Material taken from under the piston crown using a small carbide bit on a dremel. Luckily these ycp pistons were all within .8g of each other and the wrist pins were within .3g. My Nippon turbo pistons for my other crx have a 2g variance between the pistons, it's surprising how long it takes to remove even half a gram from one piston but I figured this was just one small detail that might help both builds.