Compression Release Valves are designed to lower the compression during hot-starts by reducing the pressure imposed on the starter motor and battery. Simply push the caps down to open the release valves, start the engine, and the valves will close by themselves. (They have 5-7 Newtons of closing force) The molded synthetic knobs ensure they stay cool to the touch and are easy to operate unlike other machined knobs once they get hot. INSTALLATION NOTE: These valves require you to remove the head and machine a M10 x 1.00 threaded-hole next to the spark plug. SOLD INDIVIDUALLY: 10mm x 1.0 Thread Pitch, 1-13/32" Overall Length with 10mm of Thread