The previous picture shows the piston after it has been knurled. Here is what the knurling looks like after sanding with some emery cloth. This one is sized for its cylinder.
I only took off a couple of thou to get the diameter down to where I wanted it. I mike things at about 9 spots as I take metal off on both areas of knurling: top, middle, bottom on the left side, the center, and down the right side of the knurling.
Is it absolutely perfectly uniform...like a 'new' piston? No, it is not. But,I try to get to within a tenth of a thou at the 9 spots I measure at. It is close enough to do the job.
With the knurling and careful sizing, I have restored the piston to cylinder wall clearance.
And it is where I want it: one and a half thou.