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

911 Porsche World - - This Month -

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

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