Thread: Replacing a 4sp with a 5sp gearbox?

The Santana designed LT85 is the 5 speed box also used behind the isuzu.

They are not as strong as the LT95 (4 speed). They do suffer from bearing failure due to the impulsive low speed torque from the Isuzu. Maxidrive modify them to improve the bearing life.

Someone on this forum has fitted an R380 5 speed. I expect they will read this thread and can give more information.

The R380 is a nicer box to drive than either the LT95 or LT85, and easier to find than an LT85. But the early R380's (suffix J) have a bad stress raiser on the mainshaft, which leads to early fatigue failure. The mainshaft was fixed, wider gears and larger bearings used in latter (suffix L) boxes.

The suffix L, R380 has much larger bearings and wider (stronger) gears than the LT77. I would certainly not use an LT77 or suffix J, R380 behind an Isuzu.

The LT85 5 speed o/d box really makes the County-Isuzu as a highway and long distance touring car. Comfortable cruising at 110-115kph with a bit in reserve for passing

Id leave the 4 spd in it and change the high range raitio to the .996;1 like in the rangie that way 4th gear is just overdriven and it will be good on road ..
Dad has an old s3 stage 1 with the 4bd1 and 4 spd it was happy to sit on the govener at 140 k on the open road and im sure it has the .996;1 high range and it never sounded bad at that speed not like the old series 2a or 3 were ya need an intercom at speeds over 80 k when talkin to the passenger

Gday

I run an R380 behind a 4bd1T in a Rangie. Currently have about 350 hours on it, and run it on 75/85 Transmax. Very happy and comfortable to drive. I can sit on 100 - 115 not much more as I run 35" tyres with 4:1 diffs. But it will do it all day and you can talk and hear your passengers. The D2 R380 is the box to get if you can source one. You will need an LT85 bellhousing from an Isuzu to convert to R380. I am sure someone on the forum will have one. As long as you drive it like an Isuzu powered L/R and not a race car it will serve you well. PM me for more info if you need.


Justin

Originally Posted by lokka

Id leave the 4 spd in it and change the high range raitio to the .996;1 like in the rangie that way 4th gear is just overdriven and it will be good on road ..
Dad has an old s3 stage 1 with the 4bd1 and 4 spd it was happy to sit on the govener at 140 k on the open road and im sure it has the .996;1 high range and it never sounded bad at that speed not like the old series 2a or 3 were ya need an intercom at speeds over 80 k when talkin to the passenger

I'm pretty sure the isuzu's have the .996 high range gear already ?

Originally Posted by loanrangie

I'm pretty sure the isuzu's have the .996 high range gear already ?

Your right, the LT95 for the Isuzu does have 0.996:1 high range. Thanks guys for all the info and suggestions. What a great resource you all are.
I like the R380 idea, anyone got a bell housing from an LT85 Isuzu for sale?
Thanks again

The LT95 has, in 4th. high, an overall gear ratio of3.52:1. The LT85 has overall gear ratios of 3.55:1 in 4th. & 2.82:1 in 5th. Taking 2200rpm as the point of peak torque rise and thus the most comfortable and economical engine speed gives the following cruising speeds on 7.50 x 16 tyres:-

LT95, 4th. gear- 91.8 km/h
LT85, 4th. gear- 91.0 km/h
LT85, 5th. gear- 114.6 km/h

For those interested, the formula for road speed against engine speed is:-

(60 x rpm) divided by (gear ratio x tyre revs. per kilometre)

TYre rpk is 408.5 for 7.50 x 16 and 417.4 for 31 x 10.5 x 15

My County-Isuzu with LT85 & 31 x 10.5 x 15 tyres has a theoretical top speed of 163 km/h @ 3200 rpm. A top speed it can not achieve because of insufficient power. It runs out of puff about 140 km/h and 2750 rpm.

yep thats about it the old mans stage1 would only do 140k in 4th gear with the pedal on the floor on the flat point it at a hill and the speed would drop not sure what revs the thing was dooin but it wasent screamin im sure the gouvener was the reason it would only go that fast as on a good down hill run it would pick up speed well enough to see 150k and that was dam fast in a series landy and it run 235x85x16 tyers