A knurl on an aluminum piston, running against a cast iron cylinder wall, can't possibly score the cylinder. Actually the knurl pattern traps oil and lubes the skirt better. I also scuff the high spots down with emery cloth a bit before moving on to the next piston. After several thousand miles of running, the increased diameter can still be measured with a micrometer. Back when the average re-ring job only lasted 30K miles or so, knurling pistons was a very common practice. One of the pistons in John's 261 had a bit of excessive skirt clearance. I knurled it with the help of a lathe owned by one of his Amish neighbors.
For the "no-lathe" automotive machine shops, Perfect Circle sold the "nurlizer"- - - -a hand-cranked piston skirt knurling tool.
Just watched that last video, Don- - - - -my shade tree would die of embarrassment if I tried something like that under it!
Jerry
