This is an adapter for magneto test benches. The adapter attaches to the motor shaft. The adapter includes a flex coupling to reduce vibration and noise from the operation of the test benches. There are two drive pieces that allow you to rotate direct drive or impulse coupled magnetos. There is a second allows you to test the magneto if the Lycoming drive gear is still attached. The unit can be used on 1/2 inch, 3/4 inch, 5/5 inch and 7/8 inch motor shaft diameters. You must specify when purchasing. The flanges have a 3/16 inch keyway