The R & D Flexible Coupling fits between the transmission output flange and shaft coupling. It is designed to protect a boat's drivetrain. The unit will reduce vibration, compensate for some misalignment, isolate the shaft and absorb shock loads.<p> The flexible disc is machined from a resilient and durable polyurethane. It allows the unit to flex, absorbing vibrations and temporary misalignment of the shaft and engine due to movement of the engine or flexing in the vessel's structure. HP/RPM=5 (HP x GearRatio x 100/Max Rated RPM) Capacity 4" Flange Diameter 0.39" Hole Diameter 3.07" Bolt Circle 1.97" Pilot Diameter 4 Holes