[Purdy Swoft]

The C head is said to be 4.6 and the A head is 4.22. the C head is almost a half compression point over the A head. The shape of the chamber has as much or more effect as the .4 increase in compression. The C head can be milled some but is a very thin casting and will be weakened. If you look at the valve side of the combustion chamber you will notice the area is dished way more than the stock A head. This was Fords attempt to reduce shrouding of the valves. If very much is shaved you will lose clamping area on the head gasket on the valve side and part of the gasket will be exposed to fire in the combustion area, not a good thing!!! Milling away the counter bore may raise the compression ratio to around five to one (just a guess) but would probably cause more problems . I have had C heads milled slightly but would leave a depth of at least .030 in the counterbore. The C head was the poor mans high compression head, it will give a slight increase in hill climbing and driveability power but not as much as a Snyder 5.5. I'm quite sure that there are others on this forum that have used the C head and noticed the boost and used it for that reason. People that have never run a C head on an A engine will give opinion. Opinion without actual experience isn't worth much.