The Comet 5005.0216.00 Gear Reducer is designed for directly mounting to a gas engine with a 3/4" shaft.
The Comet Gear Reduction will work with the following Comet Diaphragm Pumps: Comet MP20, MP30, P48, APS31, APS41, APS51, and APS71. It also comes assembled on the 'GR' versions of those pumps, for instance the Comet MP20-GR.
You also will need to ensure the Comet Gear Reducer is compatible with certain gas engines and meets horsepower requirements. It will work with the following brand name engines: Briggs (3.5, 5, 5.5, 6.5 HP); Vanguard (6 HP); Honda (4, 5.5, 6.5 HP); and Kohler Command (6 HP).