HOW TO: SERVICING PORSCHE’S CENTRE LOCKWHEELS AND PCCB BRAKES
Centre-lock wheels look great, and are arguably the icing on the cake for any fast-road or certainly trackday or full-on competition Porsche, but they come with a few minor technical challenges that it’s as well to be aware of before you attempt to remove
Not just any old wheels and not just any old brakes
There is a certain irony in Porsche’s espousal of centrelock wheels as an extra-cost option for its current higherperformance sports cars. (Or perhaps that should be Centerlock; that’s how it appears on the hardware itself, and also in most of the company’s own literature.) Notwithstanding its origins in the days of so-called wire wheels, it has become a system intended primarily for motorsport, in which rapid tyre changes are essential – here facilitated by having each rim secured to its hub by just one large ‘nut’, instead of four or five smaller ones. In a road car, though, it renders the process both time-consuming and technically fairly challenging – certainly for the average DIY owner – and in practice almost impossible at the side of the highway.
That is arguably not a major problem – few modern vehicles carry a spare, and even if they do it is a brave, if not foolhardy, man or woman who tackles the task in today’s busy traffic. But for Porsche owners who prefer to do at least some of their own basic maintenance, and certainly to undertake the routine brake, steering and suspension inspections that are essential to any car’s long-term health and safety, it makes it, well, all a bit of a faff. (There aren’t going to be many breakdown outfits capable of changing a Centerlock at the side of the road, either. And getting a low-slung Porsche onto a recovery truck with a flat tyre is going to be an interesting exercise. Discuss…)
As ever, though, knowledge is power. Certainly, you will need one or two items of fairly specialised (but still relatively easily obtained or even improvised) tooling, in order to be able safely to remove and refit your Porsche’s Centerlocks. And it is not difficult to mess it up, big-time. But the fact is that any recent Porsche road car that was factory-fitted with the devices should also come with the special socket ultimately required to undo the equally special wheel retainer. (Which is basically either just a single large nut or bolt, depending on how you define either term.) And beyond that it is really just a question of being very
careful, and using plenty of good, old-fashioned common sense.
Here, then, courtesy of independent specialist Precision Porsche in Uckfield, East Sussex, and not least the vehicle’s owner, we have shown the process of removing and refitting Centerlock wheels from start to finish, on a 991-model 911 Turbo ‘S’ – surely the flagship of the current Porsche sports-car range. In fact, the exercise began as a story examining the intricacies of a major service – and that’s something we shall be returning to in a month or so; we believe you will be as fascinated as we were to see the entire rear end of the car removed for access to the spark plugs – but somehow those monumental 20-inch wheels, and the magnificent ceramic brakes inside them, seemed to be crying out for coverage first. Centre-lock wheels of the same basic type shown here were first fitted to a road-going Porsche – the Carrera GT supercar – in 2003. A broadly similar system (although the parts are not interchangeable) later became available as an extra-cost option on the 997, and then on the 991 – but its significant cost, currently around £2500 here in the UK, naturally tends to limit it to the higher-performance models, and in which case it is often seen, as here, with the similarly exotic PCCB ceramic brakes. Centerlocks can be fitted in conjunction with ordinary steel brake discs, however.
Prior to this, centre-lock wheels were a feature of many competition Porsches. The principle was the same as in the later road cars, but the components are significantly different in detail. Earlier still, the 356 came with the option of so-called knock-off wheels, the term a reference to loosening or tightening the central retainer by hitting either its projecting ‘ears’ or, where it has external flats, a stubby wrench placed over it. Crucially, knock-off wheels are driven by
means of internal splines which engage with matching splines on the hub. Centerlocks are driven by the clamping effect of the central retainer’s conical face against the matching face on the wheel – and not, as you might expect, solely by the five projecting pegs on each hub.
The key – no pun intended – to removing and replacing Centerlocks is the large socket, for want of a better term, that engages with the castellation on the outer circumference of the central retainer. Any car that was factory-fitted with Centerlocks should have this stored in a cubbyhole inside the luggage compartment – which is worth checking if you are buying such a vehicle second-hand. It’s the kind of thing some previous owners might keep as a trophy. Note, however, that – within model ranges, anyway – the sockets are interchangeable from one car to another. The castellation is not an anti-theft device, as such (although it does have some practical value in that context), but a means of transmitting the required torque.
In early 997-model Centerlocks the retainer is tightened to 500Nm, but in later versions, and in the 991 as shown here, that figure has been increased to no less than 600Nm. No ordinary wheelbrace is ever going to do the job, then. In 2010 the 500Nm hardware became the subject of a recall, for replacement with the 600Nm components, and if your car has the older parts – identified by their easily visible ‘500Nm’ markings – you must contact a Porsche Centre to have them upgraded. This should be FOC against a valid Vehicle Identification Number, or VIN, that has not previously been used for the purpose.
In order to apply this torque you will need, at the very least, a roughly metrelong breaker bar, and with a 3/4-inchsquare drive peg to fit that special Porsche socket. A short 3/4-inch-drive extension, as well, to provide sufficient working clearance between the breaker bar and the bodywork. (And/or some kind of half- to 3/4-inch adaptor, depending on your breaker bar.) Over the end of the bar you will need to place another approximately metre-long piece of strong tube (scaffold pole is ideal, if rather heavy) to increase your mechanical advantage. Alternatively, and since you will need a 600Nm-plus torque wrench to refit the retainer, you could just buy one before you start. Unusually, torque wrenches of this nature and range are often designed to be used for undoing fixings as well as tightening them – but check that before you buy.
Another alternative, at least as far as refitting is concerned, is to use, say, a 100–200Nm torque wrench, in conjunction with what is known as a torque multiplier. This fits between the torque wrench and the item to be tightened, and by means of its internal gearing steps up the torque between its input and output shafts. The device shown below has a gear ratio of 1:4, and a torque ratio of 1:3.3. This means that for 600Nm you need to set your wrench to 600 divided by 3.3, or 182Nm. The only drawback is that you need to lock the multiplier against something solid. In this context the ground will suffice (with the end of the multiplier on a block of wood), but it’s a bit of a compromise; a fullstrength torque wrench is far better.
There is yet a third alternative – although this is purely a get-you-home (or to your nearest Porsche Centre) routine. Tighten the retainer to 100Nm – a feat within the capabilities of any Diy-style torque wrench – and, using a pencil, lightly scribe a line on the centre of the wheel, next to the ‘100Nm’ mark. Now, using your breaker bar and tube, tighten the retainer until the ‘STOP’ mark comes round next to the line. This will torque the nut sufficiently for the car to be driven – sedately – until you can have the job done properly. All of the retainers have a right-hand thread, which means rotating them anticlockwise to undo them, and clockwise to tighten them. They are marked thus in both German ( Lösen and Anziehen) and English.
To remove a wheel, then, first prise out the crested centre cap, preferably with a purpose-made plastic trim tool, or a small, flat screwdriver blade wrapped in insulating tape. Check that the cap’s ‘O’-ring is undamaged; if not, it could allow dirt and moisture to get into the area behind it. Lightly grease the seal with Vaseline or similar for easy replacement, and store the cap somewhere safe – and not face down on the garage floor, where you will most likely kneel or tread on it.
Behind the cap you will see a bright-plated central spigot with an octagonal recess in the middle. Observe the spigot’s position. When it, and thus the wheel retainer itself, is safely locked, you should see no more than a couple of millimetres of the splines inside the centre of the retainer. To unlock it, so that you can unscrew the retainer, push it in, against spring pressure, until it clicks and stays put via a sort of toggle action. At that stage you should see a centimetre or so of the splines. (Conversely, make sure, when you come to refit the wheel and its retainer, that the spigot is back in its correct locked position before you add the crested centre cap.)
The full weight of the car needs to be on the wheels before you attempt to loosen any of the retainers – and, given the high torque involved, it is advisable to have an assistant with his or her foot on the brake pedal. Best not to rely on the transmission and/or handbrake alone, in other words, although those will suffice in an emergency. Make sure the front wheels are pointing dead ahead, too, or you risk damaging the bodywork with your breaker bar. At this stage, however, all you are seeking to do is ‘crack’ off the retainer, not to unscrew it; a third- to a halfturn should do it. A smear of Vaseline on the tool’s own central ‘O’-ring will help it slide home more easily, and its built-in central spigot will push in and release the locking device. Now, using a low-slung trolley jack – and only the car’s under-floor bracketry provided for the purpose – raise the relevant corner and, once the wheel is off the ground, fully unscrew the retainer.
At this point, STOP. It is vital to draw the wheel off the hub at a right-angle to the latter, or very nearly so, in order to avoid damaging either the rim or, more seriously, the brake disc – and this is a particular concern with ceramic discs, which are highly susceptible to the edges chipping. One way round this – apart from experience and good upper-body
strength, perhaps – is to make up a small wooden trolley with a castor at each corner, on to which you can slide the wheel for support. Better still, use the special Porsche tool, or an after-market equivalent: a tube that screws into the central part of the hub and serves as a mandrel, supporting the wheel until it is clear of the disc. Precision Porsche technician Gron Owen does it by positioning the lift such that the tyre is just clear of the floor, thus using the latter alone as his support mechanism, but that’s probably not such a good idea where only one corner or side of the car is raised, and thus the hub is inclined slightly above the horizontal.
OK, so you’ve got the wheel off. Now what? Closely examine all of the components for signs of wear or damage, both of which are likely to be the result of driving the car with the retainer(s) insufficiently tightened. On the conical mating faces of both the retainer and the wheel you need to see smooth, unbroken witness marks – not unlike what you would expect when lapping in a valve in a cylinder head – and likewise on the large-diameter central spigot projecting from the hub/disc. Those small-diameter pegs are important, as well, and if any of those are worn then probably so will be the equivalent holes in the back of the wheel. You could, in theory, simply fit new pegs and rotate the wheel through one or two positions, but perhaps unsurprisingly Porsche advises buying and fitting a brand-new rim.
Refitting the wheel, assuming that all is well, is by and large a reversal of this procedure – with again the risk of damage to the disc if you don’t slide the former on dead straight, or if it should slip from your grasp. Wipe clean and lightly degrease all of the various contact surfaces, and then apply a fresh layer of the special Optimol TA paste recommended by – and available from – Porsche. So that’s the conical face of the wheel and the retainer, the threaded area of the retainer, and not least the back of the wheel, where it contacts the hub/disc assembly. You don’t need any on the locating pegs. Check, too, that the large-diameter ‘O’-ring around the outside of the wheel retainer is in good condition, and give it a thin smear of Vaseline to prevent it possibly snagging and tearing as you wind it home.
The tightening procedure is in broad terms simple, but in practice does require both thought and care. The first step, and crucially with the wheel still clear of the ground, is to tighten the retainer, using either your fullstrength torque wrench, or the smaller item and your torque multiplier, to 600Nm. Now loosen the retainer by no more than one-sixth of a turn, and then retighten it to 600Nm. At this point the car can be lowered to the ground. Finally, make sure that the locking mechanism is engaged, either by simply pushing against the spring-loaded centre and then releasing it or, if necessary, by simultaneously pushing and twisting with, say, the end of a half-inch-drive socket extension. You should hear a distinct click as the device extends and locks into position. Refit the centre cap and the job is done. PW