Vudozahn Although the designer was theoretically free to choose any ratio for the gearbox and final drive, there is one additional consideration which meant that the top gear of most gearboxes was 1: The final drive then took this output and adjusted it in a fixed-ratio transmission arrangement that was much simpler to build. In the U. By using this site, you agree to the Terms of Use and Privacy Policy. This is especially important because the differential gears are bathed in heavy oil and seldom provided with any cooling besides air blowing over the housing. With the popularity of front wheel drive cars, the separate gearbox and final drive have merged into a single transaxle.

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This time I drifted it down firmly as far as it would go and found a firm seat, so it should work better. It assumes the overdrive is already off the vehicle. The panel may be stuck on its cork gasket, so have a replacement ready. The shaft can be tapped out of the rear of the casing, allowing the fork to be removed. 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.

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.

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. 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. Inside the front of the gear you will find a thrust bearing and roller bearing set. 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. 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. A new seal should be fitted at this point.

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. 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.

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.

Start with the replacement of the front main bearing if required, and the front main seal. 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.

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 ball bearing to one spring and press it in, forcing the doughnut down as the balls presses in. Push the do ughnut 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.

Be very careful to ensure the shims are fitted on the casing side of each bearing pack. The complete layshaft should have a very small amount of endfloat, only just perceptible by hand.

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. Be very careful not to damage the inner seal. The rear bearing can be tapped into place with a mallet or pressed in using the rear cover as necessary. 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.

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. Fit the top cover with a new cork gasket and loosely fit the four corner bolts. Once the selector rod and shaft and rod are connected, the overdrive should be selected to neutral. 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 Be Sociable, Share!


Fairey Overdrive Overhaul








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