This is a Stant SP 21 Lever Vent Safety Cap. When you lift the Lever it Releases the Pressure & Stops you from getting Burned.The Complete Application Chart is in the Photo Section, also the Size Chart is there.This is a 14 PDS Cap. On the Cap it is marked Use On 14 LB Systems Only.It is 3 3/32" on top from Point to Point & 1 1/2" High.It fits 3/4" Deep Neck of Radiators.Thanks, Jack