Guys
On and off I have been chasing an annoying, relatively low frequency vibration coming from the rear, showing itself by coming up though the floor and into the seats. I changed the propshaft and it went away for a year, then it, or something just like it, returned. I improved the damping on the gearbox mount, and again it went away for a few months until about two months ago. Cornering stability, straight line stability seemed unaffected, but finally a decent "clonk" once the car was warmed up coming off the power gave me a clue that something was loose at the rear. I guessed either the diff in the cage, or the UJs in the driveshafts. or just possibly the "dog bones" - the casting that bolts into the diff bottom and forms, along with the cage mounts, the lower wishbone inner fulcrum, as in shown by 5, 6 ,7 and 8 this diagram:



So I bit the proverbial on Monday and dropped the axle to see what was what. before I dropped it I poked at the bolts to the dog bone, and they moved with a VERY light touch from a screwdriver! So the cause found.
Axle out and turned over with the lower wishbones at the top for easier access. I found at least, at least, 1/8 of an inch of loose play, and the alignment shims, used to align precisely the cage and bone holes for the fulcrum shaft, had disappeared or were trapped somewhere not on the bolts. In one of the photos below, you can see the air gap between the diff and the bone.
A guastly job, as everything is covered with dirty grease from years of assiduous rear axle greasing. Luckily no damage to anything important either once all was apart. There is a very complicated four part thrust washer/grease retainer/spacer/inner thrust washer/inner bearing tube assembly (10,12,13,14,15) to get in on BOTH sides of each side of the suspension arm between it and the cage or casting. Luckily the actual needle bearings were fine, as these are a bear to change. The only sign of trouble having been there at all was some evidence of hammering on the bearing inner tube, which is just a slide-in replacement. You can buy a kit with all the necessary pieces in it, which I had in stock, luckily; but I could have reused the old pieces as they were actually fine. If the bone bolts had not come loose, the arrangement was otherwise as good as it was when I fitted it to a new diff 18 years ago. So the regular greasing did some good!
Apparently loose bone bolts are a known problem. 18 years ago when I did the rebuild, I used new bolts from Jaguar with the factory Loctite on them and new bones, as the old ones could not be undone from the old diff. But loose they have come.
I have ordered up some Loctite 270 which will allegedly never come loose without several hundred degrees of localised heating, for when I put all back together. The only tricky bit is ensuring the dog bones are shimmed up dead on, so the shaft can be slid though all four holes without jamming. I shall carefully trial fit all that before I add the Loctite! Also this is much easier with a helper.
Checking the bolts is extremely hard. The spacer tube (to protect the shaft from road grit etc) between the bone's fulcrum holes (25) prevents one of the bolt heads from being tightened. The other can just be seen, but again is very hard to get a spanner on. Worse, once a couple of thou loose, the shims fall out and that is that.
If ever the axle comes out though, and mine has been for various reasons over the last few years, this is a vital area to get at and check, undoing the fulcrum shaft if necessary.
Greg

 

Good work !

I'm only two sips into my first cuppa coffee but I could almost swear that the bolts (7 and 8) are safety-wired on my car.

Cheers
DD

 

They were Doug. But then Jaguar decided to use Loctite-type stuff only, and my new bolts in 1999 were supplied with it already on the threads and the bolt heads were not drilled for lockwire. I am putting my trust on Loctite 270. Am I being foolish or not?

 

I wouldn't say 'foolish'...as Loctite is widely accepted as a good alternative to locking wire.

However, given the history, this might be a good time to apply the "belt and braces" approach !

Cheers
DD

 

One idea I learned from the J-Lovers forum, was to use shims made from cutting the corners out of camber shims. There will now be a hole, and they can't fall out. The shims are in different thicknesses.
Good luck,

Rob

 

That is one terrifically good idea. And I have some in stock!
Greg

 

Pulled the shims I had used on the dogbone, on the one side I have got ready for reassembly. Measured them at 21 thou. Then I got out my spare rear axle camber shims and, proving the existence of a God of XJSs, one was exactly 21 thou thick. Then I did as Rob Laughton suggested and cut out two corners complete with their OEM holes and replaced the horseshoe shims with the corners. Test assembled and all 100%. Brilliant, thanks again Rob.
Greg

 

That is one bugger of a job you got there Greg, I like the shim idea I remember when I did mine I didn't have enough hands to hold everything while re-assembling.

 

great info Greg, i have a slight rear vib not sure exactly what, will be doing some close checking.