These use 8mm-1.0 internal threads to attach to the valve stem.
These are replacement Rave valve pistons made from aluminum billet 6061 and anodized BLUE or RED for corrosion protection. Replace the original plastic piston that will eventually melt with Aluminum and it will be the last one you have to buy.
These are for the 787cc 2-stroke motors, 800, & 951cc carb models
These replace the seadoo/skidoo oem part numbers:
420854447
420854449.