi X r squared X stroke = 3.14 X 4.4 squared X 0.3175 = 19.3 cc's

So if the pre-combustion chamber is 3 cc give or take a few cc's then add the volume of the piston cup. I just looked at a piston from a 1.5 liter diesel and the cup couldn't be more than 3 or 4 cc's so it's going from 6 or 7 cc'c to 25.3 -26.3 cc's with the 1/8" shorter stroke. Big difference.


With the cost of parts and machining it's not worth rebuilding. Buy new or find an engine that doesn't need a crank and complete rebuild.

When looking at pistons it should be noted a direct injected diesel will have a much bigger cupped piston. A indirect injected (pre-combustion chamber) will have very little cup.

[Compass790]

Originally Posted by kmacdonald

Bore 8.8cm
1/8"=0.3175cm
volume= pi X r squared X stroke = 3.14 X 4.4 squared X 0.3175 = 19.3 cc's

So if the pre-combustion chamber is 3 cc give or take a few cc's then add the volume of the piston cup. I just looked at a piston from a 1.5 liter diesel and the cup couldn't be more than 3 or 4 cc's so it's going from 6 or 7 cc'c to 25.3 -26.3 cc's with the 1/8" shorter stroke. Big difference.


With the cost of parts and machining it's not worth rebuilding. Buy new or find an engine that doesn't need a crank and complete rebuild.

When looking at pistons it should be noted a direct injected diesel will have a much bigger cupped piston. A indirect injected (pre-combustion chamber) will have very little cup.

Yes not arguing with your calcs ( thanks for those ) but if you read post #49 it wasnt my experience with NO big end bearing left & conrod worn oval. Compression was still nearly 80psi higher than his and single cylinder engine still ran albeit sounding like a blacksmith beating on an anvil.
Guess it's the heating under compression thing again
Yes think the 2QM20 has flat top pistons but I'll be corrected if I'm wrong.

Yep agree a good runner is probably the best bet.

[Wotname]

Yep, the 2QM20 pistons are flat topped.

If I run out of other things to do, I might measure the capacity of the pre-com chambers