and install vented rotors that looks like this:

The rim of the original solid rotor is 1/2" thick.  The rim of the vented rotor is 3/4" thick, and the rotor assembly is configured so that the entire 1/4" of additional thickness is added to the outboard side of the original configuration.  The inboard friction surface remains in exactly the same location as before.  The OEM calipers are separated and reassembled with 1/4" spacers between the halves to accommodate the thicker rotors.  Since the calipers are mounted by the inner half of each caliper, they are thus positioned correctly for the thicker rotor with no change in caliper mounting.

The example in the photo is a "two-piece rotor", consisting of a custom hub and a generic disc bolted together.  The outer disc is a wear item but the inner hub will last the life of the car.  When buying replacement outer discs you can return to the original supplier of the hardware for your upgrade, but you don't have to; several places offer such discs.  This is discussed in greater detail on the two-piece rotor page.

Note that the photos on this page are from Kirby Palm's own upgrade, but the design has been revised in numerous minor ways in the meantime.  If you follow the plans on this web site to the letter, your finished brakes won't look exactly like these, but largely similar.

Upgrading the inboard rear brakes on a Jaguar E-type/XJ6/XJ12/XJ-S is a popular (and highly recommended!) modification, and three reasons are generally given for making the mod:

1. Improved brake cooling.  This generally isn't of much importance unless the car will be raced or driven down mountain roads, since the solid rotors seem to perform well enough for most street applications.

2. Improved cooling of the output shaft seals on the differential.  The inboard brakes surround the output shafts, and the brake rotors are bolted to them.  Hence, the seals may get heated by radiant heat coming off the brake rotor, or they may get cooked by heat from the rotor soaking through the output shaft itself.  Either way, vented brake rotors address the problem; they cool off faster after hard braking, so less heat gets to the seals either by radiation or conduction.  Also, they should increase the airflow in the area by sucking air from around the differential housing and blowing it out the bottom of the car, so the oil and metal inside the differential should run a little cooler.  All goodness in theory, and there is some evidence that the differential output shaft seals -- notorious leakers -- do last longer on cars fitted with vented brakes.

3. Avoiding oily brakes.  When (not if!) the output shaft seal leaks, the oil runs down the inboard side of the solid rotor.  Since the inboard side friction surface and pads are thus well lubricated, they provide almost no braking action, and all braking from the rear has to come from the outboard side of each rotor.  Typically, it is evident that this is going on because the inboard pads are not wearing at the rate the outboard pads are wearing.  Once vented rotors are installed, oil leaking from the output shaft seals goes through the cooling passages instead of running down the inboard side, and therefore gets out the bottom of the car without lubricating the brake rotor friction surfaces on its way.  It also might help due to the airflow caused by the vented rotors; in pulling air from around the differential housing and blowing it out the bottom of the car, it may tend to draw airborne oil droplets into taking the same route.  Owners having performed this upgrade have reported the brakes seem to work better when cold (there would be no change if oil on the friction surfaces wasn't an issue), and there is evidence that cars with vented rotors do not demonstrate the unequal brake pad wear that is so common on cars with solid rotors.

In general, there are two ways to upgrade your Jaguar inboard brakes to vented rotors:  Buy a kit or fabricate the stuff you'll need yourself.  Since this is a common upgrade and there are quite a few Jaguars out there that have the same basic rear brake arrangement, there are several outfits that offer such kits.  Note:  If you can't find it listed under XJ-S, try looking in a supplier's E-Type section; the rear brakes are the same, but some of these suppliers forget that owners of later models might also appreciate the upgrade.

XK's Unlimited offers a kit, catalog number 17-1214, that utilizes a two-piece rotor similar to the photos above.

Judging from the photograph in their online catalog, the kit offered by Terry's Jaguar Parts, code TJP190, uses one-piece rotors.  This means that when the rotor is worn an entire new one must be purchased -- and such rotors are not likely to be easy to find elsewhere so you will probably need to go back to Terry's for replacements.  Of course, you could opt for installing two-piece rotors then.

BG Developments in the UK reportedly offers a kit; their rotor is part number BGVD035-13.

Theoretically, you could actually get the parts necessary for this upgrade from Jaguar; the TWR XJ-S came off the showroom floor with two-piece vented rotors.  Be forewarned, however, that TWR owners have some trouble getting parts to maintain their brakes from Jaguar, and sometimes opt for some of the ideas presented here for finding parts instead!

Kirby Palm did this upgrade by fabricating some of the hardware and having the rest made for him.  Pictures and guidelines are presented here in two pages; one page describes the two-piece rotor itself, including the outer disc, the inner hub, and the bolts to hold it together.  The other page describes the small parts needed for the job, most of which are involved in spacing the caliper wider.

Here's what Palm's brakes looked like when done:

The upgrade described herein reuses the original calipers.  This works fine -- but most people don't even get involved in their rear brakes until there is a problem, and that problem as often as not is that the calipers need rebuilding or replacement.  Hence, you might consider installing an all-new high-performance brake system such as those made by AP or Wilwood rather than simply upgrading the existing brakes to vented rotors.  The cost of renovating the original calipers plus the parts needed to widen them would go a ways toward paying for a really nice set of aftermarket high-performance brakes.