Gear Box, Again

I stripped and rebuilt the gear box with new bearings for the main shaft and the back of the lay shaft – they all looked fine, but the main shaft rear bearing (the big one) had a small amount more play than its new replacement.  I refitted the old primary pinion that hadn’t been jumping out of gear, just in case a defect in the newer pinion was the cause,  but kept the newer bearing as it clearly had less play and ran smoother, so can’t have been the cause of the 4th gear jumping.  After reassembling the transmission and during fitting of the overdrive’s coupling (equivalent to the Fairey clutch sleeve), a new problem became apparent.

As the main shaft’s rear nut was tightened the last 1/4 turn, the gear box would seize up.  It could only be something to do with the first gear wheel, as the nut does not affect end float on the second and third gears (and shouldn’t on first, really), and a check through the top of the box after removing the selector shafts confirmed that the second and third gears could still be rocked a little, to the limits of their mesh with the lay shaft, by hand.  It was fairly readily apparent that the first gear wheel was being pinched by the synchro unit and the rear thrust washer and the only way to get at these parts is another strip down.  A salutory lesson in such things – don’t just look at the bits you know are broken and ignore the parts that haven’t shown issues; check the lot carefully and suspect everything!

Thankfully, I was able to remove the main and lay shafts without separating the transfer box, just like when I replaced the rear bearing, but the bell housing has to come off, so it’s still a significant task.  What I found was that the thrust washer was worn in its centre, where it is pressed by the compression of the rear nut against the transfer box gear/overdrive coupling, oil thrower and rear bearing against the ends of the splines on the main shaft and was also marked, though not worn, on its rear face where it sat against the rear bearing’s inner race.  These splines hold not only the hub of the 1st/2nd synchro, but also the 1st gear bronze phosphor bush inside the first gear wheel.  With the inner part of the thrust washer worn, the washer was pressed in further by tightening the rear nut to the specified torque, and so the outer, unworn part of the washer that the gear wheel runs against was pressed hard against the gear.  The gear, and indeed the bush, should have some end float against the thrust washer to make sure that the washer is pressed only against the shaft splines on tightening the rear nut.   I suspect the thrust washer became worn when I had recurring problems with the Roverdrive’s special main shaft nut, which sits inside the coupling and is clamped by two grub screws in the sides of the coupling, unlike the standard accessible castellated nut that can use a tab washer for locking.   With the small amount of spline play on the coupling, each application and removal of engine torque causes the grub screws to drag the nut on its threads, and it works like a very fine ratchet wrench to slowly undo the nut.  Using high strength thread lock on the main nut and medium strength on the grub screws (I sheared an allen bit trying to undo one of them after using high strength threadlock on the grub screws) seems to deal with the problem.

A new thrust washer was all that was needed, and the unit has been rebuilt ready for installation into the car.  The overdrive coupling was left off to allow the use of the standard rear bearing and carrier on the transfer box in an effort to keep all the shafts in correct alignment for refit – the overdrive will be installed after the rest of the transmission is back in the car (it’s all a bit long for manoeuvering into the chassis with the OD installed anyway).  Unfortunately, I have to hire an engine crane again, so it’ll have to wait until a day off in the working week.

 

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Comments

  1. Hi Nick,

    You must be sick of lifting that lump in and out by now mate, Land Rover life is never simple is it. Hopefully this time when it goes back in it will stay in for a while.

    I finally got around to swapping the challis on the Td5 last week. Took an hour and a quarter just to get the seven mounting bolts out of the axle case, tight to the last thread was not the word! Got to love Solihull’s extra-super-mega-strength studlock.

    Managed an oops-ouchie moment while at it as well. Using a larger spanner locked over another to get the leverage to turn the bolts the spanner snapped as I was pulling down on them resulting in two fat lips as I collected a spanner jaw on each one. That will teach me! The offending , and broken spanner, was last seen on a low and fast flight across the vehicle park. That will teach it.

    I hope all goes well once the gearbox is back in this time.

    Regards,

    Neil.

  2. Ouch! That sounds damned sore, Elvis! Apart from the frustration of doing the job so many times, it sounds like I came off best!

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