Dropping the Diff Ratios – Salisbury

Though the 3.54 diffs in the coiler axles gave me much better cruising rpm and fuel economy (10%), they didn’t drive nicely and I strongly suspect they were a major contributory factor in the failure of the gear box.  So, I have replaced the rear 3.54 diff with a SIII 109’s  4.71 ratio diff.  The front Rover diff will subsequently be replaced as soon as I collect the unit I have located.

The Haynes manuals for the SII/SIII and Defender all say that working on a differential, especially a Salisbury, is beyond the ability of the DIY mechanic.  I beg to differ, but you do need a few special tools.  The good news is that if you just want to replace bearings, then their tolerances are so good that you can strip and reassemble the unit using the existing shims, making careful note of the preloading nuts before the strip starts.  In a Rover diff, the pinion bearing preload is set by shims, so the big nut securing the flange is set to a very tight torque, and missing that torque will not affect bearing preload.  The bearing caps for the main carrier have a pair of big nuts inside that set the carrier preload, and so their positions must be marked if you want easy reassembly.  Not so the Salisbury, which has shims for carrier preload and mesh, shims for pinion mesh and pinion nut position for preload.

The Salisbury is reputed to be much more difficult to work on, but other than it being heavier and more awkward in that the housing is integral to the axle, it’s not harder, it’s just different.  I started with two units – the 4.71:1 unit from a 109, and the 3.45:1 unit in the 110 axle fitted on the vehicle.  Both are identical in construction and share many common parts (bearings, shims, seals, gaskets, cover plate, collapsible tube, flange, nut and washer).  Whether rebuilding an existing unit or swapping from one to the other like me (and it doesn’t matter which way you’re swapping), the technique will be the same.

As I was doing this with the axle fitted, I jacked the vehicle up, propped the rear chassis on stands and removed the wheels before lowering the axle on its springs.  This was to lower the top of the axle below the level of the rear fuel tank – if you have no rear tank, then you can do the job with the vehicle on its wheels (chocked).  The prop shaft was removed from the rear diff pinion and hung from the cross member above with a bungie and both half shafts were removed.

The rear cover plate was removed using a 17mm hex socket and ratchet.  This exposes the diff carrier and crown wheel.  The carrier bearings are secured by a cap on each side, each with two bolts.  These are removed with a 19mm hex socket.  Make sure you mark the caps with which side and orientation they have.  Some carriers will now be removable with a little prying, but they should still be held in place by the axle case if the case and diff are still in decent condition.  A spreader is needed to widen the aperture and release the carrier bearings from their seats.  there are two types, one which locates into the circular holes either side of the aperture and forces them apart as the threaded bar is turned.  The type I borrowed was the other sort that comprises of two thick bars which sit above and below the the diff housing, with threaded rod between them.  As the nuts are tightened on the rods, the bars are drawn together as a big clamp, squeezing the axle case top and bottom, forcing the sides out.  With either type, use the minimum pressure you can to get the carrier out, and release the pressure as soon as possible to avoid distortion or damage to the casing.

As the carrier is removed, the outer bearing races will fall off.  Make sure you note which came from which side if you plan to re-use them.  Be very careful as you withdraw the carrier – it’s very heavy and you don’t want to drop it, damaging it and yourself.   The carrier and races were cleaned up by soaking and brushing in a large bucket of old petrol.

The pinion was next up for removal.  Make alignment marks on the nut and the end of the pinion in a way that won’t come off and will allow you to refit the nut at exactly the same position later.  Undoing the big nut is not normally that hard.  A 32mm socket and a ratchet should do the job, with one of the prop shaft bolts through the flange to wedge a pry bar against to stop the pinion turning.  Once the nut is off, recover the washer and pull the flange off the pinion.  With these clear, you can now pull the pinion out from inside the axle case.  It will come out with its bigger inner bearing race, just next to its gear, which is a press fit onto the pinion.  The other inner bearing race and an oil thrower disc will remain inside the diff housing nose, just behind the seal, along with both outer races.  The crushable spacer for the bearings should come out still on the pinion shaft.

All the mesh adjustment in a Salisbury diff is set by shims behind the rear pinion bearing outer race and between the diff carrier and each carrier bearing.  Here’s the good bit: the pinion shims are to take up tolerances and variations in the diff housing, with the pinions being machined accurately enough that they will only rarely need shim adjustment on being changed.  So, whether you are simply replacing worn bearings, replacing a damaged pinion or swapping to a pinion of different gear ratio, you keep the existing shim pack fitted to you current axle case.

In order to inspect the other bearing behind the seal, the seal had to be removed.  A bit of a shame, as the seal was working perfectly and had done less than 5,000 miles, but necessary.  As it turned out, the bearing race was fine, but this presented me with the opportunity to set the bearing preload, which is critical and had been previously mis-set (making and markings on the pinion and nut worthless).  I think this would be wise if fitting a different pinion anyway, just in case of any small variation in machining.  So, I tried assembling all the pinion components, less the seal, and then tightening the nut to compress an new crush tube.  This might be possible up on a lift with a 4′ breaker bar and the correct pinion flange locking tool (a lever with lugs that engage in the bolt holes), but it’s not possible with a crow bar and torque wrench on a drive way.

I came up with a cunning plan:  refit the pinion without the crush tub and nip the nut up until the preload is right, make the nut and pinion alignment, strip them out and then reassemble them for setting the collapse of the tube in the bench vice, using a bolt through one of the flange holes to stop the flange turning in the vice jaws.  This allowed enough space and a better position to apply the 250’lbs force to crush the tube, turning the nut until the alignment marks lined up.  The whole lot was then put to one side while I set about the carrier.

After comparing the dimensions of  both complete carriers with their bearings side by side (the black carrier is 4.71, the red is 3.54), using a digital vernier and selection of bars and blocks, to see if any shims would need addind or removing from the 4.71 diff, and being satisfied they were similar enough as they stood, the 4.71 carrier and its outer bearing races were fitted to the 110 axle.  The spreader was slackened off and the carrier checked for end float and rotation.  It was free to rotate and had no excess float – it was an immediate perfect fit.  The total shimming was right for this axle case too, but it remained to be seen whether any shims would have to be moved from one side to the other to get the mesh correct.

So, a new seal was obtained, the smaller bearing and oil thrower lubricated and installed, and the oil seal drifted carefully into place with a bead of RTV sealant around its outer edge.  Once that lot was in, the pinion was pushed through from inside (bearing pre-lubricated), and then the flange, washer and nut fitted (having lubricated the seal lips).  The nut was tightened up in the same manner as before with the wrench and pry bar until the marks aligned.  The new crush tube was holding the bearing races apart, as is its function, and so I still had a little too much end float despite the earlier efforts to preset this, so the last little effort had to be made on the vehicle.  It was very hard work to turn the nut tighter, but with just a little over 60 degrees (one more flat of the nut), the play was just eliminated.  It is critical not to go too far when doing this – the crush tube will be unable to do its job and the preloads will be wrong.

With the pinion complete, the carrier unit was fitted again and the bearing caps installed with their bolts just nipped to have everything held in place.  The cleaned and dried crown wheel teeth had to be tested for mesh, given the transplant (a simple bearings change would not require this), and not owning any “engineer’s blue”, I had to improvise again.  Emulsion paint works well enough for this task, and four teeth of the crown wheel were given a moderate covering.  The carrier was then rotated several times until an even covering of emulsion had worked its way around.  I was very pleased to see a good pattern of gear mesh in the paint straight away.

The paint was removed, the cap bolts torqued up to 100’lbs, the rear cover fitted with a new gasket, the drain plug, prop shaft and half shafts re-installed and the diff filled with 2.5l of EP90.  My drive is on a slight slope, so having put 2l of oil in, I had to drive the vehicle onto the road to turn it around to park it nose-down.  The remaining quantity was then added.

I had researched the job in advance, and found two other accounts of a similar swap, though fitting 3.54 diffs into 109 axles, the reverse of my swap, in both cases saying they didn’t need to change any shims.  Additionally, Kam Differentials’ own website specifically states that pinions are accurate enough that when swapping them over, even for ratio changes, they don’t need shim adjustments (they give a figure of 95% not needing adjustment, but this might just be to cover their backsides).  Kam say that you can expect to adjust the carrier shims, but my experience matched the other two amateurs in that the whole carrier and bearing assembly and pinion and bearings set transferred from one axle to another with no adjustment required.  I would still recommend you check yours, though.  I found that my pinion sat within 0.02mm of its original depth in the new axle case, and the overall thickness of the carriers and their bearings was within 0.2mm of each other (all measured with the carriers removed).

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  1. Red Rogue says:

    Thanks for the great post. I’m thinking of doing the reverse swap of ratios. This gives me a little more confidence!

  2. Hello Nick. Congratulations for the post.

    Would it be possible to change only the ring and the pinion with ratio 4.7: 1 to the differential of the defender (ratio 3.54: 1) keeping the red case of satellites and planetariums (defender)?

    I want to know this because I want to install a salisbury axle diff lock for Defender 110” on a series III 109 ” salisbury axle and keep the ratio 4.7: 1 (original ring and pinion of series III 109 ”).

    Specifically this diff lock: https://www.paddockspares.com/da5711-detroit-truetrac-rear-locker-24-spline-salisbury-axle.html

    Thanks and regards.

  3. Hi Jaime,

    It is possible as long as you use a spacer ring between the crown gear and diff carrier. This is what has to be done with most locking diffs, as they are generally made to Defender rather than SIII spec. I intend exactly the same ATB for mine, either with the existing 4.71 ratio or the 4.11 Dana set I bought (yet to confirm they fit).


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