We use a Rottler F5A boring mill. It is basically a semi-automated version of the old boring mill design they have used since the early 1970's. Rottler produces much more expensive CNC boring mills and multi-function machines. Our mill is fairly accurate but has it's limitations. The new bore is located off the original bore, so offset boring with this machine is very difficult. It can bore to +-.0003" out of round and about .0005" top to bottom. I leave a minimum .007" to do a 3 or 4 stage plateau hone on most blocks. Cast pistons and even some hyperutectic pistons run pretty tight. I build some street engines at .002-.0025. If they are round and straight with the right surface finish, no issues and very quiet. By contrast, the funny car we run at .013-.014". It depends on what you are doing with the engine and what the piston manufacturer wants.
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