Alright Duke, here's how I size a piston seal, put the seal on the piston, put in the tube, work the piston back and forth about 10 to 15x pull the piston out, you should see a black or burnished ring around the seal where its making contact with the tube, that's where the tight spot is on the seal, only try to remove the burnished area, install the piston and seal and check it for fit, repeat this process until you get the desired fit your looking for, I leave my seals a tic tight because they will break in, don't over do it removing material, I do this by hand! powered tools will remove to much material to fast and and you can destroy a seal in short order! I use a jewelers file and 1500 grit sand paper to fit my piston seals, I usually find the tight area just behind the parachute edge, I try not to remove any material off the sealing edge unless it absolutely have to! I leave the seal on the piston while doing the work take your time!!!! it sometimes takes me an hour or longer to properly fit a seal! This why I try to find a seal maker who makes seals close to proper fitment to start with, like ASA