The OEM 34091-85 inner bearing race has been used on Big Twins since 1985. The OEM 9135 roller bearing rides on this race. By design, this race press-fits onto the transmission mainshaft. This system works fine for stock motors, however, 100 HP+ motors will make this bearing walk on the mainshaft. If it walks towards the tranny, it will damage the maindrive gear seal and cause a transmission oil leak. If it walks towards the clutch, it will cause a primary oil leak. This ball bearing kit eliminates this condition.