Yanmar 2QM20 running issues

[jimbunyard]

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.

My approach to figuring the difference would be different, because everything else except the stroke is the same, and stroke + combustion chamber volume yields compression value.

Rounding the 1/8" to 3 millimeters, given then that there is three mm less stroke, 3 mm equals about 4% of 72 mm (the stroke of the 2QM), so there should, since all other things are equal, be about 4% less combustion pressure.

Using 400 PSI as the combustion pressure, 4% of 400 is 16, so there should be, using this logic, 16 psi less combustion pressure.

Don't know which is correct.