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. Hi Phillip

    My comment about the Fairway design not being robust enough for the TDI is speculative, but they do suffer bearing failures and spline problems behind a 2.25 diesel, so an engine with twice the torque and twice the power is going to make it suffer. I’m sure it’ll be ok for a while, but I wouldn’t use it in high range except in fourth gear to keep the input torque down.

    I think you’ll also be overstressing the gear box. I had a third gear failure caused by using 3.54 diffs in conjunction with overdrive, and it’s just too much for the gears to handle. A friend recently suffered exactly the same failure (stripping two teeth of the main shaft third gear) after running 3.54 diffs and overdrive behind a Tdi and disagreeing with me, perhaps, some woukd think, tempting fate but I think confirming my diagnosis. Personally, I found the 3.54s a horrible drive with the Series transmission, over geared, sluggish, no better on fuel economy, and needing a revvy their gear around town rather than a lazy fourth.

    I have found a 4.1 Dana 60 gear set that should fit the Salisbury axle (need to get teh dimensions of a Salisbury pinion for confirmation) and will get a matching front Rover set from KAM or Ashcroft. But even with that middle ratio, I’d only ever select overdrive in fourth gear when in high range in order to protect the gear box. Low range does not provide the same resistance and gear box wind up, so is safe for the gear box and overdrive in all gears.

  2. Phillip says:

    Many thanks for that response mate. I have to agree, albeit begrudgingly, that the 3.54’s are disappointing around town. The gearing is all wrong and gaps between all gears are awkward. You are dead right regarding 3rd being too revy and 2nd being too brief. I will most likely reinstall the original diffs.
    Fuel economy was also disappointing on an 800 km trip last month.
    I wonder if the original diffs with the fairey o/d will give better gearing on the highway. I will cross my fingers and be careful with the o/d and hope to get some life out of the gearbox.

  3. If found no discernible difference in fuel economy on long trips with the 3.54s; the engine is working so hard with the tall gearing that the benefit of reduced rpm is wiped out. It is quieter at 70mph, but not enough to be worth the dreadful low speed driving, poor acceleration and cumulative gear box (and overdrive) fatigue damage, which will take months to result in a failure, but is inevitable all the same.

    It’s a shame, because the 4.71s are undergeared for the TDI and improvements are to be had. If you haven’t got too many mods to retain the VIN and registration, fitting Defender transmission with the 3.54s would give nice gearing (with the modified 2wd LT230, a CV front axle or just FWH on the front to counter the steering kick of 4wd on hard surfaces) is probably the way to go. For me, I am limited to changing the diff ratios if I want to avoid IVA, reregistration and loss of tax exemption. Right now, with the consultation on applying Euro6 emissions standards to all new IVA vehicles pending, I’m not sure I can ever fit the TDI Range Rover auto and transfer box I bought.

  4. Colin Timbrell says:

    Hi Nick, On driving my Landy Series 111 with overdrive. I lost my gears on 3rd & 4th gears. ( i was not in overdrive) After driving for a quarter of a mile, while still trying to engage 3rd & 4th gears, the gears went back to normal. I was told it could be the castled nut come loose on the overdrive. Before i take off the overdrive to check for a loose nut, is there anything else i could check before hand?.

  5. Hi Colin,

    If you could still use 1st and 2nd, then it’s not an overdrive fault. By “lost” 3rd and 4th, do you mean you could select them but had no drive, or couldn’t select them? The latter is a common fault as a gear box wears, and I have had it a couple of times, once documented on the blog.

    As the three leaf springs on the 3rd/4th synchro wear, they weaken and eventually one or more will shatter. When that happens, they can jam the synchro hub in whatever position it is when they break. That can lock you in or out of those gears until the shards clear. If you lose two of the springs, the synchronisation on gear change will be weak, so though the shift will feel easier with reduced spring resistance, the gears will be more prone to crunches. Big all three fail (and they will eventually, as each failure puts more load on the remaining springs), then you will have no synchronisation on changes and will get bad crunching.

    Draining the oil through a seive will reveal any spring parts. They’re dull grey sheet steel, very hard.

    Some claim to be able to replace the springs through the top cover, but I tried and couldn’t manage. Most have to strip the box to do the repair.


  6. Colin Timbrell says:

    Hi Nick thanks for the great advice. I could not engage 3rd or 4th gears.
    Now i know its not the overdrive. I did take off the top covers on the overdrive & all the gears showed no signs of wear. I will do some short journeys & see how the gearbox go’s. Thanks again for your help. Colin.

  7. If you still have the top off, look for the three dull grey leaf springs on the 3/4 synchro, tucked under the bright steel rods from bronze ring to bronze ring. I’ll bet one is missing or broken.

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