The Ford Barn - 49 heads

There is no set amount to mill the heads. There are production variances and they may have already been surfaced. The above poster is correct in that the piston head will contact before the valves (at least with a stock cam). What you have to do is aim for a set clearance between the piston crown and the head. You do this by using aluminum foil balls attached to the top of the piston with dabs of grease, installing the heads (with used gaskets) and turning the engine over a couple of times. You then measure the compressed thickness of the aluminum balls. You want .045 to .050 clearance between the top of the piston and the head. Let's say most of the balls measure at .070 compressed. This means you want to take.020-.025 off the heads. What I do is measure in seven or right spots on each cylinder, and then come up with an amount to mill the heads to get close. Please note that the amount taken off of each head may be different. I shoot for something that gets close with a few tight spots. I then use a die grinder to relieve these areas until a consistent clearance is established. I like to end up with a consistent .045" to .050" clearance which will give you the maximum compression with those heads and provide a proper "quench" area on top of the piston.

There are variations of this scheme; using clay balls instead of aluminum foil, bolting the heads on with no gaskets and trying to get 0" clearance (because compressed gaskets are between .050"-.055" thick). It takes a little practice, but it's fun and makes you feel like a real "Hot Rodder". It is also the single most important thing you can do to improve flathead performance and economy. (It's a lot easier with aluminum heads, but stock heads can be made to perform as well as the fancy finned aftermarket units.