S3 Star Head compression insert for the TRS trial motorcycles. Uses the best alloys available to obtain a better power output and improved traction. The advantage of these heads is that they can hold more water and have thicker walls which helps with the cooling rate thus improving low revs power. These inserts allow you to regulate the compression in your engine for the various terrain you ride. Inserts fit with S3 Star Heads and the stock TRS head and also provide increased cooling capacity.