XJS / XJ Rear Caliper interchangeability

[richardpierce]

I am restoring an XJ12 S1

I have a pair of late 80's XJS rear brake callipers which were on the IRS that came with the car but I've noticed they are slightly different from S1 rear callipers

I can see they are slightly different but my question is will they fit straight onto my Salisbury 4HU diff and will braking performance be affected (I intend to refurbish all callipers on the car)

Also can anyone confirm if they are different seal kits (I think they are), pistons & pads?

If anyone fancies a swap XJS for XJ let me know please

[mike020150]

Hi Richard,
The later callipers are a direct swap for the earlier version. Series 1 and early series 2 are the same and then later series 2 and series 3 adopted the later version. XJ-Ss changed in similar fashion. The rear calliper threads remained imperial even after the fronts went metric (at a slightly different time from the rear calliper changes). The rear seal kits are definitely different. Perhaps someone else can confirm or deny it but I have a recollection that the piston dust boot sealing arrangement was the reason for the change so the seals were changed along with the pistons. There was no difference in the pads or braking performance. Mike.

[richardpierce]

Thanks Mike

[oldtimer]

Hi Mike,
Interesting stuff, you say the rear caliper pistons were changed. Do you know if the piston diameter changed?

Many moons ago I replaced the 3 pot front calipers on my very early Series 2 for 4 pot calipers from an XJ-S.

A bit late to ask now, some twenty years or so after the event but just out of academic interest I wonder if the rear caliper piston diameter altered with the introduction of the four pot caliper to achieve an improved front/rear balance!

Oldtimer

[mike020150]

Hi Oldtimer,

I'm 95% sure (old memories can play tricks!) that the rear piston diameters were unchanged. Jaguar did replace the original 3 pot calliper combination valve (which, unlike 4 pot equipped cars included a rear pressure reducing valve) so that change would have effected a brake balance change between front and rear. Equally, brake balance changes could have been achieved by the master cylinder changes. Mike.

[oldtimer]

Mike thanks for the reply,
This topic has the making of a long running serial but perhaps not ay.

Oldtimer

PS. I still have my '73 XJ ---- No road fund license next year!

[mike020150]

Hi Oldtimer,

Glad to hear you still have the XJ in anticipation of a 'free' road fund licence but since that depends on the whim of politicians, I'm tempted to suggest that to avoid disappointment you might be better off believing in fairies! Mike.

[jBlackwell]

Just to confirm,
The government have NOT re-introduced rolling tax exemption. They have just moved it, currently to its selected date.

James