The 'primary gear' in a Mini application is the critical gear which the clutch rides on. It is the first gear that connects the power from the engine to the trans. These helical cut gears take a lot of abuse over the years and while the teeth seldom fail, the bushings are always worn. All 1275cc engines use the same gear dimentionally but they are not all interchangable. The 22G1053 gear is the stock one used in all 1275cc engines until the advent of the A+ design. These gears do NOT have a groove cut into the center of the gears.