Chatting today to my mechanic Daniel at Langs In Etobicoke, Ontario.
I have been very impressed with his work so far on my XJS acquired last fall.

While he's got it up on the hoist I asked him to check rear rotor and pad thickness, as they will need doing sooner rather than later.
He mentioned in passing that one option is to switch out the rear brakes for the outboard ones from an XJ40. Upfront cost in doing so, but then pain free brake jobs going forwards.
Curious as to whether anyone has done this conversion on their cars and experience in doing so?

Cheers,

Quentin

 

I've done it on my 87 XJS. It requires the XJ40 hubs, calipers and discs, half shafts, spacers for the outboard end of the lower control arms, emergency brake pivot levers unique to the late XJS, inboard lower control arm shafts, again unique to the late XJS, unique emergency brake cables for the outboard brakes, rerouted brake lines, and probably other things I forgot. You might have to trim the diff output flanges to clear the half shafts to get the rear camber correct.
I highly recommend getting a complete late IRS, because finding the unique components is a PITA, as they are all NLA from Jaguar, except for the emergency brake cables. The IRS most likely has to come out to do the swap properly. You will likely want to rebuild the hubs and half shafts if they come out of a junkyard car. You'll also want new discs and rebuilt calipers.
Would I do it again? Maybe, if I got a complete IRS. Piecemeal, it's not worth the aggravation. Rebuild your inboard rear calipers with stainless pistons, rebuild your parking brake assemblies, maybe convert to inboard vented discs, put in a remote bleeder system and be done with it.

Jon

 

Jon,
Thanks for your detailed reply.
So you would suggest getting an IRS of a facelift model already equipped with outboards as best option. Makes sense.

Quentin

 

I would recommend getting a complete late XJS IRS but you dont have to. You will also need the factory late XJS or XJ40 rear brake pressure limiting valve. The rear brakes will be so much more effective that they will tend to lockup much earlier than the fronts, this can create a loss of control in many situations.

You could Simply leave the inboards for the Ebrake and spacer functions. Then you only need XJ40 half shafts, rear hubs, brake lines. Your mechanic will need to make a 2 hard lines. One end it will be a SAE 3/16 male threads and the other a metric m10x.10 female to accept the brake hose.

 

ICS A
Very interesting, what is the reason why the outboard brakes more effective?

 

I haven't experienced the lockup issue, and I have the vented outboards of the very late XJS' with the appropriate calipers. Of course, my 87 doesn't have ABS, so that may affect the brake balance.
I wouldn't even contemplate leaving the inboard brake junk in place. Twice the rotating mass, horrible parking brake design, and more weight. And it may still be impossible to get the camber correct with the XJ40 half shafts without mod to the output flanges of the diff. I had the Dana diff, and I had to remove a portion of the centering portion of the flange, as well as shorten the studs for the half shafts. I had less of an issue when I changed to a Salisbury diff.

 

I am about to start this project on my 1992 4.0. I have just bought a 1994 XJ40 rear suspension set up (and diff) so was about to drop out the rear cage this weekend and start comparing. Before I read this I was hoping it would be a straight forward swap, XJ40 half shafts hubs, disks and calipers I thought would just bolt on with no adjustments or fetterings other than the brake lines, but wouldnt the xj40 lines match up to some extent?

 

Your XJS only has one brake line running to the cage and it feeds both calipers. You have to run a line to the other side of the cage. In addition, the rubber brake lines from the late XJS have to be used. Note that the rubber lines for the unvented disc calipers are different than the vented caliper lines. The metal lines have to be fabricated, and have different fittings on each end. You will need brake line flaring tools to do it. You also have to fabricate hard line mounts where the flexible lines attach to them.
The diff from the XJ40 is completely different, and unuseable in the XJS.
The parking brake cable setup on the XJ40 is different than the late XJS. The late XJS uses a dual lever setup that fasten to longer inner lower control arm shafts to pull on cables to the individual parking brake drums in the XJ40 brake setup. The levers and shafts are hard to find.
The outer lower control arm shaft for the rear hubs on the XJ40 is smaller in diameter than on your XJS, so spacers between the shaft and XJS lower control arm have to be fabricated. I used stainless tubing of the correct ID and OD to fabricate the spacers.

 

Is there a reason why people take XJ40 components and then do a significant amount of fabrication and change? Other than easier availability of the XJ40 parts?

Surely (if you really want to do the change) it's so much easier just to fit a complete cage out of a late XJS? Or am I missing something? Or if you're going to drop the cage anyway, why not just refurb the inboard brakes, fit remote bleeders, as mentioned, and then you're good for another 10 years?

Paul

 

It's easier with the complete cage, but are much harder to find and are usually more expensive. Also, the late cages have a different diff ratio (3.54 vs 2.88) to match the 4 speed trans in the late cars. So it all depends on what you can find. I was able to find XJ40 parts at my local LKQ for cheap. Over time, I bought the other parts I needed off ebay.
I don't care for the inboard setup as you have the exhaust in close proximity, is hard to work on, and discourages maintenance as a result. Just replacing the discs is a major effort. The difference in unsprung weight on a heavy road car makes little difference in real world handling. My car needed the inboard setup completely rebuilt, and I decided to do the changeover instead as I was going to have the cage out anyway. I can now change pads in half an hour, and discs in an hour, without getting under the car.

 

That would be my choice....but availablity on complete outboard brake cages might not be that good. Not that I've looked, mind you

Perhaps more than ten years.

From purely an ease-of-repair standpoint I'm not really sure the swap pays off....considering how little most of these cars are driven nowadays. It would've been easier to justify the swap 20-25 years ago when the cars were still in regular use and wearing out parts regularly.

Cheers
DD

 

You could Simply leave the inboards for the Ebrake and spacer functions. Then you only need XJ40 half shafts, rear hubs, brake lines. Your mechanic will need to make a 2 hard lines. One end it will be a SAE 3/16 male threads and the other a metric m10x.10 female to accept the brake hose.

Good idea!

You could remove the main brake pads, and just use the E brake. Yes, there is extra weight there, but not too much.
I've always read that the outboard E brake is a big issue with this swap.
Also, I have an outboard IRS that I stole the 3.54 from. I thought the rest would be nearly useless, but now I see it could be used if you left the diff and brakes on.
Food for thought.
Rob

 

The 1995+ XJ items use a caliper has a larger bore pistons and the rotors is larger. That combo of more clamping force and leverage adds up to more braking force relative to the front. This is less of a problem with the earlier (94 and down) XJ rear brake items with the solid rotors but they are still noticeably more effective. These have a smaller caliper bore and a smaller rotor. Both the early and late rotors are still larger than the inboards.

Rear Brakes - Rotor Size -- Caliper piston size
Inboard, - 10.375 inches - 42mm caliper piston
Outboad solid (up to 94) - 11.5 inches - 36mm caliper piston
Outboard Vented (95+) - 12" inches - 48mm caliper piston

You can see the inboard and early solid are close when considering the smaller piston with larger rotor. The 13% lever arm increase offers up a bit more mechanical advantage but the smaller clamping force of the piston mitigates that effect somewhat. On the car the brakes are much more effective and rear locking will occur earlier. This wasn't so much of a problem on the heavier sedan which also had larger front discs. Both the later XJ40 and XJS uses an inline pressure valve to compensate when fitted with rear outboards.

On the 95+ items Jaguar went full kill with the large rotor and 48mm piston bore. These offer up tremendous clamping force with the combination of increased lever arm and piston bore advantage. By this time the ABS systems were faster acting, more reliable and less intrusive

 

Thanks ICS A.
This bears out what Norman Lutz said and recommended about the inboeard rears. They could be bored out to 48mm diameter pistons and this would geartly improve the braking capacity. His calcs are attached.

 

As ever I am in awe at the collective wisdom of the kind folks on this forum.
One other piece of crucial XJS info I am after:
When Tom Walkinshaw came 3rd in the '85 Bathurst 500 downunder, what colour were his underpants?

 

Brown, he was absolutely furious to have lost ...

 

He didn't wear any, it was far too hot with that v12 up front!

 

I have been comparing workshop manuals for the XJ40, XJS 3.6 and 4.0, my 4.0 manual is up to 1993 so still shows inboard, I do have a 96 manual on dvd but cant get it to open.

I thought it would be interesting to compare the 3 and see what similarities I could learn, Brakes I can work, suspension messing is a new thing for me, it need to do more research.

After 33 years of owning a Series 3 XJ6, and having 33 years of cheating death under a very highly jacked up car or at least losing all the skin off the backs of my hands, I thought this upgrade would make life simpler only having the one set of inboards to worry about,

I am considering seeing what 95MY parts I can source

 

My thoughts exactly!