This valve seat is a custom part made to be used with the 28.5mm ADJ-1122 Stainless valves. The inside diameter of this seat is .145″ larger than the stock seat. Simply put, a bigger hole flows more than a smaller hole. Note that air cooled OHV engine heads get HOT and a tighter interference fit will be necessary than what would be acceptable on a water cooled OHV or flathead engine. Valve Seat Dimensions: 1.020″ ID x 1.209″ OD x .250″ HT