Overdrive service guide

This is what I wrote:(I am sure I got the web reference from on here somewhere!)

you can get them from http://www.nickslandrover.co.uk/2007/03/04/overdrive-overhaul/

His article is very good, but there are a couple of things I would do differently now!

Firstly, take the selector fork out and once you have got the back cover off and removed the spring clips (both of them!), you need to loosen the mainshaft from inside the synchro hub by putting the overdrive face down on the out put gear and knocking the rear of the shaft (where you just took the clip off) until it is free. This means you are less likely to have the synchro hub dismantle itself and lose bits!

When putting it all back together, I used a piece of ca ½" dia rod to hold the bits of the layshaft together whilst putting the mainshaft bits in (like he suggests), only to have the bottom thrust bearing bits 'escape' due to the rod moving. I found you need to jam the rear end of it in the casing with a piece of cloth until everything is together, then stand the overdrive on its nose, take the cloth out, use the rod to align all the parts, then fit the layshaft.

Yours could have a number of defects, but whining is most likely to be a bearing problem. Most of the parts are available, but it is a case of cost overall! If the main output shaft has worn into grooves, it may be uneconomic to repair (this can happen where the bearing on the clutch sleeve runs, if it is allowed to run dry). Also, if the engagement teeth on the clutch sleeve are worn, then you really need to replace both this and the mainshaft which will cost you about £220. My clutch sleeve had come loose on the gearbox mainshaft and worn the inner splines badly, the engagement teeth were about 30% worn, but I replaced both parts anyway! There was no wear in the bearing run on the output shaft, just a shiney area where the bearing actually goes.

You should replace the seal kit at the least, you can re-use the rear large O ring and use generic O rings on the selector shaft, but an O ring kit is advisable. Mine came from Craddocks. When fitting the inner seal inside the output shaft, use the mainshaft to pull it into place as far as it will go (mainshaft fitted as it would be in the complete unit) then turn the mainshaft round and push the seal about another 1/8" in so it is clear of the wide part of the shaft and runs on the area machined for it. If this is not correct, then oil leaks out this way into the transfer box and causes bearing failure!

If you have excessive endfloat (it should be barely perceptable by hand) you may have a worn thrust bearing. If the bearing is OK and its thrust washer has worn a groove, you can simply turn the washer round. I also swapped the front to rear thrust bearings where possible as the helical gears put a thrust in one direction when in forward gear, so one bearing gets more wear than the other. The bearings might be discoloured from overheating - a golden colour is OK, but if they are blue, they need replacing.

Another trick is to reverse the positions of the synchro rings (these should still have grooves on the inside faces and minimal wear on their teeth) as the one for changing out of overdrive gets more wear than the one for changin up. This is because when changing up, the gear slows naturally, whereas changing down, you need to speed it up with the synchro ring.

The biggest problem is that you will find that the parts end up in a heap when you take it to bits. The best thing is to clean them all up and lay them out, and put them into sets. It should be possible to identify what goes where and lay out the parts.

There are no real tolerances given - endfloat should be barely perceptable, using shims if necessary. In case of wear, any ridges in bearing races that you can feel with your nail as opposed to just being shiney, should lead to replacement.

There should be no visible wear ridges on the gears as these are quite robust.

There may be wear on the teeth of the bronze synchro rings, or the splines of the synchro hub, but the bronze synchro rings should still have sharp ridges on the inner cone faces. The technique for endfloat on the layshaft is as Nick suggests - use a feeler gauge if there is end float then select the nearest shim from the set.

In the case of the main shaft this can be shimmed last thing before re-fitting the spring clips, You could use a feeler gauge between the thrust washer and the washer for the spring clip and then insert the most suitable shim. What you don't want is preload on the bearings.

There will always be a bit of whine when you install an overdrive as the ouput gear is not matched to the transfer box gear it meshes with - this might be worst on overrun, but should fade after a few thousand miles. It should definitely not scream!

Sunny Jim

	Author	Topic: Overdrive service guide (Read 1650 times)
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