The Whiteline Flat Out rear differential mount insert or "positive power" kit for the Subaru WRX and STI is an easy to install solution to get rid of the "clunk". Have you ever experienced that terrible clunking sound during shifts? That is your rear differential moving around! The Whiteline rear differential mount insert kit will get ride of that annoying sound and get more power to the ground. The rear diff mounts in factory form allow the rear diff to move around a lot. The clunk is most noticeable during acceleration from a stand still and shifting gears quickly. The Whiteline rear diff mount kit will provide the driver with a precise gear shift and predictable clutch engagement. This kit installs very quickly and easily with simple hand tools. Whiteline includes all necessary hardware for installation as well as a grease packet and instructions.