93 disco 5th gear problems

[rotordogg]

Hi, my work truck is a '93 Landy discovery, diesel manual.
We're having problems with 5th gear, about 6 months ago the circlip holding 5th gear on main shaft broke and the whole 5th gear assembly tried to exit out the back of the box... Fixed it and been driving nicely since then. Now it's done it again! Any ideas on what's causing this to happen??

Truck spends most of its time cruzing at 90 on open road, used a little for offroad and a little for towing (have been told not to tow in 5th, so don't.)

Thanks,
Dogg.

[nakimud]

LT77s Gearbox? usually its the helical thrust created by the gear that smashs the thrust plate against the circlip(think everytime you let your foot off the gas in 5th then on it again) next time you have the extension housing off to fix it, check the float between the synchro housing and thrust plate, mite be a bit sloppy.

[nakimud]

Like the lt85 box which does this as well, the circlip isn't man enough for the job too. a H55f landcruiser box for example has a far beefier circlip