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The LT85 is a good strong gearbox. I'd happily put another one behind my 4BD1 and I did just that this year. If it lasts as long as the last one it's good for another 20 years!
Most are good for 300,000 Kms behind a NA 4BD1 before a rebuild is required - I don't think that's too bad at all given the potential damage a 4 pot 3.9 diesel can inflict via torque impulses. Many need a re-build not because there's anything wrong with the gearbox, but because they strip the splines on the output shaft. This is more of a LT230 transfer box input gear issue than a gearbox problem and is solved by fitting a cross drilled input gear, others recommend fitting splash lubrication devices as well.
My car was still fitted with the original box and did 265,000 km's before it got noisy in 5th and then another 15,000 km's before it got noisy in all gears - but it still worked to the day it was taken out and never let me down or failed catastrophically, it was just noisy. The previous owners had had no work done to the gearbox, I know this because it's only had two previous owners and I know them both.
Anyway, the main point of my post is that there are 2 versions of the LT85 - the '20C' solid case version and the '22C' lightweight divided case version. It's generally regarded that the earlier 20C solid case version is much stronger than the divided case one. I have the solid case version and I think all factory Isuzu 110's came with the solid case version, even the later 110 Isuzu's were fitted with the 20C despite the 22C being available to LRA.
Asides from the case construction the main difference between the two is that the 20C has an oil pump and pressurised lubrication delivery, the 22C doesn't and it's my understanding that it's this box behind 1000's of V8 powered landies worldwide from about 1988 to 1991 that got the LT85 a bit of a bad name.
The main complaint is that the LT85 will flog out the layshaft bearings if 5th gear if used extensively under full load... it's my understanding that this is true for the later divided case 22C which has no oil pump and suffers with lubrication issues to both the layshaft and mainshaft bearings. But not correct when discussing the 20C solid case version that does have an oil pump and thus a pressurised oil feed.
In fact - I'd like to know if anyone has got less than a quarter of a million km's out of a 20C LT85 behind a NA 4BD1, excluding military vehicles that get 'driven like it's stolen' by a gazillion different drivers. It's starting to brass me off how many people bag this gearbox and most of it seems to come from stuff they've once seen on this forum being regurgitated.
Some interesting reading from the following link: Land Rover gearbox history
1984 through 1991: The LT85 (85 mm shaft spacing) was used in all V8 110 and 90 Land Rovers. The LT85 was originally designed under contract by Land Rover for Santana in Spain and was considered to be a strong working gearbox. Rover purchased LT85 gearboxes directly from Santana. It is also known as the Spanish Box. This gearbox was used as originally designed until 1988. A cost reduced, lighter weight, divided case version of the LT85 was introduced in 1988 and used through 1991. The main short coming of the divided case version was its main and lay shaft bearings. The loading on the bearings in 5th gear caused frequent gearbox failure when cruising for long periods of time under high throttle. There was also a bearing quality problem at one stage of manufacture and the gearbox was sensitive to the type of oil used.
In 1991 the relationship between Rover and Santana ended and the LT85 was no longer available to Rover. They needed to find a quick short term solution while they developed a new stronger gearbox.
Jon
Originally Posted by justinc
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