milling caddy heads ???

well i am going to jump into the hp race today, am going to get a eldebrock intake for my 500 caddy and also while i am that far going to take the heads off and get the bad exhaust manifold bolts fixed.
the original head gaskets were a thin style.
how far could i mill these heads to first make up for the thicker gasket my investigation was 20 thou , how much more can one mill heads till one has possible valve interferance. the pistions not flattoped as have the figure 8 dish in them. factory std lift cam in this thing.
i am assuming one can mill the heads 60 thou without clearnance problems but want to make shure befor i take them to the machine shop and get into trouble.
what would the std rule of thumb for comp increase for milling heads on a big block style motor ie 20 thou = 1/2 comp ratio ? :) :) :)

it has alot to do with the volume of the head if your is .017 to .020 thick gasket and if the new ones are .034 to .040 then your would cut .017 to .020 off to be the at same place but this will chang the cc a bit .the head would be were it was on the block with old heads gaskets . best way to go is cc the head when on the mill to know how much you have and what a cut is in cc .to many thing s can change thing . you can cut off .060 off but you will have to check the intake for fit and may need to be cut. valves hitting the pistons will need to be check to. you will need to find out the the cc of the old head gasket the cc of the new gaskets and the cc of the head and go from there and the cc of the piston at tdc

thanks pat i dont think i will be trying to get more comp out of my caddy i did some investigating re type of piston in my car. it was always in question what year the motor was as the previous owner thoguth it had been changed before he got it to a early year 500 71,he thought.
the pistons appear to be from a early engine with 10 t0 1 comp as they are not a actual dished piston by have a figure eight flycut in them.
pics off the caddy site say 10 to 1 if you ave these pistons.
this would make sense as this is the most timing sensitive engine i have ever owned even in a light car if you put the timing anywhere past 10 degrees advance instant knock.
i thing i am going to ahve the machine shop take 30 thou off the heads to make up for the new thicker gasket and put them right back on.
i am now even thinking of just keeping the old intake also as i dont know how much more i will gain in this case, i have been told 20 hp so i will check it out.

took the heads to the machine shop and have decided to do the cheap manifold modification as per other sites i have visited and clean out the center of the carb side of the intake manifold to open up the intake.
spent about 20 minutes last night with the grinder and got a awful lot done
do that a little more tonight and then get the die grinder out to ream and smooth the rest of the manifold out