Jaguar XJS Racing X300 powerlock diff swap

X300 DIFF PROJECT

The snows of last winter proved that with a little nerve and a steady right foot, an X300 Jaguar with 50kg of your old dumbbells in the boot is quite a capable car in the snow. Conserve your momentum and plan it out, and you’ll be going offline to pass slithering hatchbacks whose pilots are too dumb to realise that a snowy hill might need a run at it.

But, lose your momentum, and the lack of grip provided by an open diff and old Pirelli 6000s turned the car into a large maroon skip dumped in a snowy car park, it arrived in snowy Barnsley one morning with vigour, and 8 hours later that same vigour had to be applied to the shovel to get back out of the parking space, one rear wheel uselessly polishing its own tyre marks.

With lift-off oversteer apparent on the motorway, and a rear wishbone that quite plainly had front to rear play, a clunk coming from the rear end during low speed manoeuvres and wallowing snap oversteer when enthusiastically piloted in the wet, it was time for an upgrade.

There’s no powerlock diff for an X300, so I’m told, this was the car that Ford made everything cheaper, and certainly having now broken a plethora of XJS and a pair of 300s, the difference in concept is clear. So, no limited slip. Which was a mistake, but one of Jaguar's mistakes that we can fix.

XJ40s did get a powerlock on specific models, and Ebay located me an XJ12 lsd. A trip in the van brought me to a cowshed full of assorted carnival folk headed by a man I used to call Van Helsing because of his distinctive hat and coat ensemble. You might know him as Ralph Hosier, former XJS racer and a man with a death wish – when we arrived the XJ12’s front and rear axles, and indeed gearbox, were loosely arranged beneath a Triumph GT6 bodyshell. Yes, he really was planning to. Brave boy. Deranged even.

The subframe had been at about ten fathoms for about ten years, but there is little that a hint of the hot spanner cannot loosen, and it was not long before the component parts to this thing were scattered about the Kutuka garage amidst their own rust.

The pins that hold the massive wishbones on are, inevitably, stuck. Heat and a big hammer, whilst applying a Bear on the end of a long bar finally get them rotating, but the bushes that tear out are beyond redemption.

The hushed casket was unsealed, and the spinning oily parts were mercifully intact. A new oil seal was in order, but that aside only a good clean was required. The output shafts, predictably, had one duff bolt each, and though you can buy the bolts, they can’t be removed without breaking apart this sub assembly, meaning new collapsible spacers etc. The XJS isn’t the same, there is a difference here, and we note that even the bolts aren’t quite the same either, so we can’t steal XJS parts to repair this.

Seems a shame to take apart a correctly-assembled piece just to swap a bolt, indeed our ethos usually is that if something works, it’s correctly shimmed etc, and it’s not broken, don’t take it to bits. Fred the File creates a small recess in the offending flange. Only small, maybe 3mm taken off, but it allows the bolts to pass, and without interfering with bearings, seals etc. Simples.

The XJ40 and X300 subframes do differ in the details, but generally speaking most of the parts are a straight swap. That said, I happen to have an X300 subframe lying about with a rear anti roll bar on, and clearly we need to gain that too.

With two subframes in bits, Bear embarks upon a blasting exercise, having finally coaxed an old shotblast gun into life, and using an old fridge as his cabinet. It’s not pretty, but it works. The pins are spun in the lathe with emery cloth to clean them up, and then anything that can be plated is sent for a coating of bright zinc.

The subframe itself is cleaned up and painted, as is the diff casing. As this is only a road car, we settle for Hammerite. I hate the stuff, I think it’s utterly useless, but for sheer ease and speed, and for the simple fact that it’s got to be better than putting bare rusted steel back together, it gets a chance to prove me wrong.

What we then reassemble is, in our opinion then, the best bits of XJ12 and X300. What it means in practice is X300 wishbones and anti roll bar strapped to a powerlock diff.

New bushes pose a problem. Those long pins are rather crucial. You can buy the front ones, but not the rear. We reason that the rear ones are the same internal and external diameter, and the same width. To my little brain that makes them the same bush, surely? Why the various websites that sell these things fail to mention that I don’t know.

The Bear lays out the whole jigsaw puzzle, every bolt, spacer and nut across the garage floor. One by one he then substitutes a cleaned and plated version. It is by far the most careful and thorough I’ve ever seen him be.

Reassembly is fast, and deceptively simple. Much easier than an XJS, you only need one pair of hands. Wishbones first, brace bars next, shocks will be dead last. The shocks are from another X300 subframe that came in 2 years ago, and the standard test, the classic leaning on them on the garage floor, showed that they appear to function. Everything gets a tickle with the paint brush. Pointless, but why, really, not? Odds are I’ll be next one to take this apart, I might as well give myself half a chance.

Not fitted at this point are drive shafts. We plan to take them off the existing subframe, complete with shims and spacers. Although I don’t think she’d mind gaining a degree of negative camber…

The Jag then comes in to lose the old subframe. We stole a motorbike jack off Mr David earlier in the year, and it’s perfect for this job. With the car jacked right up in the sky, and forty-six axle stands underneath it (Philip Comer’s car having fallen off two stands earlier in the season and almost killed us both) the motorbike jack allows for a controlled and stable removal from the car. Much easier than my usual trolley jack and piece of timber balancing act.

Disconnect handbrake and prop, and remove the brake callipers from the subframe, we can leave them on the car and thus avoid re-bleeding the brake system. I’m in two minds about fitting the car with the V8 rear brakes though, I have a set we reconditioned, and X300 items fit our race cars…. But at that this thing is ready for out.

This X300 is unusual, in that it’s a bit rusty underneath. We’ve broken two previously, and they amazed us with their lack of corrosion. This one, annoyingly, seems to have spent some of its life at the coast, because it is not at all happy about giving up some of its nuts and bolts. So difficult does it prove that we disconnect the shocks from the subframe using a spring compressor, and leave the shockers behind. It takes half a day to get this thing out, it’s a real pain.

The exhaust won’t come apart at all, and that therefore comes out with the subframe in one giant lump. There should be a button we push in the garage to play some sort of fanfare for moments like this as the subframe is dragged away to its grave.

We want its driveshafts and uprights/hubs, complete with handbrake cables. They prove to be incredibly hard to remove. I know that we are spoiled usually, working on the race cars that come apart when we whistle, everything clean and greased and dismantled every ten minutes, but I’ve never encountered hub nuts like these. It takes our longest torque wrench, and it’s a big bugger, make no mistake, and a full 95kg Harrison swinging on it to move the nut, and it does this all the way to the end of the thread. The nut comes off so cleanly that inspecting the shaft confirms that it left its sacrificial second thread behind.

The hubs come off the shafts only with a great deal of persuasion. By which I mean WD40 and a sledgehammer. Clearly it had never been apart before.

The old shocks can now be removed from the car, there is now access to swing a hammer, and the six-sided socket can now be beaten on to the rusty nuts, filling your face with fifteen years of road crap and rust. But she gives them up, and peering up at the shiny steel above them that’s when I remember where the fuel tank is on this car. Lucky we didn’t try the hot spanner on these isn’t it…

Whilst we’re in here, Bear swaps the fuel filter. Well, why not?

Time for the exhaust, which was very pious. There are 4 holes in the downpipe alone, all of which are easily repaired with the hot metal glue gun, also known as the Mig. The intermediate silencers showed a large crack at the outlet end, which also accepts a gob of the stuff, and the rear silencers have holes in so large that you could put your head in them, which is why there is a hole burned in the rear bumper.

The system won’t come apart, it simply refuses. The system off the last car we broke has intact silencers but is broken somewhere at the downpipe end, and we can’t get this apart either. A rather crude solution presents itself, and the bandsaw chops both the old silencers off both systems. The good ones are welded to the best of the two systems, having vaguely marked their orientation. The welding is not my best work. Wrestling a complete X300 system is not simple, you need 8 hands.

This exhaust is the first thing to be refitted. It strikes me that if we remove the subframe exhaust mounts there is nothing to stop this being refitted, and the thing is therefore slung in place temporarily with tie wraps, wrestled, levered and beaten back onto the refitted downpipe and manifolds, until we have what looks like a gas-tight join. Which would be the first time in about a year.

The original rubbers are used, but supplemented with stainless steel tie wraps in places that we consider the system needs additional support. That gearbox crossmember is just begging to be used.

Bear has fitted the driveshafts, having selected slightly thinner spacers to give the car a hint more negative camber. It’s almost nothing, but it’s there.

He fills the new diff with Millers lsd oil, and the new shocks are fitted to the subframe, much easier off the car with twin spring compressors and plenty of access. Not as easy as XJ40 though, I do struggle to understand why they changed the design at all, I can’t see where the money saving is doing it this way.

Suddenly the subframe is ready. Heaved onto this genius jack and wheeled under the car, refitting is amazingly simple. In the background the tv is chattering away about the McLaren technology centre and the cars they are making, with sticky doormats to keep the place clean. As the Bear and I lie in the accumulated filth of this project, it does strike me that perhaps we’re not going to be in direct competition with them anytime soon.

It is as simple as pushing the subframe under and jacking it up, it goes swiftly into place after one aborted attempt as we realised the diagonals had ended up the wrong side of the exhaust. With it lined up and in place, the prop waggled into line, the ten bolts that hold this on are quickly buzzed into place, and the damned thing is in.

Prop bolts in, handbrake reconnected and adjusted, and brake callipers refitted. Shocks are allowed new bolts, because all the studs came out with the nuts, and as far as I’m concerned a stud that is fused to a nut is a bolt.

Fitting those exhaust mounts is not simple, it requires strength and profanity, but it goes together. Job, we think, done. Little nervous about the ABS sensors that we reassembled, I can only hope they still work, but screw it, what’s the worst that can happen? I don’t know. What I can’t work out even now is how the hell the speedo works. We’ve now had the engine, gearbox, prop and rear axle out, and not yet seen the sender. How does this witchcraft work?