I’ve been meaning to follow up on this post, here are a couple of pics I took showing the differences in the cylinder heads if folks are trying to identify the 18/20 heads vs the 25hp heads. Note that if you accidentally install a 25 head on a 18/20 motor, I had compression readings between 180-200 PSI (normally about 115-130 with the right head). Can’t imagine it’d take long to blow up a motor that way.
Forget about the parts #’s, just look at the inside of the cylinder heads. Seems that later parts actually had the number 20 stamped near the inside of the plug hole (cleaned off the one on the left to reveal the stamped number better for the picture…these were off of dead motors). I tried looking up part numbers before realizing it’s easier to just look at the inside cupping. Pretty easy from there on.
