When I did this I put bearing grease all the way around the pistons, like gobs of the stuff, so any sanding stuff that got past whatever I'd stuffed in above the pistons wouldn't get down in the gap between piston and cylinder wall. Then I cut foam rubber well oversize and jammed that in above the pistons. When I was done I scraped the bearing grease out. I knew a mechanic that told me to do that. He put a helicoil in an aluminum head without getting chips in the combustion chamber by a fairly similar method: a lot of grease and removing small amounts of metal followed by removing and replacement of the grease.