Jaguar XJ-S. Manual - part 112

 

Figure 24 - Hub for Vented Rear Brake Rotor 

80°

 

100°

 

ø3 3/4

 

ø7/16

 

4 PL

 

ø4 3/4

 

ø6

 

1/8

 

ø5/16

 

8 PL

 

EQ SP

ø5 3/8

 

ø2 3/4

 

ø7/8

 

2 PL

 

ø4

 

3/8

 

ø7/8

 

2 PL

 

ø4 3/4

 

 
 

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Make very sure to clarify to your machinist that all three flat surfaces on this part must be flat and true, as close as he 
can get them.  That means that all three surfaces must be machined; you cannot hack this part out of a rusty plate with 
an acetylene torch and expect acceptable runout when you install your discs.  Also, insist that the flat surfaces be 
machined on a lathe or turntable that rotates the part, not on a milling machine where the part sits still while the cutter 
moves back and forth over it.  Milling was one of the problems with the GT Jaguar kit, as the hub would warp a bit 
during the milling and cause runout. 

As a guide to cost estimating:  When I had a pair made, the shop charged me for 5.5 hours machining time plus a 
pittance for the metal.  Mine were made from 1/2” steel plate; you might be able to save a little machining time by 
starting with 3/8” steel plate, since the machinist won’t have to hog off so much metal.  However, by the time the hubs 
have been “skimmed” on both sides to make sure they are flat and true, they will be a little thinner than 3/8”.  As a 
result, you will need to add a couple shims C33440 to avoid altering the rear wheel camber. 

You can make these hubs out of either steel or aluminum.  Aluminum costs a few cents more in material, but the 
machine shop might give you a break on the machining since aluminum is easier to cut. 

The two 7/8” holes closest to the OD of the hub are helpful to those with the Dana final drive unit.  The OEM Dana 
rotors have two similar holes in them.  Note that the Dana access holes shown are located differently than on an OEM 
Dana rotor; they are rotated 90° around the hub.  Brian Schreurs developed this idea for relocation to make it possible 
to remove the calipers without removing the rear axles first.  With the original location, the U-joint on the axle obstructs 
the access hole.  With the location shown, the U-joint still obstructs the access hole -- but once the axle is disconnected 
at its outer end, you can swing it out of the way without removing it.  The shims on either side of the rotor obstruct this 
revised access hole location, so trim the shims prior to installation. 

You might look for a 5/8” socket with an external hex on the back end so you can turn it with an open-end wrench.  
Spark plug sockets have this, but they’re too long.  There are some odd tool designs that feature an external hex on the 
back end of a short socket for one reason or another, though.  If you find such a socket, you might be able to snake it 
under the U-joint and into these relocated access holes and turn it with an open end wrench, allowing you to remove the 
caliper without having to disconnect the outer end of the axle from the suspension.  Alternatively, you might just grind 
flats on a standard socket, or drill cross holes in it and turn it with a bar. 

The two 7/8” holes closest to the ID allow for removing the brakes from a Salisbury final drive as a unit along with the 
output shaft bearing assembly, same as with the aftermarket Brembo rotors as described above. 

Note that, if the vented disc has a continuous ID of 4-3/4”, it will obstruct all these access holes.  To make them usable, 
the disc needs to be scallopped between holes.  Fortunately, Essex scallops their discs without even asking. 

Read the notes above on making sure you have a 5/8” socket that will fit easily through these 7/8” access holes.  If you 
prefer, make the access holes a bit bigger in your hubs, like 23mm or 15/16”. 

If you have a Salisbury final drive unit, you can omit the Dana access holes when making the hubs; they will do 
nothing for you.  If you have a Dana final drive unit, you can omit the Salisbury access holes; they will do nothing for 
you -- although you might consider the possibility that your Dana gets replaced with a Salisbury in the future.  Finally, 
if you have either the Salisbury or the Dana, you can omit all the access holes if you wish, since it is possible to rebuild 
the brakes on either final drive without access holes; simply remove the bolts holding the outer disc to the hub and 
remove the hub, leaving the disc in place within the caliper, and this will provide all the access you need to get to the 
bolts behind the rotor.  In fact, Alan Heartfield’s TWR came new with a Dana final drive and two-piece vented rotors 
with no access holes.  Having the access holes makes things easier, though. 

Ask if the company providing the discs will also make the hubs for you if you send them the drawing.  Brian Schreurs 
found that Essex will, and for an excellent price, too.  In addition to simplifying the procurement process and obtaining 
a good price, they can also make sure the two parts fit together properly. 

Note that if you keep this car long enough to wear these rotors out, the only thing you will need to renew is the discs 
from Essex -- send that form in again.  The hubs will last forever. 

To assemble the discs and hubs, you will need 16 bolts and 16 nuts -- 5/16” Grade 8.  1” is the length needed, but 1” 
bolts are typically threaded all the way to the head.  Since it is good engineering practice to have a smooth shank within 

 
 

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the joint between such parts, you’d be better off to find bolts about 1-1/2” long and cut them down to 1” to get a smooth 
shank between 1/4” and 1/2” long.  You need self-locking nuts, but don’t get nyloc -- things might get too hot here for 
the plastic inserts. 

There are other ways to obtain suitable rotors for this project.  According to Chad Bolles, “The rear rotors on a Maserati 
Quattroporte are vented and are the same pattern as the Jag.  The calipers are the same as a Jag, but with a spacer to 
make up for the wider rotor.”  Unfortunately, a www search found a price for Maserati rotors well over $400 each.  
There are reports that Aston Martins also use a similar rear brake except with vented rotors; Bill Sotiras says, “Aston 
also moved the rear caliper setup from in front of the final drive to the rear, for better accessability (from under the car). 
 They used to have an inspection plate under the back seat.”   Unfortunately again, there are also reports that owners of 
these cars converting to the Jaguar solid rotors because their replacement vented rotors are so expensive!  If you know 
of any Aston Martin or Maserati owners, you might tell them about Essex and the two-piece rotor design described 
above; it might fit their car perfectly and save them considerable money at rotor replacement time. 

Terry’s Jaguar also sells their one-piece rotors individually, apparently as wear replacements for owners who have 
installed their kit. 

On to the other brake hardware you will need to use your vented rotors.  You will need spacers to widen the calipers.  
These can be made from 1” wide 1/4” thick steel stock, or from any 1/4” steel plate you happen to have lying around.  
They can also be made of 1/8” thick or even 1/16” thick stock; just make more of them and stack them up.  When you 
split the caliper halves, it will be apparent where these spacers go and what shape they need to be. 

You need 8 new bolts to hold the caliper halves together, longer than the originals.  3/8”-24, 2-1/2” long, Grade 8. 

You need pad retention pins that are 1/4” longer than the original pins.  You can buy such pins from Jaguar; the ones 
for the front of the XJ will fit perfectly!  They are part number 12798.  Or you can just make some pins out of long 1/4” 
bolts (you’ll need a smooth shank about 3” long) or even from 1/4” steel rod if you want.  If you go with steel rod and 
therefore end up with headless pins, you will need four more retaining clips just like the ones that are in there. 

The main calipers have a bridge pipe that connects the sides.  Since you have inserted a spacer, this pipe needs to reach 
a little farther.  Fortunately, it is a simple matter to bend the pipe a little bit to make it fit. 

You’ll need to deal with the bronze fork on the handbrake calipers.  This fork is supposed to hold the handbrake pads 
away from the rotor, but just a little bit -- and to take a permanent “set” as the pads wear.  Since you must cut this part 
in half to get it on the widened calipers, you must come up with a way to hold each half still so it cannot turn around the 
pivot pin, which would defeat its purpose.  Jan Wikström just brazed in a spacer.  Other ideas include fashioning a plate 
to go over it with a lip that prevents the halves from turning, or ordering four bronze forks and cutting a long half from 
each one in such a way that they interlock when bolted into place. 

Alan Heartfield’s TWR has a bronze fork in which the holes had been slotted outward to the ends.  I do not recommend 
this; the tines of the fork are attached here, and cutting open the loop at each end of the fork weakens this attachment 
and results in the fork bending here as the handbrake pads wear rather than bending over their length as they are 
supposed to.  Obviously, you also have to bend the fork to begin with with this idea.  Better to cut the fork in half and 
space it. 

If Aston Martins and Maseratis came with similar brake calipers already spaced, perhaps parts suppliers for those 
makes can provide a bronze fork of the correct dimensions. 

You also need to address the issue of locking the handbrake pivot pins so they don’t unscrew; the original locking 
plates won’t work.  You can drill the heads for lockwire -- if you’re really good with a drill; for most of us, that job will 
require a drill press.  Or you can make your own locking plates from sheet metal.  You can combine this homemade 
locking plate with the plate needed to hold the bronze fork halves if you want. 

Alan Heartfield reports that his TWR does have a locking plate.  Apparently TWR either found or made a locking plate 
to fit.  No known Jaguar part number, though.  Again, perhaps finding out what Aston Martin or Maserati did here 
would be informative. 

Some seem to feel that just omitting the locking plate and tightening the pins securely is good enough.  I would not 
recommend this; the length of the pivot pin between the thread and the head is far too short to expect enough “stretch”