May be used to ease starting on high compression engines, especially Strokers. Releases are activated by pushing on the top before starting, which reduces cranking pressure, and allows the engine to spin over easier. After the engine starts they automatically close to restore full compression. Installation of these compression releases require matching of the cylinder head. This operation should only be performed by a competent machinist. Sold in pairs. Fits all Screamin Eagle Twin Cam Cylinder Heads.