"Indexing" is a way of self centering. When I use this in context with a transmission and a bellhousing... The hole in the bellhousing has to be the same size as the base of the bearing retainer. The 2 are machined for a tight fit to center the transmission. The 4 bolts that hold the transmission to the bellhousing are not enough to guarantee a perfect centerering of the transmission. Those bolts hold the transmission to the bellhousing once it has been centered by lingning up the circular base of the bearing retainer with the hole in the bellhousing.
I know what you are thinking... what centers the bellhousing to the engine block?
That is usually done with the aid of 2 aligning dowels which locate the bellhousing to the engine before the bolts are tightened.
The transmission "bearing retainer" is the "hat" that is usually held on by 4 bolts to the front of the transmission. This protects the input shaft. The throwout bearing rides on the snout of the bearing retainer, not on the input shaft.
