Pack of 5 (20mm x 1mm) nylon AOE spacers / washers. Not always needed but handy to have if you're looking to correct the AOE. Included is a slightly longer (18mm) piston head screw. Stock screws vary in size, ZCI generally 16mm and SHS / Rocket 12mm. Generally 2+ shims requires a slighly longer screw but every build is different.
* As with all piston heads I recommend you use some threadlock on the screw.
AOE Guide (not mine)