Help - Need advice Re crossflow compression

[Bruce G]

It's been a long time since I posted. I am a collector of vintage fiberglass boats - the kind that look like the cars of the 50s and early 60s. I like tinkering around with the old outboards the most though. I am in process of making a cool sleeper engine - it has the hood & outer skin from a 1958 Evinrude Fat 50, but the powerhead is a 1968 100hp. The 68 100horse actually burns less gasoline that the other early V4s did.

I plan to use the heads from a 1970 100 horse motor, as it and my 1968 were both 90 cubes. The 1970 heads have a improved combustion chamber design. Most everyone who has experience with the old crossflows knows that the compression wasnt very high. I know that a motor with cranking pressure from 90psi to 110psi is all they had when new. I packed one of the combustion chambers with plumber's putty and rocked the crank over to have the piston smash the putty into the head. I then removed the head & measured the thickness of the putty left in the dome of the combustion chamber. I discovered that there is no less than .250" or 1/4 of an inch from the piston crown to the cylinder head in any given place that I mesured. I also had the head gasket in place when I did this. I am thinking that milling .125" or 1/8 inch from the heads would give a good increase in compression, while leaving no less than .125" at the minimum between head and piston. I have milled heads on the old V4s up to .060" without having pistons melt.

My question is this - how sensitive are the old cross flows to compression boost, nd how much is too much? I plan to run regular pump gas, and this will be a cruiser, not a racer. I can always run surface gap plugs to keep her cooler. I just don't want to blow holes in pistons. Did OMC keep the crossflow compression low for reliability? Am I blowing it if I mill these heads this much?

Sorry for the long post, I just can't get a real answer anywhere else & these questions are eating at me & stalling my project.

Thanks for any advice.

Bruce G

Redding CA