Grooves on pistons and head tolerance?

Good observation. That worried me too when I took the pictures. I had used some brake cleaner to try to clean a bit so you can even see lint (blue Scotts shop towels). So it’s very reflective in the picture and shows more artifacting that you won’t see in person. I also used the flash so that creates some interesting reflections. The cylinder wall is quite smooth.

I do need to find a way to extract some of the carbon that I broke off though… it’s gotten into the piston/wall crevaces which I imagine will create scratches when the cylinder is moved.