Designed to lower the compression for the purpose of reducing extreme pressure imposed on the starter motor and battery. simply push the caps down to open the release valves, start the engine and the valves with close by themselves. These valves require head removal and installation next to the spark plug by a qualified machinist. 10mm x 1.0 thread pitch, overall length is 1 13/32 with 10mm thread length. Note: As with any machining process, verify for sufficient material thickness before any machine work is done. sold each.