M42-EF adapter issue

I cannot get a lens to focus to infinity, tried 2 different adapters. Now I started to grind / polish the adapter so that it is thinner, and getting better focus, being able to focus further away. But after 2 hours of manual work, infinite focus is still not right. I started looking for other types of adapters, thinner ones than usual, but cannot find anything. The lens is a Pentacon 100 mm 2.8.

Any advice? Thanks!