Ok, so I have a 72 914 with 5 lug all around, redrilled rear 914-4 hubs, 914-4 calipers.

I am having a problem setting my rear brake caliper venting clearance. The brake disc wobbles slightly as I rotate it, meaning that if I set the clearance to 0.004" in one spot, it does not maintain that clearance all the way around as I rotate the disc. In some spots there is too much clearance, in others it goes to 0.000" and the pad drags against the rotor. It is more pronounced on one side of the car but both sides do this to some extent. What gives?

I guess I should add that I just did the following things which may have resulted in this situation:

1. replaced pads, rotors
2. rebuilt calipers
3. replaced rear wheel bearings, meaning I had to take out the stubs axles and hubs and pull them back in. the stubs aren't torqued to 300 ft lb yet but they are probably at 150+ (the most I could easily put on them with the car in the air)

Is the runout of the rotor due to the fact that my stub axle isn't torqued? Did I install the hub slightly crooked somehow, or the rear bearing crooked, or ??? Or is this normal? I'm not really sure what to do about it...

Oh there is one more possibility - the new rotor was a tight fit on my redrilled hubs, it may not be seated correctly? I torqued down all 5 lugnuts directly on the rotor to seat it, you would think that would work but who knows? I had to ditch the little screws since they do not fit on my hubs.

Help!


PS: Porsche specs disc runout at 0.008". If venting clearance is set to 0.004" (according to Capn and DD) isn't that in conflict with the runout spec?

Hey

I would check fist and formost - disc and hub. The bearing and everything back would cause centering problems, not a wobble.

You need to get a runout dial guage, and check the hub with everything bolted less disc and brakes.
Then pop on the disc ond bolt it down and try again.

I suspect its the disc to hub seating, any interferance will turn things bad quick.

QUOTE

the new rotor was a tight fit on my redrilled hubs, it may not be seated correctly?

you know I hate to say it, but rear rotor always seem to be wobley, you put it together, run it and things seem to place after a hot cold cycle. But you got to get the other stuff checked, and make sure about the machining.

Good luck with it
later

I agree that the hub to disc fit is critical, and that is where I would look first. Even a tiny bit of dirt or a burr is too much. If the disc is hanging up on a stud, then you will have to clearance the hole.

One would hope that the hubs are running true. As long as the torque is getting up there, it should not be the problem. Mark the tight spot on the rotor, then advance the rotor one stud, and try again. If the tight spot follows the stud, then the hub is to blame. I really hope not.

Were the discs new? Used discs can be warped from heat cycling. There will be a bit of wobble in the disc, but should be less than 0.004" I'd guess. Just a little less than the venting clearance. The front discs depend, to some degree, on the wobble to help set the venting clearance

The disc is new. It did hang up on the studs so I whacked it a couple times with a mallet and then torqued the lugs to get it fully seated. Maybe that wasn't the right thing to do in hindsight!

I'll pull things apart and check out the hub, I have a dial indicator/mag mount base lying around in the garage, just need to figure out how to use it.

Do you normally adjust venting clearance to be 0.004" at the tightest spot? The disc is always going to have some runout but the shop manuals don't make any mention about rotating the disc when you set the clearance. I find that strange. 0.004" in one spot might be 0.002" in another etc.

Thanks for the tips guys.

QUOTE (davep @ Oct 10 2005, 06:02 AM)

I agree that the hub to disc fit is critical

you have to make sure the hub is spotless clean, even the smallest piece of dust can result in a wobbly rotor motion ...

also, you got all 5 lugnuts torqued to spec, right?
Andy

wacking the rotor to the hub might have raised a small burr on the stud hole, which might cause excesive runnout. Removing it, and use a file/counter sink tool to clean up the surfaces, and try again. Full torque on the hub nut might also help some, but burrs or dirt between hub and rotor seem likely cause

Torque the hub and stub axle first. If this isn't torqued to spec it would be easy to conceive the problem your having.

Studs can be pressed in and might be a tight fit he first time. They usually straighten out once the wheel is torqued to spec. That's probably not your problem.

Check the surface of the hub for imperfections around the base of the stud. I haven't noticed this on mine but others have.

After you're sure the disc is fully seated on the hub, set the clearance to .004" on the high spot. That should do it. The Cap'n

I had the same wobble when I bought new rotors. My hubs had some rust on them
and made the mating of rotors to hubs uneven. I couldn't believe new rotors
would do this but the problem was the rust on the hub (and maybe the hub itself).

So what I did to fix this was:

1) Mate rotor to hub in one orientation (did this with magic marker).

2) Mounted the wheel on the hub and tightend the lugs to seat the rotor.

3) To measure runout since I don't have a runout tool or anything like it,
I got a small stick about 3/8 x 3/8 x 6 inch and jammed it in the hole in
the trailing arm used for caliper venting clearance. I jammed it with a paper
towel which allowed the stick to move back and forth in the hole with some
resistance.

4) Then rotate the wheel and push the stick towards the rotor until it scraped
on the lowest part of the rotor runout. Then continuing to rotate the wheel
pushed the stick back out. At the widest gap between the stick and rotor,
I measured the runout with a feeler guage and marked the opposite side
of the hub from where the rotor rubbed the stick. I marked the hub with a
piece of tape.

5) After determining the high side of the hub, I removed the wheel and rotor
and gently filed half of the hub that was marked with the tape. Theoretically
this will reduce the wobble. Held the file flat on the hub to make sure the
filing didn't distort the hub flatness. File gently and not much per measurement.

6) After filing, I repeated 2-5 until the wobble dissappeared.

OK, this isn't too scientific but it worked real well although took about 2-3 hours
per rotor to get rid of the runout. File gently and not much per measurement.
With this method, I was able to get the wobble down less than 0.003 in.
I think 0.008 in is the maximum allowable.

Spoke

with Eric. You gotta torque the axle nut to spec. I also agree with the Cap'n, torque the nut and go with the high spot.

It is critical though that the rotor is sitting flush and perfectly on the hub. I always test fit the hubs & rotors before mounting.