Some really interesting info in there!
Originally Posted by http://www.expeditionlandrover.info/gearboxes.htm
The factory built a small number of all syncro series IIA gearboxes just prior to converting the box into the series III gearbox. The all syncro series IIA gearboxes can be identified by a "S" prefix on the stamped number located on the transmission top cover. Since top covers can move about when multiple gearboxes are rebuilt, and a IIA gearbox is most likely to be rebuilt in the earlier documented style, the 'S' stamp on the top cover does not guarantee the gear box under the top cover is still all syncro.
The Series III gearbox has syncro in all gears as well as lower first and reverse gear ratios. The transfer case remained the same as was used on the late IIA. Series IIA gearboxes (D suffix & later) are generally considered to be stronger and longer lasting than early series III gearboxes (The ones imported into North America). Though a lot of this conception may have to do with people's shifting habits. Series III boxes prefer that you hesitate briefly in the neutral position while you shift gears. This puts less stress on the syncros and enhances their service life. Suffix letters on the Series III gearboxes restarted with suffix A.
There were changes made to the SIII gearbox during its 14 years of manufacture, mostly to make them less prone to wear and to keep them from jumping out of gear. The suffix D and later Series III gearboxes (manufactured after Land Rover left the US) are considered to be the most robust of the Series Land Rover gearboxes. The principal changes were to the reverse idler gear, bearing and shaft and the syncro units.
In general the Series gearbox is considered to be strong enough for about 120 HP, 160lbft is about the limit on torque. Early gearboxes are considered to be weaker than later IIA gearboxes. Early gearboxes tend to beak lay shafts and main shafts at the circlip groove positions. Late 2A lay shafts without the circlip groove are stronger than the early ones. According to Bob Shannon "Here is the fix for the earlier gearboxes. The lay shaft from suffix D onwards gearboxes (part # 556040) has a stepped shoulder that fits into the rear of the D-onwards series lay shaft 2nd gear. So you don't have to buy a matching set of gears (D-onwards is part # 600916) you can take an earlier lay shaft gear from the pair (Suffix A-C is part # 245766) and machine out the relieved section to replicate the later gear's greater clearance. Also a suffix D lay shaft is probably cheaper than the suffixes A (part # 09917) and B-C (part #528703). Make sure the main shaft is set up correctly!!! "
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