I milled mine .030 at work. I haven't figured out exactly what compression this will give me just yet, but I have been told by a couple people that it will be between 9.5:1 to 9.7:1. 40 should put you close to 10:1 I would guess.
Summit sells a tool that you use to test the volume in the chamber. I can't remember what it's called. Maybe it was a barret or something like that =P
Someone needs to take a stock head and mill them in .005 increments and record the volume each time so people can access this info because I tried to find out when I did mine with no positive answers.
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