Used to change valve springs, top collars and valve seals without pulling the heads on Milwaukee Eight engines. Works with engine in frame. Rocker cover and rocker arms must be removed. Tool installs on the rocker arm shaft. Cylinder must be filled with compressed air to keep valves in the head once spring keepers are removed. The mechanic can rotate the tool and thereby compress the top collar of the valve spring, exposing the keepers to remove the top collar, valve spring, and seal.