This particular gear box from Hypro is designed to fit 5.5 HP gas engines and fits pump models 9910-D30, 9910-D303 and 9910-D403. If you want to use pumps like 9910-D50, 9910-D503 or 9910-D813, just ensure that they don't exceed this boxes 5.5 HP threshold. This is a 6:1 box and comes with a 3/4" shaft for ease of use.