I have always used the old pin because I dont have any new pins and its not a load bearing pin just a keeper to stop the piston from rotating. When I change the piston I screw the new piston in until it bottoms out then wind back until the holes line up plus half a turn. Put the pin back in and peen over the end and finish flush with the carrier. I have to do this to comply with strange rules and regulations. I have to use a new piston when I build a semiauto to make it compliant with the laws. There is no differences in size and function of the pistons just point of origin but thems the rules.