This is an o ring seal that goes between the C5 gearbox and the gearbox mount. This ensures that no grease escapes from the box. The seals are often damaged when removing the gearbox or are simply lost when a motor is supplied without a gearbox.

TOP TIP: Molyslip MLG Moly E.P Lithium Grease is the best grease to use for Sinclair C5 gearboxes. Apply a small coating to all services when rebuilding a gearbox.