What you are seeing is one of the pistons after it has been knurled on a K-line knurler. We are looking at the major thrust side of the piston. The minor thrust side looks exactly the same. The knurling is done to extend just beyond any scuffing mark. That way the raised surface area will be all of the surface area that comes into contact with the wall.

The purpose of knurling is to increase the diameter of the piston skirt: at the surfaces that come into contact with the cylinder wall. That diamond pattern raises the surface of the skirt about four thou [0.004in] on each side; resulting in a diameter increase of about eight thou [0.008in]. Eight thou is about the max you can go; but eight is plenty for this motor. The cylinders had about 2.5 thou [0.0025in] of belling. Honing the bores to get things cleaned up nicely ended up at about three thou [0.003in] larger than STD size.

That means that I had about five thou [0.005in] of knurled metal to carefully remove, using fine emery cloth, so as to end up with the piston to cylinder wall clearance that I want: right at one and a half thou [0.0015in]. That does not happen in five minutes. It does take time and patience.

Pistons have to be carefully examined before you knurl them: they have to be usable. The ringlands have to be in excellent condition. The wristpin bores through the piston have to be unworn and undamaged.

The first step is to clean the pistons and rods before disassembly: to facilitate the wristpin removal without damage. And only when clean can the piston be examined and measured, and closely checked for any cracks or other evidence of damage.

If everything passes muster, the pistons are knurled to the eight thou diameter increase: called "blowing them up" eight thou.

Then I get to REALLY clean them; especially the ringlands. No carbon residue is allowable; and no ring scraper is allowable either. The pistons are soaked in carbon softening solvent, and the ringlands are scrubbed with toothbrush size brass bristle or stainless steel bristle brushes. Sometimes I actually use an old toothbrush: the plastic bristles won't damage the aluminum alloy, especially in and around the wristpin bore.

When thoroughly cleaned, the fit of the new rings in the ringlands are checked and verified.

Then I start the resizing to fit the bore procedure: sand a bit; clean; mike it; repeat. Once I get to about zero wall clearance, then it gets more intense, and the metal removal is done in smaller amounts...until I get to the right wall clearance.

Done carefully and done right, knurled pistons can perform as well as brand new pistons.