Disc run out .005-6 what am i missing???
 

Need help with his one. I am rebuilding the front end of a 74 911 - my dads car.
No matter what I do, I keep coming up with .005-6 disc run out, near the outter edge of the disc. I had no problems with pulsation or steering wheel shake before the rebuild:
New Zimmerman discs, bearings, 10.9 bolts, hardened washers, self locking nuts, torqued, twice, disassemble, rotated ~ 1/2, reassembled, still have .005-.006 disc run out. .
I find it hard to believe that the stock hubs of a gently used car could both show .005-6 run out.
Back ground: cleaned high heat paint off of rotor face and mounting surface, Used a fine file to insure that the mounting surface of the hub was clear of burrs, carefully snuck up on the torque for the disc to hub bolts, disassembled the disc/hub, rotated it about a half turn, reassembled and retorqued but stil have the run out in the same marked location. Then froze the new races, warmed up the hubs and installed new bearings. remeasured, same run out, same location???
Before I take the hub-disc to have it "trued" on a brake lathe, I am hopeing there is something I have missed.
This is humbling, (not a first), but I want to get this right.
Help,
chris

Bearing seated off?

Can you hit the high spot with a mallet and wood block?

Had the same on a ford truck. New everything. Lots of runout. Just had the brake place lathe it...

It is possible to add shims between the rotor and hub......

You should be able to pinpoint the source of the runout by marking, measuring, then moving the disc onto the hub. If it's the hub, have your brake people throw it on the brake lathe and take a light cut. At least that's how we used to do it. If they've never trued one up, they might look at you funny, but it's entirely possible to accomplish on a brake lathe.

And when you put it back together, use hand tools and a torque wrench to reinstall both the disc and the wheel. Impact wrenches are kinda hard on the aluminum hubs.

Disc off the hub, machine base of the hub. If all else fails, which it has....

Do you still have the old rotors? As in, reinstall them and see if new rotors are the culprit? John.

I tried repositioning the rotors on the hub, both sides, high point remained the same.
Thinking I might have not fully seated one of the bearings, I removed and reinstalled the races, no change in high point or run out.
I thought of shims, but didn't have the stock and I wanted a more solid fix.
The brakes are going to take a beating, (track), so I didn't want to try trueing them up with a block of wood and a BFH.
Old rotors long gone.
Looks like it is time to get a trueing cut on the hubs.

Thanks guys

Axial or lateral runout (a definition thing)? If the rotor runout is random at different mounting positions, then it's probably the rotor. If consistent (e.g. at 2 o'clock), could be a slightly bent axle spindle. The caliper mounting surfaces must be at a right angle (90º) to the spindle axis (or parallel to the rotor face). Have I got that right?

What's easier to check, take rotor back to the machine shop or check yourself? Better if you can definitely confirm.

S

How would a bent spindle change brake rotor runout?

Nice catch. Thanks. It wouldn’t, except the rotor wouldn’t be concentric within the caliper if the spindle was bent.

I put front struts from a '72 911 on my 914 and had some runout on one of the new discs. All surfaces were clean so I marked the disc position on the hub and gently filed the mating surface of the disc to reduce the runout. I must have installed and removed the disc about 10 times as I didn't want to get too aggressive on the filing. I filed the entire surface of the disc mounting area with pressure just on the side which I wanted to remove material to keep the surface as flat as possible.

I agree with your assessment of gently driving the car as I also drive the same way. However, I have hit a couple of nasty potholes that had me cursing for a minute. I think one big hit like that could bend or distort a hub.

Spoke must be much more careful and precise than I ever could be. I wouldn't trust my own hand filing at all but would much rather have it chucked up in a lathe where the runout can be carefully measured and trued up.

If I had a new disc run out problem, I'd return it and ask for another, or send them both back and try for a better quality disc, like a Stoptec, curved vane rotor.
Spoke, I admire your patience and metal working skills.
Chris

It's not hard to check runout.

http://forums.pelicanparts.com/uploa...1562457806.jpg

Next, check runout on the disc mounting surface. Disc off of course.

If you do see runout on the wheel surface or disc mounting surface, how would you remedy it?

Post #5 from John Walker. Machine the hub surface.

http://forums.pelicanparts.com/uploa...1562469021.png
Think, what has/was changed from previous. Look there. To change the surface of the disc mounting surface on the hub would show tremendous distortion. My bet is poor machining on the “new” discs. The Porsche limits are above.