Probably have to cut heads 50 -60 thousandths to get 60 cc. I've milled pass car heads down to 55 cc a few times, had to mill intake a like amount of course.
Yep last J heads I cut to 59 ccs was .050" removed. Those had flat faced valves. Notice the calc says 10.75, I will cut .045"-.050" and if it winds up @ 10.3 so be it. J.Rob
