milling heads for comp ratio increase

Thanks for informative reply Mike. My 273 is all stock so it has the original CR of around 9:1 But because I have to use 318 head gaskets, which are slightly larger bore and so will reduce the CR to about 8.8. It is a 67 block but has the later 1968-71 318 heads which are open chamber 60cc. Valves: 1.78 / 1.5"

It also had a hydraulic cam whereas the 1967 273 engines were supposed to be mechanical (and with 57cc chambers). I don't think it had ever been pulled down so it may have been an early example from the factory with the later fitments. However I am fitting a small solid cam (0.47 lift) and the 273 mechanical adjustable rockers etc to bring it back to what a 1967 273 should be. Also it is getting a Carter AFB (4 barrel) and M1 intake

But I want to increase the CR to over 9:1, around 9.5+ would be good. Not too much as it doesn't have the hardened exhaust seats. But I will run an additive for that.

I could put in stainless flat head valves which would give that increased CR (over the stock tulip/dished style valves) but the cost is too much. Buying the valves plus getting the seats cut for the 1.88 /1.6 valves. Best option is to mill the heads.

It's an easy question to find the answer for, I will keep looking through the internet or call some engine shops. Just thought someone might know off the top of their head. If there was some basic rule of thumb about it.

However I have found a little useful info which agrees with your post TMM. From an article by Paul Pitcher on how to port LA heads: "For any open chamber head, the chamber volume is reduced about 0.2cc for every 0.001" planed off the deck surface of the head. If more than 0.010" is removed from the deck surface, then the intake surface will need to be milled to allow the intake manifold to fit. Mill 0.0095" from each intake surface for every 0.010" milled from the deck surface of each head."

So that has answered most of the question. I just need to to find the formula to work out CR from combustion chamber volume and maybe about 15 thou off the heads might be enough.