8.15.4 - Rebuilding the Differential ( Martin Violette,  February 22, 2002 )

Before I bought my ’91 XJ40 a couple of years and 60,000 miles ago I was lucky enough to find Jag-Lovers and the XJ40 buyer’s checklist, on which as I checked over the car I duly noted a faint whine in the differential. Well, after some procrastination (ha ha) and a whole lot of noise from the rear end, I finally got around to dealing with the problem. Here are some notes on what I did to rebuild the differential (‘91MY GKN, non-Power-Lok). Please note, although this is a bit of a how-to, I’m not a pro, and Jag gives virtually no guidance on this procedure, so try this at your own risk. And I am assuming that you aren’t a novice, so I haven’t gone into detail on some standard procedures.

Removing and replacing the diff is covered in the on-line book (very well, thanks to Brett Gazdzinsky), in Haynes, and of course in the shop manual, so I am not going to describe that procedure here. But there is no info in Haynes and very little in the shop manual about the diff because Jaguar’s official policy is to replace, not rebuild.

But I thought that since I had to remove it anyway, why not have a crack at rebuilding it. The shop manual gives info on tearing down and reassembling the Power-Lok version to replace bad clutch plates, and the info there is critical, so it should be referred to. After the diff is removed it should be securely anchored to a heavy work bench, the two lower wishbone studs removed (30mm box end required, and these are installed with 200ft lbs/270Nm of torque and Loctite 273 so they are difficult to remove. The shop manual states these are to be replaced but it seems you can only get them as part of the Power-Lok clutch kit, so I didn’t. I am guessing that the kit has an updated version of the studs, but I don’t know for sure.) Then remove the rest of the cover bolts and the cover. From now on everything should be scrupulously clean.

While the diff is bolted to the bench (use the two bottom case to subframe bolts) remove the pinion nut, washer, and drive flange. If you haven’t already removed the output shafts remove them now, and keep their shims marked as to which go on which side with which bearing carrier.

At this point the case must be expanded to allow removal of the crownwheel and output bearings. There is a Jaguar special tool, but two pipe-type furniture clamps and two pieces of angle-iron served me well. Use a dial indicator as shown in the manual to be sure you aren’t overdoing the case stretch. When you are ready to remove the crownwheel assembly be prepared to capture the bearing cups, which will otherwise fall off. Then remove the pinion gear shaft, and drive out the inner and outer pinion bearing cups. Be sure to keep any shims with notes, illustrations or whatever you like to use to put them back in the right places. (N.B. If you are only replacing bearings, and not shafts or gears, you will reuse the old shims- there is no need to re-shim the assembly or check teeth mesh and backlash, as none of this changes.) Mark the output flange to case and pinion shaft, and the crownwheel position relative to the case for reassembly.

The output bearings will need to be removed by an automotive machine shop, unless you have a large hydraulic press. Be sure to instruct them to keep the shims with the appropriate side of the crownwheel- I marked one side of mine with paint. Take the output shaft assemblies with you and have them taken apart as well, and take your new bearings to have them pressed on the shafts. Total cost for this in my case was less than $50.

Examine the old bearings for edification, even though it should go without saying that you will replace them all. In my case, the starboard output bearing was the most damaged, the inner pinion the next worse, but all were shot. Interesting wear patterns, but that’s another story. All gear teeth looked like they were brand new. A note on bearings: you can buy rebuild kits from the usual sources, or for about half the cost you can buy bearings and seals from a bearing house, which is what I did. But one warning: both the pinion shaft bearings are special. Mine were SKF, and the codes after the size and duty numbers indicated (when I finally found them on the SKF Web site) that they were produced especially for application as differential pinion shaft bearings. The other bearings are pretty standard fare. I paid about $150 for all, including seals.

While all that is going on you will have cleaned up the diff case, and all the subframe parts, so you are ready to put the diff back together. The first step is to properly install the pinion shaft and output flange assembly, which is the most difficult part of the operation. The pinion shaft nut has to be tightened to bring the bearing into a preload state, and either too little or too much preload will cause premature bearing failure. This bearing preload is achieved by tightening the nut on the pinion shaft to crush a steel sleeve until the desired preload is reached, so that the assembly is as rigid as if it were solid. This means it is a tighten and measure, tighten and measure operation. And it’s one-way - the nut can’t be loosened, if there is too much preload you have to start over with a new crush sleeve (I bought two in case, although I didn’t need the spare). Factory assembly uses some very sophisticated computer controlled air driven machinery to do this which just can’t be replicated in a shop. It is worth doing some research before attempting this job, and in particular I recommend visiting the Timken Web site for technical info on bearing preloads.

Bolt the case back to the bench, assemble the pinion shaft, including the new crush sleeve, install it and fit the output drive flange. The threaded part of the shaft must be dead clean, as well as the nut. You are going to need an inch-pound torque wrench, Loctite 273 (I couldn’t find this so I used 271 which is almost identical), a clock or watch because you are going to be working against the adhesive cure time (get the data sheet from Loctite), and a torque multiplier. This last tool is expensive (around $400), so try to rent one if you can find one. I work on farm stuff, so this gave me an excuse to buy a used one on e-Bay. I used a 4-to-1 multiplier, and I still showed about 125 ft-lbs on the torque wrench I used to drive it, which means the actual torque on the nut was over 400 ft-lbs. This is the torque it took to crush the crush sleeve. You could do it with a large pipe on the end of your socket wrench, but you won’t have the necessary control over the pinion bearing preload, which is what the crush sleeve is for- the idea is to tighten the pinion shaft nut just enough so the frictional drag on the bearing is between 35 and 55 in-lbs. To achieve this requires precise control over the amount the pinion nut is tightened, and checking the turning torque after each 2-5 degrees the nut is turned, which would be very difficult with a long breaker bar. To see how much you are going to turn the nut, measure the difference between the old crush sleeve and the new, and divide this by the pitch of the shaft threads. For example, if the pitch is 13 tpi, then one turn is one-thirteenth (.077) of an inch, and if the crush sleeve difference is say .040 inches, then you will turn the nut approximately one-half a turn to get “in the zone” of the required preload. But for a number of reasons you will start measuring preload early, in fact, right after you have tightened the nut far enough to take up any slack (you will feel it when this happens). I started measuring about every 5 degrees of tightening. Tighten, remove the multiplier setup, attach the in-lb torque wrench and measure, repeat. With about 30 minutes of cure time for the Loctite, you have can’t waste a lot of time. As soon as you get the preload you want, stop! If you have consulted the Timken site you will have noted that too little preload (as long as it is positive) is a whole lot better than too much.

After this things are easier. Stretch the case again and insert the crownwheel and bearing assembly, and then everything else is pretty much cut-and-dried, just reverse what you did to take it all apart. Be sure all shims are in their correct positions and that you have followed your alignment marks. And don’t forget to fill the case with new lubricant!

I now have over 600 miles on the rebuild and it is still dead silent, so I’m pleased. It wasn’t easy, and if I had to do it again I would seriously consider replacing it, especially since a couple of our well-known dealers have brand-new Power-Lok versions available for a great price. But it was interesting and I ultimately saved enough to do the SLS conversion at the same time and then some. And a few new tools for the box.