[Alpines] How reliable is an overdrive unit?

[£auri]

Hi, folks,
I bought an overdrive gearbox from ebay. It is from a later Sceptre, so
I have to change primary shaft from my old box.
When looking pictures from my workshop book, I am quite reluctant to
take all pieces apart. 
Is there a way to examine overdrive and gearbox without total overhaul?
I am very happy if I must not remove mainshaft or baulking rings etc.

Lauri Lehtinen
journalist, engineer
mobile: +358 (0) 400 851988