Base circle is the round part of the cam lobes. that's where you check your valve lash when the valves are closed. I bought a few of the Tecumseh billet rods off EBay hoping they would fit in the HS Tecumseh's no go... to long and wide.. must be for the H50's. I've heard the Billet rods for the ohv engines will work in the HS engines... anyone try this yet?