When making the pistons, I mounted them on
their rods for finishing. This ensured that
they were concentric with the rods. Here,
you can see that I've stuck the rod in a
collet for turning the ring grooves.
The tool is ground to the width of the rings
to ensure that there's no slop in the groove.
Depth of the groove is less critical, but I
was still pretty careful about it.
Two grooves per piston. (By the way, if you
make your own rings, make twice as many as
you need. You should plan to break some, and
you'll likely need some for heat treating
experiments and whatnot until you get the
whole process figured out.)