Originally Posted by zephyrc
i have a sbc 408, pro 1 heads, 10.8-1 cr.. and i would like to raise the cr to around 11.5-1,, the heads i have are 64cc and i have 210 thou piston to valve clearance. how much would i have to take of the heads to get the increase that i want and would i have to skim the intake
About .045 inch. Don't forget the intake also has to be milled sides and bottom. The ratio for material removal from the intake is about 75 percent of the head deck cut for the intake side and about 2 times for the bottom cut
Forgot to mention that as the head moves toward the crank center, you may find it necessary to make adjustments to push rod length, so be sure to check out the rocker sweep on the valve stems and make corrections if necessary. The other thing you need to watch is distributor engagement in the oil pump drive and whether or not the distributor sits tightly enough on the intake to maintain and oil tight seal. There are shims sold for the purpose of making this adjustment if it becomes necessary, though doubling the distributor to intake gasket is usually sufficient.