An inner primary bearing, that supports the transmission mainshaft, was added to the aluminum primary housing with the introduction of the 1965 Panhead. Before that the tin primary housing was used and there was no option to install any aftermarket bearing support to support the maindrive gear bushing, that is the cause of leaking mainshafts, worse than with the 1965-up transmissions. The Overbearing 2.0 is actually a hardened replacement for the stock mainshaft lock nut (35211-36), with a precision ground roller bearing inside. When installed the Overbearing 2.0 keeps your mainshaft and main drive gear concentric, eliminating maindrive gear bushing wear which drastically reduces, if not altogether prevents mainshaft leaks. This narrower 2.0 version is now fully compatible with all regular stock and aftermarket clutch hubs.