Annoyingly, the overdrive sprung a leak somewhere, dumping its oil into the transfer box. I suspect it was the smaller seal between the input and output shafts – I didn’t know where to locate it within the output shaft on the initial rebuild at the start of the 109 project as there is no obvious shoulder for it to sit against. This time I drifted it down firmly as far as it would go and found a firm seat, so it should work better.
I have taken a great deal of photos to document the whole strip and rebuild to create this as a “how to” feature – there are a lot of people buying up second hand overdrives from forums, shows and e-bay, so hopefully this will help them refurbish their units cheaply and effectively. It assumes the overdrive is already off the vehicle.
The first task is to remove the top panel by removing the four 1/2″ bolts. The panel may be stuck on its cork gasket, so have a replacement ready. Next, remove the selector fork and shaft by taking out the 1/2″ pinch bolt in the fork and undoing the selector shaft detent and retaining grub screws almost entirely. The shaft can be tapped out of the rear of the casing, allowing the fork to be removed. Retain the fork’s brass pads – they’re unlikely to need replacement. The selector shaft front opening’s plate can be removed for O-ring replacement by removing the two small bolts.
With the fork removed, undo the four 1/2″ nuts on the rear cover. The cover should come off with careful prying. Be careful not to damage the mating faces of the cover or main casing. There is a large O-ring around the rear cover that should be replaced on rebuild. Keep the cover’s nuts and spring washers in the cover during the work.
Now the back end of the input shaft is visible. Use circlip pliers to remove the small circlip. Put this to one side, followed by the washer beneath it. This exposes a second circlip, which must also be removed. Then may follow one or more shims, followed by a three-piece thrust bearing set. Place all these parts in order in a container or tray.
With these clips and bearings removed, the input shaft can be removed from the unit. This is the the shaft that can be seen by looking inside the geared output shaft on the front of the overdrive. It can be pushed out from the back. Once the front end emerges from the output shaft, withdraw it completely and put it to one side. There may be a thrust bearing set comprising of a thick washer (with a bevel on the inside edge), a thin washer and the needle roller race near the thick end. Retain these on the shaft. This bearing may have stayed inside the output shaft, in which case you should remove them now. There will also be a pair of tubular spacers and a roller bearing in the rear gear of the overdrive – remove these and place in the same container as the other input shaft parts.
Next, to get the output shaft and rear units out, you have to free the layshaft (its teeth will interfere on the mainshaft gears’ synchros, causing damage if you try to force them out with the layshaft in place). To do this, lever the rear end of the layshaft out with a screwdriver until there is enough exposed to grab hold with a pair of grips. It can be resistant as a vacuum ban develop in the forward case’s shaft recess. Rotating as you pull may help. Withdraw it completely and allow the layshaft gear cluster to drop into the corner of the overdrive sump.
Use a soft faced mallet to tap the output shaft aft, drifting out the rear gear and bearing set. It’s very unlikely that the outer ball race bearing will need replacement, so don’t be tempted to remove the big criclip holdin the rear gear in its bearing. Inside the front of the gear you will find a thrust bearing and roller bearing set. remove them and lay them in sequence with a container for the whole rear gear assembly.
As you remove the rear gear unit, the synchro hub is likely to drop. This is not a problem, but make sure you retain the spacer ring that sits between the rear gear and the synchro hub with the rear gear. Remove the synchro hub complete with brass rings, and then withdraw the output shaft from the front of the casing. There is a further thrust bearing set between the front face of the synchro hub and the output shaft. Keep these together. Note that the outer part of the synchro and the two brass rings are symmetrical, but the inner hub is not – the nosed side is to the rear, while the flat side for the thrust bearing faces the front on reassembly.
Next, withdraw the output shaft through the front of the casing. If the complete thrust bearing kits came out with the inner input gear and the synchro hub in the earlier stages, there will be no more metal parts on the output gear – the only remaining attached part will be the blue rubber seal inside the front end (quite far in), which should be removed and binned. Make a careful check on the thrust bearing faces at the back end and deep inside the front (inside of where the seal was) for thrust bearing washers – you really don’t want to loose those and they do tend to stick with oil vacuum.
Remove the layshaft cluster. There will be two roller bearings and a tubular spacer inside, and a thrust bearing pack for each end (probably still in the casing). These thrust bearings are likely to have matching shims to set the layshaft cluster endfloat. In this case, one shim can be seen next to each thrush bearing kit.
This should leave the casing with only the front bearing and seal. Lever the seal out, ensuring that the casing is not damage in the process. Clean and inspect the casing and bearing. Again, it’s unlikely that the bearing will need replacement, but if it does, it should be drifter out from behind and a replacement squarely and gently pressed or drifted in from the front. A new seal should be fitted at this point.
Check each component for wear or damage. Some wear to the splines on the shafts and synchro unit will be evident, and is unserviceable if more than 1mm has been eroded. The bearings are usually OK, but if there is evidence of any scoring, pitting or corrosion, renew them. Polishing of the bearing tracks is acceptable as long as there is no wear ridge.
Close attention should be paid to the condition of the clutch sleeve (the spline part that replaces the gear on the vehicle’s gearbox main shaft and the input shaft – the fine spline wear, and if they wear too much thy’ll strip, leaving the vehicle without drive regardless of overdrive or gear selections.
The plastic mesh bush inside the input shaft supports the end of the gearbox mainshaft, substituting the bearing in the standard aluminium housing on the back of the transfer box. If it is damaged, it will cause rapid wear of the gearbox mainshaft’s bearing surface. Damage or wear to the splines or bush require replacement of the complete part.
All seals, O-rings and circlips should be replaced as a matter of course, and are sold in kits. Shim packs are available cheaply. The bigger components are more expensive, and some of the bearings are specific to the unit and could be difficult to obtain, but Rovers Down South are now manufacturing the overdrive new and can supply parts direct in the US or via John Craddock in Europe.
In Haynes’ finest tradition, reassembly is the reverse of stripping.
Start with the replacement of the front main bearing if required, and the front main seal. The selector shaft’s front O-ring sited inside the lozenge shaped cover on the right hand side of the casing should be renewed now too.
Also fit new O-rings to the rear cover, layshaft and rear end of the selector shaft. Use the input shaft as a drift to set the new inner oil seal in its correct position inside the output shaft.
If the synchro unit came apart during the strip down, or if you find missing or worn out detents, here’s how to rebuild it. First, If you’re going to open it up, lay it on top of a towel, and lay a heavy cloth over it. this will stop the ball bearing shooting across the room and into oblivion as it comes apart. Lift the outer ring and sharply tap the inner hub down. The two should separate, allowing you you with draw the three springs for check or replacement.
To reassemble the synchro, lay the outer ring flat and hold the inner hub up inside it. Fit the springs and square doughnuts (making sure the curve of the doughnuts matches the curvature of the hub) and allow the inner hub to sink so that it’s sitting on them. Fit the ball bearing to one spring and press it in, forcing the doughnut down as the balls presses in. Push the doughnut down as far as possible so that the ball bearing is retained by the outer sleeve. Repeat for the other two detent balls, then sink the inner hub down fully into the outer sleeve.
Next is to set the end float on the layshaft. Oil and fit the bearings and spacer inside the layshaft gear cluster. Using LM grease, assemble the front end layshaft thrust bearing kit and any shims that were present with it on strip down. Use just enough grease that parts stick together. With the overdrive casing sitting rear end up, fit the front bearing/shim pack to their seat in the casing, then install the cluster. Next assemble the rear thrust bearing/shim pack like the front, and slide them into position. Be very careful to ensure the shims are fitted on the casing side of each bearing pack. Then use a long handled tool down the layshaft aperture to “stir” the bearing packs and cluster in to alignment and insert the layshaft. The complete layshaft should have a very small amount of endfloat, only just perceptible by hand. If it’s obvious, it excessive and more shims must be inserted. If there is no endfloat, remove a shim (very unlikely). Once the endfloat has been correctly set, remove the layshaft and fit a long narrow tube, bar or screw driver that will allow the cluster to move laterally while holding the bearing packs roughly in place.
Now fit the output shaft through the front of the casing, oiling the seal and seal land first. Oil the input shaft all over and ensure its thrust bearing (against its wide section) is set the correct way around – thick washer against the shaft’s “cup” end (bevelled edge against the shaft’s radius corner), needle race, then thin washer – and insert it it carefully through the output shaft. Be very careful not to damage the inner seal. Stop with the input shaft’s rear end just protruding from the back of the output shaft by 1/2″.
Oil the synchro’s thrust bearing and refit to the rear of the output shaft before fitting the synchro unit, complete with brass rings, behind the output shaft. Next fit the rear gear spacer ring on the synchro hub’s nose, then fit the rear gear unit, complete with its internal thrust and roller bearings, through the rear of the casing. The rear bearing can be tapped into place with a mallet or pressed in using the rear cover as necessary.
Next, “stir” the layshaft cluster and bearing packs again to align them for layshaft insertion. If you have a pool of oil in the bottom of the layshaft aperture, try to remove it by inverting the overdrive or wicking it out with a cloth. This is to make the shaft insertion easier – with the shaft’s locating hole full of oil, the shaft is difficult to press down fully and springs back out. If you can’t get all the oil out and the shaft’s tail is protruding excessively, don’t worry – the next step of fitting the rear cover (after installing the new O-ring on the cover) will draw the shaft down into its proper position. the oil in the aperture will be slowly forced out of the way. Just close down the rear cover squarely and slowly, turning each nut an equal amount in sequence, using a short handled 1/4″ drive socket only – air tools will draw the cover down too quickly, forcing the shaft down faster than the oil can dissipate, likely causing hydraulic cracking of the overdrive casing.
Finish by inserting the selector shaft (with its new O-ring) through from the back of the casing so its end is just showing in side the top hole. Fit the fork and feed the selector shaft through it. Make sure the elector shaft is rotated so that the detent grooves face downwards, the long slot up, parallel to the head of the grub screw, and tap it through until the slot for the fork’s pinch bolt lines up with the fork’s bolt holes. Insert the bolt, but don’t full tighten yet. Titghten the top and bottom grub screws fully and back of 1/2 turn a dab of thread lock is useful). Fit the top cover with a new cork gasket and loosely fit the four corner bolts.
The final tightening of the fork pinch bolt has to be done once the overdrive is fitted in order to allow the selector shaft to rotate to align with the selector lever link rod – if the pinch bolt has been tightened and the alignment is slightly out, the link rod’s clevis pin will not fit through the hole on the front end of the selector shaft. Once the selector rod and shaft and rod are connected, the overdrive should be selected to neutral. with the shaft held in neutral by the detents, carefully hold the synchro hub in exact centre so that an equal amount of the brass teeth an be seen each side, and tighten the pinch bolt. Smear both sides of the cork gasket heavily in LM grease and fit the top cover tightly. Fill the overdrive unit with the correct amount of EP 90.