Fairey Overdrive Fitting

Having rebuilt the Fairey overdrive, this post shows how to fit the overdrive to a standard transmission. It shows the whole procedure except for the installation of the selector lever and bracket, but I’ll try to explain that further on. This guide is applicable to refitting an overdrive or fitting one to a vehicle for the first time.

Start by chocking the wheels and cleaning the outside of the gearbox and transfer case. It doesn’t have to be spotless, but you don’t want any debris falling in while it’s open, so scrape of all the loose rubbish, degrease with “Gunk” (or similar) and a stiff brush, and give it a thorough rinse with a hose.

Inside std transfer boxstandard mainshaft rear bearingThe standard transfer box has a dished circular plate at the rear which holds a bearing for the rear end of the gear box’s main shaft, secured by six studs and nuts. The nuts on my unit were 15mm, but I suspect most units have 9/16″ nuts. There are two inspection covers on the top of the transfer case, each secured with four studs and 1/2″ nuts. The rear bearing carrier and upper (left hand) inspection plate need to be removed.

castle nut and tab washerWith these two panels removed, the rear end of the gearbox main shaft and the transfer box input gear (on the main shaft) are visible. The gear is retained by a castellated nut, which in turn is secured with a tab washer. With the gear box and transfer box in neutral, rotate the main shaft until the one tab engaged in the nut. Once this tab is at the 12o’clock position, select 1st gear to lock the main shaft in place.

bits to be removed for safe keepingUse a hammer and drift to lever up the tab and then undo the castellated nut. This nut has a standard right hand thread but is on very tightly. LR have a special tool to fit the nut, but a hammer and drift (large flat screwdriver) work equally well through the top inspection aperture. Remove the nut, the tab washer, the spacer washer and the gear. Retain these with the bearing carrier.

mainshaft splinesInspect the main shaft for wear – look at the splines and the rear bearing surface. It’s unlikely that there will be any major faults here, but it’s always worth checking. Also make sure that the condition of the “clutch sleeve”, which is the overdrive’s replacement for the input gear, is in good order – the fine outer splines are prone to wear. If more than 25% worn, I’d strongly recommend replacement. If you have a new tab washer for the overdrive, make sure you use this on assembly, as the standard tab washer’s tabs are too long and foul the inside of the overdrive’s shafts. If you do not have a dedicated overdrive tab washer and can only source a standard type, file 1mm off each tab end.
sleeve and nutcltch sleeve Thoroughly grease the overdrive’s clutch sleeve with LM grease internally and externally, or better still (if you can find it), Rocol Anti-Scuff Paste. Make sure that the exterior bearing is well packed as it will get little lubrication once the overdrive is installed. Fit the clutch sleeve with the outer splined end towards the rear of the vehicle, followed by the spacer washer, special tab washer and castellated nut. With the gear box still in 1st gear, use the special tool or hammer and drift to tighten the castle nut as much as possible and then tap in the tab which aligns (you may need to select neutral and rotate the main shaft again to find the aligned tab/slot, and even tap the castle nut slightly tighter or looser to get the alignment perfect).

Once the tab is engaged in the castle nut’s slot, refit the top inspection panel with a thick smear of grease on each side of the gasket. Fitting this panel now will help prevent foreign objects falling into the transfer case.

Now prepare for the overdrive fitting. The overdrive main shaft (input shaft), recessed inside the larger output shaft, must have a generous application of LM grease or ASP in the gearbox main shaft bearing (the perforated plastic surface as far in as possible) and around the splined areas where it engages on the clutch sleeve. Fit the gasket, again with a heavy smear of grease on each side, to the transfer case studs. Make sure the six nuts and lock washers are easily to hand. Ensure that the gear box and transfer box are in neutral.
fitting Lift the overdrive into position from below the vehicle, carefully feeding the geared output shaft through the aperture. It should push most of the way in with little resistance, but may stop short. If this hang up occurs, it’s because either the output shaft gears have not meshed with the transfer box intermediate gear cluster or the overdrive’s main shaft and the clutch sleeve splines are not meshing. Either way, rotating the casing of the overdrive back and forth while easing it forward should cure the problem. If this is not possible because the casing is already on the studs, flicking the starter motor with 1st gear will do the trick. If it hangs up with about 1/2″ to go, then this is the castle nut tab washer catching on the main shaft – remember the standard tabs are too long.

fittedawkward studWith the overdrive butted up against the transfer case, fit the top pair of lock washer and nuts, only turning the nuts on a couple of turns to prevent any possibility of the overdrive falling out. Then slide the overdrive out against the nuts. The reason for doing this is that two of the studs have sections of overdrive casing behind them, and it’s impossible to get the nuts onto their studs with the overdrive fully seated. Once these two washers and nuts are fitted, wind them on, reseat the overdrive and fit all the washers and nuts, tightening them up in an “opposite pairs” sequence.

lever bracketFitting the lever is relatively simple with the box and floor in situ. Fit the bracket to the back of the gearbox upper housing with the two existing bolts. Drill a pilot hole at a point 108mm above the foot well floor and 54 mm(4cyl engines) or 41mm (6cyl engines) forward from the floor’s rear corner (not the back end of the tunnel as the seat base slopes) through the tunnel cover and just into the bracket.

Remove the bracket and measure where the drill mark is in relation to the bolt hole. Transfer these measurements onto the tunnel cover and these will be the pivot hole centre. Use a tank cutter to cut a 2″ hole at this point.

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  1. CCKW363A1......Phil says:

    The nuts for the rear bearing housing are 3/8″ BSF which is approximately 19/32″AF spanner size in the absence of BSW/BSF spanners.

  2. What are the tubing arrangements onto the cover plates – is it some system for auto-oiling?

  3. Hi Paul,

    They’re just for a remote breather, as I felt that the existing small hole would allow easy dirt and water ingress. the silicone pipe joins a gearbox breather and runs up into the engine bay.

  4. Prithvi says:

    Are the overdrives available on any site for a Series 3 LWB 109 & a DEfender 110 station wagan?

  5. Hi Prithvi,

    The Fairey design OD for Series Land Rovers is now produced by Rovers Down South in New Orleans, USA. The UK distributor for the unit or parts is http://www.johncraddock.com . For a only slightly more money, you can also get the Rocky Mountain “Roverdrive” (SX model for Series), which is quieter and tougher, though slightly harder to engage. Their website is http://www.roverdrives.com , and includes contacts for ordering. The Toro model (similar to the Fairey) is obsolete, and very hard to get parts for.

    For Defenders, you have two choices – the GKN overdrive, which is a solenoid actuated unit (a switch is added to the main gear stick, with no extra lever), or the Rocky Mountain DX, which is very similar to their SX model, and is lever actuated.

    I now use the RM Roverdrive SX, having sold this Fairey unit, as the older model would not cope for long with the torque of the Tdi engine in my 109. So far, I’m very pleased with it – it’s quieter, and doesn’t need weekly topping up with oil.


  6. John Hope says:

    Just to thank you for all your work in providing the info on overhauling and fitting a series 3 O/D.
    Am just about to start on mine, and your info will be most useful.

  7. Simon Miles says:

    We have just reviewed your page on fitting an overdrive, and have come across the problem where we 1/2″ to go (As detailed in one of the paragraphs), but have tried what you have suggested. Is there anything else that could cause this this?

  8. Hi Simon,

    First of all, see if it’s just the splines on the input shaft and clutch sleeve misaligned. Try engaging first gear (so the engine will hold the clutch sleeve splines in their position), high range and overdrive “engaged” or”disengaged” – anything but neutral. Then rotate the transfer box gears (by rolling the vehicle a foot or so, or by jacking up and turning a wheel) while gently pressing the overdrive housing forward. This should align the splines and allow the unit to slide forward against the transfer case.

    At that 1/2″ gap, the other likely culprit is the locking tab washer between the conical castellated nut and the clutch sleeve *the input piece of the overdrive that replaces the transfer box input gear). It’s probably one or more of the un-bent tabs fouling the inside of the overdrive’s input shaft. Fairey (and subsequent licence builders) used a special tab washer that was identical to the standard LR supplied one except for having shorter tabs. I would suggest you either remove the current tab washer and file the tabs down a little, or use a vacuum cleaner while filing to catch the particles as you file it in situ (the former would be the preferred method).

    The only other cause that I can think of is that one or more of the mounting studs has unwound when the nut was removed, and is now sticking out of the transfer casing more than it should and is pressing against part of the overdrive casing. It’d just need winding back in with pliers.

    Good luck – let me know if you still have any trouble.


  9. Robert Mason says:

    thanks mate this is a great resource. it has made the task of fitting an overdrive to my ex army series 3 109 not such a daunting task.

  10. First, thanks very much indeed for the information: I had literally no idea of what is involved in getting the overdrive installed. I’d had it done ‘professionally’ with the previous two gearboxes (!); hence installing the third gearbox myself…

    Second, are there any tricks involved in getting the overdrive OFF the old gearbox? I’ve undone all the nuts and it seems to be glued in place.



  11. There are no real “tricks” as such Roger – it’s all relatively simple.

    Chock the vehicle and put the hand brake on. Drain the overdrive. I put the gear box, overdrive and transfer box in nuetral so that the helical gears can freely rotate while the unit is withdrawn. Then disconnect the selector rod and shaft by remoing the split pin and clevis pin.

    When you undo the overdrive retaining nuts, you’ll find that some of them hit the overhanging parts of the casing before they’re fully off. Don’t worry about that – undo all the nuts as far as possible, and you will then be able to shunt the overdrive partially away from the transfer box to finish off the last few turns on the awkward nuts.

    Thye unit should pull straight off, but a lot of people use sealant on the gasket, so you may need to tap it off with a soft mallet. I use grease on gaskets – it seals just as well, but doesn’t stick or make a mess.

    To undo the nut retaining the clutch sleeve on the main shaft, rotate the main shaft by hand until you can see the locking tab which has engages the castle nut, then engage 1st gear. Use a hammer and small chisel (or cheap screw driver) to bend back the tab, then undo the nut. If the shaft rotates as you try to undo the nut, then the engine is turning over. Using a flywheel locking tool will solve that problem (a scrap R380 reverse light switch works well). If the clutch is slipping, then you’ll have to remove the the gear box top cover to try to jam the gears with a sutable tool. The clutch will also need attention.

    I hope that helps.

  12. Hi,
    Fitted my overdrive yeasterday and all is working well. Thanks for the instructions. I was more than ready for the job after reading them.

  13. Kevin Mckenzie says:

    Hello Nick.
    Was wondering if you can help me with my Axle Rebuild on my swb series 2a landrover. im at the stage of fitting the diff bearing and i know for sure that there should be 1 x shim fitted onto the shaft before fitting the bearing. The problem is i have got 2 shims which look identical but cannot remember where it come from.Is there a chance the 2 x shims were just stuck together or should the other shim go on after the bearing?

  14. Hi Kevin,

    I assume that it’s a Rover diff, being a SIIA and also an 88″. There are shims against each bearing on the pinion. The “inner” end has the bearing fitted first, followed by the shim, and the “outer” end (with the pinion fitted to the diff casing) has the shim followed by the bearing, followed in turn by a spacer (539745) of similar diameter but thicker than the shims, a larger washer (FRC4586) and then the pinion flange. The only other shims in the diff are on the planet gears which accept the half shafts, set to mesh correctly on the non-shimmed pin axis planet gears. The diff carrier position and end float are set by adjustment of the big castellated nuts that press against the carrier bearings.

    It’s possible that the shims you have were added together to meet the required distance of one single shim of a thickness that was unavailable, or it may be that they were erroneously put together on one side of the bearings. The only way of telling is to assemble it the way you think it came apart and testing the mesh and backlash of the pinion and crown wheel. Sorry I can’t be more specific.

  15. Steve Willis says:

    Hi Nick,

    I came to this page as I am about to remove and refit my Fairey overdrive – many thanks for the commentary, I first fitted it about 25 years ago and no longer have the instructions.

    Re Kevin’s comments on the diff bearing (presumably the pinion bearing) I am not sure about what you replied.

    I have just finished my axle rebuilds and replaced the oil seals on both Rover diffs. On one diff there were four shims behind the outer bearing and two thin ones had broken because they had been incorrectly assembled with the thin shims against the bearing race. The LR manual on setting pinion preload requires two of the thicker shims (about 30thou each) and then smaller ones in between – as required to set the pinion preload to 7-12 lbs. This is measured by fitting the pinion nut to 85ft-lbs and then measuring the force required to turn the pinion.

    On one diff I needed two 6thou shims and 1 on the other to get the preload right. In each case this was in addition to the 30thou ones.

  16. Thanks, Steve – I was working from the LR Parts Catalogue, not having done a diff rebuild myself.

  17. hi nick. am so glad i have found the info on fitting and overdrive unit. i have an eagle rv fibre galss shell on a range rover classic chassis. i have a 4spd manual and also a spare 4 spd manual ( with overdrive) ready to take off the donor gear box and fit, firstly the truck is in ibiza and i fly out in 5 weeks time. i plan to remove and fit in 1 day and then tinker (on other bits of the truck ready for the trip)for another day then drive it back to the uk to finally finish it (thinking of a wade ro34 supercharger etc). (the truck had been out there for my summer hols for 5 yrs)

    what would you reccomend i replace when removing from the donor one and installing on the truck one. ie gaskets ,bearings, the appropriate tap washer etc etc.will not have time to overhaul the over drive unit though ! also while i am in that area i would like to service the gearbox. oil filter (if one ?) and overdrive seperate oil ?(or is it shared ??

    so in a nut shell. is it do-able in the i day i have and any info or experience on the gearbox and overdrive i have would be great. i dont know what make it is. it just has a lever as you have explained to be pushed back and forward to be engaged etc. it was taken from a v reg range rover 4 spf 3.5 v8. i also have the exact engine and box in my truck but minus the overdrive. so i just want to put it in.

    thanks shaun

  18. Hi Shaun,

    Unfortunately, the RRC has a very different transmission to the type featured in this blog, and though the Fairey Overdrive for the RRC is broadly similar in design to this one, I don’t know what differences it has. I would expect it to suffer spline wear on the clutch sleeve in the same way as the series unit, so that could be a useful spare to take with you. Other than that, it’s impossible to predict what servicing it needs without testing and inspecting it. Good luck, though!


  19. thanks for the advise nick , will look into it further and see if i can find someone who knows that type well

  20. Peter says:

    Great work Nick. Quick question if I may. Have just bought a 2nd Hand Overdrive to repair aling with an empty casting.
    Both look the same but only the empty casting has the Fairy name details on the case.

    Any idea who made the case with no name?



  21. Hi Pete

    My guess would be that your casing was made by Superwinch – my Superwinch overdrive had no markings and I believe they were the only licence holders for the design after Fairey disappeared except for the current holder, Rovers Down South (of New Orleans), who mark their casings with the raised letters “RDS”.

    The casings and innards should be interchangeable, needing only the variation of the shims used at the back of the main shaft to account for production tolerances, but it’s not something I have ever tried. RDS still manufacture all parts for the overdrives, so can supply any parts you need.


  22. Thanks for your Prompt response Nick!



  23. Hi Nick, just found puddle of oil under my overdrive. Had closer look and overdrive unit covered in oil. Took off top plate and unit half full of oil, to top of dipstick. Assume oil seal (part no RTC7173) has failed causing oil to migrate from transfer box and escape through the breather hole on top plate of overdrive unit. Would you agree, and if so is it much of a job to replace this seal assuming I can still get hold of one.

  24. Hi Huw,

    A failed oil seal will normally allow oil to migrate from the overdrive into the transfer box, not the other way, because the transfer box oil level should be significantly lower than the overdrive’s. However, I’ll wager that your main gear box oil has migrated back to the transfer box, as is extremely common, overfilling the transfer box and back filling the overdrive until it loses oil through its top cover.

    There are several things you can do to deal with this, and all have been covered in the transmission section of this blog.

    1) Use a remote breather system on the three gear boxes, using silicone tube and ideally some steel unions (brake bleed nipples would work well);
    2) Regularly check the oil levels of the three units, draining down the over-full transfer box and topping up the other two as necessary (made easier if you fit the military type gearbox top-filling cap and tunnel cover and fit Rocky mountain dipsticks to the gear box and transfer box);
    3) Replace the gear box’s main shaft rear seal and ensure the rear bearing carrier is fitted into the gear box casing with Locktite bearing seat compound (similar but not quite the same as thread lock) – it is this joint that causes most of the oil migration from gear box to transfer box, not a faulty seal, and it is common for the bearing carrier to spin in the casing, wearing it further and creating swarf in the transfer box, main shaft play and excess end-float, eventually causing gear box issues like jumping gears or difficult selection in addition to a worsening oil migration problem. This can only be undertaken with the gear box removed and stripped down, though, so you’d best live with 1&2 until a rebuild is required. Be warned – almost all recon units, including LR factory units, have omitted bearing seat compound and will require stripping again to rectify, so you’re better off getting the manuals and tools or an experienced friend and doing it yourself. This was covered in the most recent post about rebuilding the gear box(early 2009).

    Good luck.

  25. Many thanks for you reply Nick. I shall do what you suggest and will get the gearbox out ASAP and treat it to an overhaul. Regards, Huw.

  26. You’re welcome, Huw, but before you do go to the trouble of removing the transmission and overhauling it, do check the three oil levels – I’d put money on the cause of your trouble being the main gear box oil moving aft, but it could be something else. If you find the main box low and the transfer box over-filled, it’ll confirm the diagnosis.

  27. Hi Nick,

    Ive had my S3 SWB for 15yrs now (1st n only vehicle), it came with an OD unit and is a Fairey OD type. About 3yrs ago the lever inside the cab for the OD started to rattle quite loudly whereas before it didnt. It moves freely back n forth about an 1.5inches when the vehicle is parked, never used to.Driveing at 50mph this becomes v v noisy, engaging it into any gears at any time is not a problem and it doesnt pop out of gear at all. As I dont use it much the above isnt a problem as I dont use it for around town (30-40mph). The noise only happens when engaged.Im guessing something has worn or a spring inside the unit has broken not keeping the lever taught ?!?! not sure. Any ideas Nick?

  28. Lever rattles are usually down just to wear in the lever links and pivots and result from a harmonic frequency being matched by the engine or other sources of vibration. The two clevis pins in the bent link shaft from the OD’s selector shaft to the lever pivot should have a plastic strip folded around on itself acting as noise reducing washers or shims, but these are frequently missing. I used the lid of a Tupperware box to make up washers to fit between the linkage parts to stop the rattle after the original strips wore out.

    Another likely issue is worn selector fork pads, which should be easy and cheap to replace – you’ll need to remove the top cover, rear end of the linkage and the fork’s pinch bolt, , the detent ball and spring and the stop grub screw, slide the selector shaft out the back of the unit and then lift the fork out. The pads are a loose fit in the fork, so try not to drop the old ones inside the unit as the fork comes out.


  29. Hi Nick. fitted a second hand overdrive to my land rover.Ifound your guide very helpfull thanks.
    On testing with the overdrive off there is no drive however in the on position every thing works fine.just a slight whine.before fitting i drained the oil and no foreign bodys were found. took the inspection cover off and all looked good.Wonder if you have any ideas to the problem. Once again many thanks. John.

  30. Hi John, and thanks for your interest in the blog. I’m glad you have found it useful so far.

    It sounds as if the unit might not be selecting the disengaged position properly, but before you start stripping anything down, you are pulling the selector lever all the way back, aren’t you? The overdrive has a neutral stick position, not just engaged and disengaged. It stumped the AA once when Helena wanted to use the 109 and I was away – they diagnosed a complete clutch failure but the overdrive was just selected in neutral!

    If you are moving the lever to its limit, then something may be restricting its movement. As memory serves, the top of the lever should move about 8″ or so, with a detent for neutral right in the middle of the arc. Things that could restrict it are the lever’s (cranked type) rotational adjustment kinking it too far aft, in which case the top will hit the seat base, middle seat or cubby box and it just needs the bottom locknut slackening to rotate the lever to lean sideways rather than aft, then tightening the locknut.

    Another issue could be the selector fork not being the right way around or its pads running forward of the slot in the synchro hub. Another cause could be a problem with the selector shaft detent or the lock grub screw underneath it (which could be a little too tight and binding on the shaft), or something stuck between the casing and the bend in the connecting rod between the selector shaft and lever.

    If the selector mechanism is all correct, then the fault must be stripped teeth on the front end of the synchro hub or back end of the output shaft. I’d be surprised at that, though. My bet is on the selector mechanism not travelling through its full range.

  31. Thanks for the info Nick.
    I will try and sort it tmz and let you know the cause.
    beer time now try and get my head in gear.ha.
    Thanks John.

  32. Hi Nick.
    your advice on none selection of drive with overdrive not selected worked a treat
    Had a good look at the selector fork; and guess wot; no brass shoes or guides left. [not sure of there technicle name].so as you suggested the selector would not move the requred distance to select.
    Bad mechanics; but learnt my lesson not to trust sellers that a second hand unit is fine.
    On the plus side; new guides fitted; And it works as it should.
    Hope this comment may help other land rover owners in the future.
    Once again many thanks Nick. John.

  33. Glad to hear you have it sorted out, John. Sadly, my experience of second hand or even professional reconditioning matches your experience with this overdrive making me very distrustful of second hand or rebuilt assemblies. In fact, the only reconditioners I trust apart from the original manufacturers (like buying engines or transmissions directly from LR or PAS steering boxes from Adwest) are Turner Engineering. I learnt a long time ago to treat all second hand units as suspect and always strip them prior to use, reconditioning them myself, just like the Tdi engine and the new axles for the 109.

  34. Hi,
    Great blog, very helpful and well presented. I have a Fairey o/d unit but no levers. Can anyone tell me the lengths of the operating rods so can make up a set?

  35. I don’t have the Fairey unit any more, so can’t help with dimensions, but I think making up the levers is a bit unrealistic – the bracket and lever itself would be simple enough, but making the pivot assembly and accurately machining the connecting rod’s slots and clevis pen holes is not a DIY proposition unless you have high-precision equipment. Craddocks sell the parts, and Rocky Mountain can help with some of them too.

  36. great write up very useful (including comments) thanks – word of warning – don’t expect lock tabs to be the right length on parts sold as an overdrive fitting kit on ebay – i still had to file my tabs down to get it to fit (the dreaded 1/2″ stoppage)


  37. John Cassaboon says:

    Hello Nick, I hope you’re still giving advice…I purchased a re-built transmission and transfer case from Rovers North. I want to transfer the O/D unit from the old gear box and place it on the new unit. Is it just a matter of unbolting the O/D unit from the old gear box and bolting it to the new? Also, upon inspection, the old copper breathers have been snipped and crimped off. What do these breathers do? If I replace them with rubber tubing like you have, where would I run the breathers to? Thanks in advance for any help or advice you can give me. This is my first stab at replacing a transmission in my ’72 series lll.


  38. Hi John,

    You will need to start by removing the lever assembly; take the split pin out of the clevis pin between the link rod and the selector shaft coming from the back of the OD unit, and then the clevis pin itself, and the undo the two longitudinal bolts in the top of the gear box/transfer box interface that hold the lever bracket. Then undo the six nuts that hold the overdrive to the transfer box. You will find that you have to undo the clear nuts before those that are part hidden so that the unit can be slid out partially to allow those part hidden nuts to continue to unwind, otherwise they will press against the overhang before clearing the threads. The same will apply when refitting – you’ll have to fit those part obscured nuts before pushing the overdrive all the way on and wind the in before closing the gap on the gasket.

    Once the overdrive is off, you need to remove the “clutch sleeve”. It is a splines bearing carrier on the gear box shaft that couples to the inside of the overdrive, and replaces the transfer box input gear. To remove it, open the top cover plate and use a nail punch or screwdriver and small hammer to gently fold back the tab of the locking washer between the castellations of the main shaft nut – only one of the three tabs will have been used.

    The nut is best undone with the special tool, a socket made to fit the nut’s castellations, but these can be hard to find. In all likeliness, Rovers North will have one and may be willing to lend it to you (don’t be surprised if they refuse or ask for some sort of security; the tools are expensive). Failing that, you can use a mallet and stone chisel to undo the nut. It does damage the nut, but not critically. To stop the shaft turning with the nut, you can either use Stilsons on the input shaft and put it in gear, or more sympathetically, remove the gear box top cover, take out the interlock rods and simultaneously engage 1st and 4th to lock it all up.

    Remove the nut, lock washer, spacer washer and clutch sleeve.

    You will need to do the same on the new unit, removing the rear bearing carrier (funnel shaped housing that sits where the overdrive goes) and the input gear. Note that the tabs on the locking washer are a little shorter on the OD version, to clear the inside of the OD input shaft, so don’t mix them up.

    Check the fine splines on the clutch sleeve. They wear, and if they are narrower than the gaps between the splines, then it needs replacing. Rocky Mountain sell them cheaply.

    As always, refitting is the reverse of removal. Remember to use new gaskets, and if you are not selling the removed unit on, retain the rear bearing carrier and gear in case of future overdrive failure.

    The breathers are unimportant as long as they allow air passage and are small enough to exclude dirt. My transmission breathers were originally just plastic grommets with small holes, and the OD breather a hole with a split pin to keep dirt out, but I installed the tubes to reduce oil seepage and prevent water ingress, subsequently uprated with banjo unions and plastic fuel hoses.
    Good luck.

  39. John Cassaboon says:

    Hello Nick, Thanks for the advice. I removed the O/D unit from the old gearbox today. I checked the fine splines like you said and I’m not happy with them. WhenIi slid it into the large gear from the O/D unit, it felt too sloppy for me and since I only want to do this once, I’ve set out to find a new O/D coupler. The only problem is I don’t think I will be able to source one in the U.S. I called Rocky Mountain and they don’t carry any Fairey parts, only Roverdrive. I called all over and there is a company in Orlando, Florida, Safari Heritage Parts. They bought out all the inventory from Rovers Down South in Louisiana as he went out of business. I’m waiting on a call from them tomorrow and if hey can’t help me, I’ll have to source it from the U.K. I found a company who sells the whole kit, outer bearings and all. It will hold me up for a few days but I don’t mind. I contacted RN to see if they would loan me the tool to take off the castle nut and I never heard back from them. I think they’re sending me a message. It’s OK, I took a small screwdriver like you said and the castle nut came right off. Thanks again for your help. It’s invaluable to guys like me who are novices in working on transmissions and such.

  40. John Cassaboon says:

    Hi Nick, One more item…what is your opinion of packing all the bearings with red synthetic grease?


  41. Glad to help, John and glad you’re making good progress. You have a good attitude to it – do it right and do it once, even if it exercises your patience and wallet to do so. I think the UK arm of Rocky Mountain Spares does the clutch sleeve if your sources don’t work out. The bearing is rarely a problem, but if the new clutch sleeve comes with it, you may as well use it. Keep the old parts as spares, just in case (even if you don’t need them, someone else invariably does, but they are the most wear prone part of the Fairey OD, so might be of benefit one day).

  42. If you want to add grease to hold parts together during reassembly, then I’d recommend using smaller amounts of LM grease. I wouldn’t want to artificially affect end-float by having excess grease, especially thick grease, in the bearings or between races, shafts and shims – it’d cause you to leave out shims when setting up end float which will then cause play as the grease dissolves in the oil. I’d also be concerned that large applications of grease could stop oil flow through those parts – while the grease will provide lubrication, it won’t transfer heat away like oil. A thin smear of LM grease works well for assembly.


  43. my overdrive whines when engaged what is usual cause which bearings should I check or is it end float

  44. Hi Andrew.

    They usually whine when engaged, so unless it’s especially loud, I wouldn’t worry. You can expect a small amount of increase in backlash when engaged, but it shouldn’t be much – most overdrive backlash is from worn splines on the clutch sleeve and inside the input shaft (that wears slower than the clutch sleeve, so I suspect the clutch sleeve was deliberately made sacrificial as it’s cheaper and easier to replace).



  46. You’ll see the mesh on the lay shaft to main shaft gears before you strip it down. The new shaft should aim to have the same mesh. You will need to set the end float of the new shaft by trial and error, but I’d start first by using the same shims the thrust bearings had, and if the old bearings are in good order, re-use them; there is no point in binning expensive parts if they are in good order. The lay shaft is likely to be slightly different dimensionally than the original, so with the shaft fitted, turn it by hand to check for free rotation and try to get a feeler gauge in between the end of the shaft and the bearings. I’d adjust the shims to get about 0.15mm float to prevent the bearings from overheating.

    If the output shaft is sitting further into the housing than before, then the only likely reason is that the bearing race between the main shaft to output shaft thrust bearing (radial needle bearing that sits up against the wide end of the main shaft) was missing – that’s about 2-3mm or 1/8″, and I couldn’t find mine on reassembly, needing a replacement, as did another reader of this blog. I suspect later units may have been made with the bearing running directly against the back of the main shaft section. If you had to omit a lot of shims under the rear end circlips, then this would all seem to tie in.

  47. Stuart Hunt says:

    Hi all
    So how much oil should live in my fairey?

  48. Hi Stu,

    It should take a little under a pint or half a litre to fill the level to reach the notch on the filler plug dip stick. You may find the level drops a little if you run an overdrive briefly after filling from dry as the oil works I to various cavirltues and voids that are trapped with air when static. The design is probe to losing oil gradually into the transfer box and through the breather and selector road seals, so it needs regular level checks.


  49. Moses Ndunda says:

    Thanks for sharing these suggestions. I’ve been looking to buy a series 3 with OverDrive but havent seeon one yet. This gives an option to buy and fit the OD myself.

    Thanks again.

  50. Phillip says:

    Hi Nick,
    You mentioned that the 200tdi was too torquey for the fairey overdrive. I have sourced a fairey in really good condition and plan to fit it to my series 2a original box. I have also changed over the diffs to ex-disco diffs for longer legs. I plan to only engage the unit on the highway as I do a fair amount of 5 hour trips out west and would like to cruise on lower revs and better economy.
    What are your thoughts on this set-up?

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