The Piston Spring is installed inside of your P-Series Booster Assembly to allow cycling when paired with a P-series piston on a tilting-barrel handgun. This Spring is a replacement part. This spring is already included and ships inside the booster housing and piston on the Odessa, Ghost, and Mojave. The SD500 and PB500 Booster Assemblies already ship with a booster spring installed as well.