Knurling is mechanically expanding the piston skirt using amachine to emboss a a pattern on the piston skirt that in effect raises a portion of the skirt with tiny crosshatch ridges somewhat like the handle of some tools. even on a low buck job like this it is wise to have the crankshaft checked and at least polished and new bearings put in at least on the rods.
Clean it, if it's Dirty. Oil it, if it Squeaks.
But: Don't fix it, if it Works!
