A fairly crucial part of how the idler gear operates is its tolerances and running clearances. End float is a continual problem as folk often ignore it, and because many units left Rover with incorrect (usually too big) clearances.
Assembling the engine to gearbox (transmission) is explained and simplified in this article. Contrary to popular mis-guidance, all BBUs will fit all four-syncro gearboxes. It was only the three-syncro boxes that varied in casting dimensions causing hassles where 1275s were applied to 850/998 gearbox casings. The longer throw and bigger crank and rod assembly dimensions fouled the sculpted areas in the gearbox case,