Alternator woodruff key RMS Vespa, 15.3 x 6.45 x 4mm

Woodruff Key

What is the Function of a woodruff key?

A woodruff key is a semi-circular key that due to its shape is often called half-moon key. It sits on the crankshaft shaft, which has a recess to accept this small key, and drives the ignition rotor, which also has a recess for it.

In short, the ignition wedge provides the connection between the crankshaft and the ignition so that both work in unison.

Woodruff keys are very small, but absolutely essential to produce a spark every time the crankshaft rotates.

Offset keys

The woodruff key is factory seated on both the crankshaft and in the rotor at a specific point so as not to interfere with ignition timing and thus engine behavior.

Shifting the rotor relative to the stator by a few tenths of a millimeter shifts the ignition timing. An offset woodruff key can be used to advance ignition timing to achieve more power at high RPM.