I know I am going to regret asking this, but - if the engine was not already misfiring/quitting on regular fuel, why would you want to hog out the head? You just give up power on an already pretty weak engine. If it's too aggressive on the break (which I have never seen on any Brodak 40), it takes something like .005" of shim to make a pretty big change, tripling the head volume is 10x more than that.
I don't understand - In the entire 40+ years I have been flying stunt, I have seen stunt engines "overcompressed" for conventional fuel exactly twice - Discovery Retro that wouldn't run properly on even 5% (2004 WC) and ST46s with extruded head gaskets that still ran OK about half the time. Otherwise, I and other people have run 25-35-40% nitro in otherwise completely standard stunt engines with no ill effects except for running out of fuel after the triangles. Stunt engines, and even RC sport engines, are generally nowhere close to being "overcompressed" and it just about never needs any changes just to get it to run. But adding all that head volume, or adding 20 head gaskets, or whatever, just kills the power. What is it that people think they are accomplishing?
The later Brodak 40s are pretty good runners with OK power, if you get a proper venturi. Is this all trying to get it to run with the stock insanely oversized venturi?
I am really not trying to be a smart-ass, but I just don't understand where this obsession and abject terror of being "overcompressed" came from, Brodak 40, or any of the hundred of other engines that people seem determined to stack 20 head gaskets into (that patently don't need it).
Brett