=The Eccentric= on an engine is for the purpose of moving the valve
back and forth upon the valve seat and has a throw equal to the travel
of the valve. The throw of eccentric is caused by the wheel or plate
being bored to one side of its true center, and generally equals
one-half the travel of the valve. If more or less, the difference is
caused by the use of rocker arm or similar devices for increasing or
diminishing the throw of eccentric as the case may be. The eccentric is
accurately fitted and fastened to the main shaft of the engine with set
screws or key.