When Kirby Palm decided to assemble the parts necessary to install vented rotors on his '83 he decided to go with a layout very similar to Heartfield's TWR , but with a few detail differences.  For one thing, the entire 1/4" of additional rotor thickness will be located on the outboard side of the solid rotor location.  This means the mounting of the inner half of the caliper does not require any modification -- which means the brake lines to it are unchanged and the handbrake cable attachments don't move.  The spacer between caliper halves will take care of the location of the outer half.

To accomplish this rotor location with a 3/4" thick disc, the 3/8" thick hub has a 1/8" recess machined into the inboard side, leaving the outer portion 1/4" thick.  The 1/8" step will provide a suitable pilot diameter for securely centering the outer disc, provided the ID of the outer disc is machined to fit it snugly.

The details of this design are outlined in the book , including instructions for ordering the disc from Essex and a diagram for having the hubs made.  Here's the disc from Essex:

The Essex "Custom Brake Disc Order Form" requires specs for the disc OD, thickness, back side ID, mounting flange ID, bolt circle diameter, number and size of bolt holes, and the offset from centerline of disc to mounting surface.  There's no mention of those cute scallops between holes; you get those without asking.  It is probable that they are a good idea for minimizing thermal stresses, even if the customer doesn't care about weight or appearance.

Essex probably isn't the only place to get discs like this.  A quick search of the www will turn up others, since this two-piece rotor arrangement is common among performance enthusiasts.

The mounting flange ID and the bolt circle diameter were both selected to clear the output shaft flange on the differential.  The OD of the output shaft flange is 4-5/8", so the ID of this mounting flange was specified as 4-3/4".  The bolt circle was specified as 5-1/4", large enough for the heads of the bolts to also clear the output shaft flange.

Palm had the hubs made at a local machine shop:

Note that the relationship between the four larger bolt holes and the six smaller bolt holes is actually not correct on these parts.  As shown in the diagram in the book, one of the small holes should be centered between the closer pair of larger holes.  This would make it a bit easier to get a wrench on the nuts, but as Palm was using small 12-point aircraft nuts anyway, this error on the machinist's part didn't cause any serious interference.

These hubs were made from 1/2" steel plate.  To save a little money on metal and machining time, you could make them from 3/8" steel plate, but by the time you got through machining the inboard and outboard sides to make sure they are flat and true the part would be a little thinner than 3/8" -- so you would have to add a couple more shims C33440 to maintain the camber alignment of the rear wheels.

You could also make these hubs out of aluminum.  Palm's machine shop claimed it wouldn't make much difference in machining time or material cost.

If you're interested in saving weight or making parts look fancy, you could have scallops cut in the OD of this hub between the six bolt holes in much the same way that Essex cut scallops in the disc.

If you're not pressed for time on the project, you should get the discs from Essex first, then take the discs along with the OEM rotors you took off the car to the machine shop along with the drawing from the book.  The machinists, like anyone else, would like to be able to double-check their measurements against some real hardware to make sure they're drilling holes in the correct places.  However, if you are pressed for time, the drawing in the book contains all the information the machinist really needs to make the parts correctly, so you can have the hubs made while waiting on the discs from Essex.

Brian Schreurs found out that Essex will make the hubs as well -- and for a good price, too.  You can send them the diagram from the book along with the disc order form.  That way, they can make sure the discs mate properly with the hubs, and all you have to worry about is the mating to the differential output shafts.

You need bolts to hold these parts together.  Grade 8 is a good idea here; this is not a good place to scrimp on cheap bolts, not so much due to load but rather due to the temperatures experienced.  Now, really, you only need about 1" in length, because the mount flange on the disc is about 1/4" thick and the hub is also 1/4" thick, so a nut will neatly fit on a remaining 1/2".  However, the joint between the disc and hub has a shear load on it, and when bolts are loaded in shear in this manner it is good engineering practice to use bolts with a smooth shank at the joint (note that Jaguar followed this policy on the input and output flange bolts on the differential).  In this case, this means the bolts need to have a smooth shank longer than 1/4" but less than 1/2".  Palm found 1-1/2" bolts that had a 3/8" long shank -- perfect.  The fact that the threaded portion is 1/2" too long merely requires a little more time with a wrench.

Socket head bolts would work just as well.  Palm went with fine thread, but actually coarse thread would also work.

When assembled, the rotor looks like this:

On the inboard side, all hardware needs to be outside of a 4-5/8" circle, and nothing should protrude above the inboard friction surface.

Those are military aircraft style 12-point silver-plated nuts.  You're not likely to find nuts like those unless you have access to military hardware -- but any good quality all-metal self-locking nuts would work.  You don't want to use nyloc nuts because the rotor might get too hot and melt the plastic inserts.

On the outboard side, all hardware needs to be outside of the same 4-5/8" diameter circle.  The hub itself also needs to be inside of a 6" diameter circle, because anything beyond that will contact the inner boss for the handbrake caliper pivot pin on the main caliper.  Anything that sits taller than regular bolt heads on the outboard side of the hub will interfere with the clamshell around the inner U-joint.  To deal with the clamshell interference, you have several options:

1:  You can cut those long bolts off.  Would probably work better with conventional nuts than with those tall military style nuts.
2:  You can turn the bolts around, putting the nuts on the inboard side.  These bolts are too long for that side, but cutting them off would work fine since there's about 1/2" of space on the inboard side.
3:  You can back the clamshell a bit outward on the axle.
4:  The option Palm chose: relocate that clamshell to a cardboard box in the attic.

As you can see, the shim sits on the inboard side of the hub itself, and there's about 1/16" of clearance to both the lugs on the disc and the bolt heads.

There are other ways this rotor assembly could have been designed.  Another idea would have been to have Essex fabricate the outer disc with the mounting surface recessed 1/8" in the outboard face, which would result in the same rotor position without the need to machine the 1/8" recess in the hub.  And, obviously, you could opt for whatever number of bolts you wish; if you choose to make the hub out of aluminum, it might be a good idea to go with more bolts to spread the load out.

Regardless of how you go about installing a wider rotor -- buying a kit or assembling your own parts -- the additional width will require a bit of grinding on the inner end of the lower swingarms.  This is not really needed for clearance while together, but rather clearance for assembling.  To install the brake rotor, the lower swingarm needs to swing down to nearly vertical to get it out of the way.  When the wider rotor is installed, swinging the swingarm back up into place causes the center portion of the T on the swingarm to hit the outer edge of the disc.  The necessary grinding becomes obvious at that point.  Looking at the inner end of the swingarm, there is a hole into the tubular center portion of the swingarm.  You need to grind an area that's roughly the shape of an eyelid, more above the centerline of that hole than below, and on both sides of the hole.

Fortunately, the lower swingarm on a Jaguar is strong enough for an armored assault vehicle, so this minor grinding won't even begin to threaten its structural integrity.

Obviously, if you go with a disc thicker than 3/4", you will need to grind more on the swingarm.  Since there's not really any good reason to go with a disc thicker than 3/4", don't.

Since Palm made this mod and took these pictures, there have been several developments.

The disc-to-hub bolt circle diameter was revised from 5-1/4" to 5-3/8" to provide a bit more meat on the inner edge of the bolt holes on the disc.

Brian Schreurs modified the design to add two access holes in the hub similar to those found on OEM rotors for use with Dana differentials.  The holes provide access to the two bolts that hold the brake calipers to the Dana differential, and are invaluable for removing the OEM one-piece rotor -- but they're merely a nicety on the two-piece rotor, since you can separate the disc from the hub to gain access to the bolts.

Rather than locate the access holes where they are on the OEM Dana rotors, however, Schreurs rotated them 90° around the hub.  On the OEM rotor, the universal joint in the rear axle obstructs the access holes, so the axle must be unbolted first; this is no problem if you're rebuilding the entire rear suspension, but if your intention is merely to remove the brake calipers for service having to unbolt the axle is an unwelcome task.  With Schreurs' access hole locations, the calipers are readily removed from the Dana differential without additional work.

Schreurs then arranged a "group purchase" for himself and others of the disc, hub, and some small parts from Essex.  Reportedly Essex now maintains the designs for all these parts on file, and therefore you can now order the "kit" from them.

Schreurs tossed the various parts from Essex onto his flatbed scanner to provide this picture:

The disc and hub are apparent.  The item at top center is one of the spacers for widening the calipers; two req'd per caliper, four total.  The Essex versions are polygonal as opposed to the curved-sided versions handmade by Palm, but that won't make any functional difference.  The item at top left is the locking plate for the handbrake pivot bolts.

You can read more about the Schreurs-designed Essex kit at http://neptune.spacebears.com/cars/engr/jagrotor.html .

Then came yet another revision.  It had been noted that Brembo aftermarket rotors all came with access holes, whether intended for use with Salisbury or Dana differentials.  Initially it was presumed that they were merely standardizing and providing the same rotor for all until it was found that the access holes on their Salisbury rotors would not provide suitable access to the brake caliper bolts on a Dana.  The access holes weren't located correctly.

Brembo had actually incorporated an ingenious innovation.  The access holes in their rotors intended for use with Salisbury differentials provide access to the five bolts that hold the output shaft bearing assembly in the differential itself.  Note that the brake calipers are also attached to this output shaft bearing assembly.  Hence, when (!) the output seals start leaking or the bearings need service, you can insert a socket through the rotor, remove the five bolts, and remove the entire output shaft assembly with the brake rotor and calipers attached for servicing on the bench.

Knowing a good idea when he sees one, Palm then revised the rotor design in the Book to not only reflect Schreurs' improvements but also to add two more access holes similar to those found on Brembo aftermarket rotors intended for use on Salisbury differentials.  To facilitate having all four access holes in one hub design, the disc-to-hub bolt circle was revised from six to eight bolts.

Note that, if you intend to actually use these access holes, the scallops in the mounting flange of the disc become a necessity.  If the flange is not scalloped, the disc will obstruct the access holes.

Don't despair at all these variations.  All of these arrangements will work on any car, regardless of whether the final drive unit is a Dana or a Salisbury.  With a two-piece rotor, the parts of the rotor can be separated to provide access.  The addition of the access holes simply makes things a little easier.