The end mill on my drill press to deepen the valve pockets was a crap idea. It would bite and release, bite and release, so very hard to control. I found a mini router with a grinding stone followed by a drum sander on my Dremel tool worked best. I must have done 8-10 test assemblies with playdoh, heads, cams, belts to get all the clearances where I wanted them. I also did the squish landings to get .050 with piston rock. A bit less would have been better but the compression of the head gaskets over time and retorques, liner movement, etc. made me a bit conservative.