Suspension Walkaround Supplement: Porsche Center-Lock Wheels

The key to removing and installing Porsche center lock wheels by yourself boils down to the right tools. The torque spec is 600 Nm (444 lb-ft +/- 22 lb-ft), a figure that's well beyond the reach of the 1/2-inch drive torque wrenches that most DIYers and track-day participants have in their toolboxes. Also, the special socket that comes with the car is 3/4-inch drive, not 1/2-inch drive.

There are two choices. You could buy a humungous 3/4-inch drive torque wrench. Typical examples have a capacity of 600 lb-ft and approach 4 feet in length. You'd also need a long-handle 3/4-inch drive breaker bar to remove the wheel nuts because torque wrenches are for finish work; they should never be used to break things loose.

Or you could buy a 4-to-1 torque multiplier like this one for a couple hundred bucks or so. This tool does just what the name implies. It is a low-range transfer case, so to speak, for the 1/2-inch drive ratchets and breaker bars you already own. Set your current torque wrench to 111 lb-ft and the planetary gearset inside the multiplier bulks that up to 444 lb-ft on the output side. 

Well, not quite. As with any transmission, there are frictional "drivetrain" losses and here the magnitude amounts to 10-15 percent. To arrive at 444 lb-ft at the nut you need to target 490-510 lb-ft and then set your torque wrench to one-quarter of that, 125 lb-ft.

Porsche's tolerance of plus or minus 22 lb-ft is a crucial thing to know here because it puts the fear of over- and under-tightening into perspective. Shoot for the high end of the tolerance band, maybe even slightly beyond, and the frictional losses in the multiplier's gearset will reel the actual nut torque back in to a point somewhere between the middle and the low end of the spec.

Torque multipliers do one more very useful thing: they accept your 1/2-inch drive tools on the input side and 3/4-inch drive sockets on the output side. There's no need to buy an adaptor because that function is built in.

The special 3/4-inch drive socket needed for the RS Spyder-style center-lock wheels comes with the car, though it's fairly well hidden behind a small access panel in the "frunk" (front trunk) of the car.

No handle is supplied, just the socket. Porsche is assuming the car will be towed somewhere in the event a new tire is damaged beyond the capacity of the supplied sealant kit, and the socket is provided in case that "somewhere" doesn't have one of their own. Even if you never plan to DIY your center-lock wheels yourself, make sure you find and locate the socket in the frunk in case you ever need tire service away from a Porsche specialist.

It's not enough for the socket to mesh with the outside of the nut; that protrusion with the o-ring is key -- literally. As the socket is engaged this bit depresses and releases the central locking mechanism that otherwise prevents the nut from unscrewing when you don't want it to.

Which brings us to step one of the removal process...

The center cap has to come off. It conceals the locking mechanism, and the hidden o-ring that holds the cap in place also keeps the workings free of grime. Here I've wrapped tape around the blade of a narrow screwdriver to protect the finish as I pry the cap and its o-ring loose from its catch.

Incidentally, earlier versions of the center-lock system had a torque spec of 500 Nm printed right next to the words "Anziehen/Tighten", but a recent recall covering all such systems produced since 2007 has resulted in a tighter torque spec of 600 Nm. No accidents were reported, but excessive hub wear was observed in a couple instances on the racetrack.

Under the terms of the recall the old labeled nuts are replaced with new nuts that lack a printed torque spec. There's also a requirement for a light grease that ensures the torque wrench is sensing actual nut torque instead of dry friction. 

Once the cap is off the socket can be seated. You'll notice I've allowed the handle of the torque multiplier to rest on the ground. A torque multiplier can't do any multiplication unless it's stationary, and the easiest way to accomplish that is to position it so that the handle is driven into a stationary object -- such as the ground -- by its own natural tendency to move in the opposite direction of the input.

My right hand is applying inward pressure to make sure the socket and its plunger remain square and fully seated. Meanwhile, my left hand is turning the breaking bar in the normal "lefty-loosey" direction. I'm getting 4-to-1 torque multiplication, but that also means I'm going to be turning the handle 4 times as far for a given amount of rotation at the nut.

Old-style "knock-off" racing wheels used to have left hand threads on one side of the car and right hand threads on the other to allow the inertia of acceleration to keep the wheels tight. But that's not necessary with a positive locking mechanism like this, so all Porsche 911 center lock wheels tighten the same way -- clockwise.

With only one hand available for pushing down I found it necessary to slide my floor jack handle over the breaking bar for extra leverage.

This setup was generating enough torque to make the front tire turn and skid against the concrete even though the full weight of the car was still on the ground. This slow motion burnout-by-hand continued until Kurt climbed inside and stepped on the brake. Only then did the nut break free.

And then it was time to jack up the car and loosen it the rest of the way. 

Here I'm holding the center-lock release plunger in with my thumb as I unscrew the last couple of threads by hand. This is where things start to get a little greasy.

Several things are worth noting here. The splines inside the nut are part of the central locking mechanism. The matching splines we see at poking out of the wheel are actually a spring-loaded part of the hub. That's the part I was pushing with my thumb to disengage the lock, the part that's pushed in by the socket's prominent protrusion. 

The screw threads that do the actual work of holding the wheel in place can be seen encircling the spring-loaded lock mechanism.

Looking back at the nut, a prominent tapered seat distributes the clamping force evenly, and that visible split is there so it can contract and maintain full contact as the nut gets torqued. Its surface is lightly greased, as is the mating face of the wheel itself.

As mentioned before, the grease is a necessary element that allows these pieces to move relative to each other as the torque reaches 600 Nm (444 lb-ft).

Now that the wheel is off we can easily see the 15 index holes that can mate to the drive pins -- the hidden secret that makes it easy to line things up quickly.

Of course only five of them need to line up with the drive pins that transmit the engine's thundering torque. Do the math and you'll see they're only 24 degrees apart (instead of the usual 72) but since you can go left or right to whichever is closest you'll never be more than 12 degrees from the bullseye.

If you take the taper on the nose of the pins into account -- and you should, because that's why it's there -- you can cut that in half again, to something like six degrees of separation between drive pin and wheel hole.

Yes, I'm going to say it: with these you're never more than a Kevin Bacon away from getting your wheels lined up properly.

New grease isn't necessary in this case because there's still an even coating. The wheels on this press loaner have probably been inspected and serviced quite recently. Porsche center-lock owners that plan to do their own tire swapping should make certain they keep a supply of the special sauce handy in case it looks thin or feels gritty.

Now it can go back on. Installation is, as they say, the reverse of removal -- mostly.

The all-important torque operation can begin once the wheel has been hung on the pins and the nut has been spun on by hand with the locking pin depressed.

At this point I've removed my breaker bar and plugged my torque wrench into the torque multiplier, which is now oriented to the left for this "righty-tighty" phase. Unlike other cars, Porsche is very specific about tightening center-lock wheels while the wheel and tire are still dangling in the air, completely unloaded. Kurt must climb back in to stand on the brake once more.

Here the car is still up on jack stands as I wait for the torque wrench to click at a setting of 125 lb-ft which, after multiplying by 4 and deducting 10 to 15 percent, will land us in the official target zone of 422 to 466 lb-ft.

But we're not done yet. At this point Porsche wants us to LOOSEN the nut once again by 1/6th of a turn then repeat the torque process a second time.

There's a post and video on one of the Porsche forums that suggests it takes but a single finger to tighten the nut with a torque multiplier like ours. Don't believe it. He's merely making the torque wrench click a second time after the nut has already been fully tightened. Tightening the nut to the point where it clicks the first time still takes enough effort to generate 125 lb-ft of torque on the input side of the torque multiplier. Normal wheels are typically torqued to 85 or 90 lb-ft, so even with our fancy tool this job still requires 40 to 45 percent more muscle than usual. 

You can tell if the locking mechanism is properly reengaged by looking at the splines. You want the plunger to be flush with pointy end of the teeth on the hub side, like it is here. It's not fully engaged if the straight ridges of the splines are visible.

Among other things, the torque-and-retorque process is meant to help the splines engage on their own. This doesn't always work out, but all is not lost. The center lock plunger mechanism has a little leeway built into it, so a little poking and cajoling will eventually get the splines lined up so the lock snaps home.  

This is probably the most finicky part of the entire process, but it's also the most crucial. Failure to get the center lock mechanism to engage can be disastrous.

With the center lock engaged, we're done. Time to reinstall the Porsche crest and lower the car back onto the ground.