This pilot bearing (also known as a spigot bearing or clutch shaft bearing) is used in both the Ford 1600 Kent (Formula Ford) engine and in the Ford 2.0L SOHC Pinto (FC/FF2000/S2000) engine.
The 21mm OD will fit most Kent crossflow-based engines (including Cosworth BD and Lotus Twin Cam with 6-bolt cranks) and Pinto-based engines (including Cosworth YB). The 15mm bore size fits just about any rear-wheel-drive transmission input shaft normally used with these engines, including Ford-based and VW-based (Hewland / Webster) transaxles.
The pilot bearing is installed in the crankshaft at the flywheel end. Be sure to grease the rollers every time the transmission is mated to the engine.
The 21mm OD will also fit 4-cylinder Zetec (1998 and later) and Duratec cranks, but the bore size is not correct for the FWD transmissions originally used with these engines in the US market. It will work for most RWD conversions and kit cars (Caterham, Locost, etc) using a Ford or VW-based transmission.