Rear rotor runout?
 

Having a very hard time getting my rear rotors below .05 runout. How important is it that I get the run out lower ? I have replaced the calipers with the o ring style. One of my friends who is a mechanic thinks I'm being a little to fussy. Most people I talk to don't know much about this runout thing. I tried to buy correction plates at napa and the guy said I'm the first person who ever asked any questions regarding the runout. And I can't get the plates. Bought brass shim stock to use to adjust run out. Not a fun job. Rotor has been on and off a hundred times and I'm still not down the .02 I have seen some guys in here are looking for. Any info help or tips to help with this problem would be much appreciated.

How much bearing play do you have and is it included in your .005" quoted above?

With O rings you will never have any trouble with .005.

The end play is .08

The op said .05 ( fifty thousanths ) not 5 (.005)!!!:eek:

Sorry I meant .008 and .005 on the runout.

It is important. I wonder if it was important back then when these cars were the daily drivers at that time. It really seems to be a big issue today and can be a real pita. From articles I have read, threads and posts on this subject on this forum; rotors were trued to the wheel hub during assembly of the vehicle so the rotor fits flush to the hub. Apparently today runout is an issue because new rotors do not sit flush with the hub and with the fixed calipers on the c3 corvette is unforgiving and the runout must be corrected. If not air is pumped into the brake system resulting in a soft pedal. They will also squeak/squeal. Not sure rotor runout is a problem with c6 corvettes as they have fixed calipers also. Runout was not an issue with c4 and c5 vettes as they had floating calipers but floating calipers have less stopping force. As for o-ring seals vs lip seals, you have two groups that support either o-ring seals or lip seals. With all of that said, if you have a firm pedal and your brakes do not squeak, you should be fine.

.013" total if I read correctly. Might be a problem with conventional seals, possibly not with o-rings. If it does present problems, I'd go after the bearing play rather than spending more time on the rotor runout.

I guess I will put it together and see what I get.

Find a shop with an on-car lathe, they can turn them on the hub and spindle to get it down to spec.

I have never used one of those machines, but I have serious questions as to whether they would actually work well on an early corvette. I understand on a modern car with zero clearance bearings they should do fine, but with .008 clearance and that thing hanging out there on the axle, might just create as many problems as it solves.

Suggestion
 

Suggestion if you are not doing this already.

put all 5 lug nuts on the rotors backwards (flat on the rotor) and tighten down.

get readings, remove the rotors and reinstall with the same shims in the same position and see if you can get repeatability of the runout with the same setup. If not repeatable, then something else is going on. I am guessing but could be bearing play, shims deforming or too small, or something else. Since the front wheel bearings are more accessible, you might start there.

I used pretty large shims (homemade cut with sissors from shim stock), largest that would fit in the space and used steel shims and got it down to about .003. Agree this can be a frustrating exercise. My friend with a '75 never worried about shims and his drives fine, but he does not drive the '75 very much either. IMHO, the less runout you can get, then the less pumping the calipers do.

Interesting, I did not know C6 calipers were fixed, but then the roof on my C6 squeaks and my old C5 NEVER did. duh?

Hope this might help and good luck.

total run out
 

I've never had to do any kind of shimming or anything like that. If you have too much total run out, there are only 2 places to correct it.
the run out of the rotor surface and the bearing play. A new or turned rotor will be right. And the bearing play should be removed. I think if you are trying to shim a rotor because you have bearing play, you are kidding yourself.
Dave.

Sorry Dave- but you've missed out the prime cause for run out problems on C2/C3 brakes. The spindle or hub face itself was not necessarily machined perpendicular to the axis of rotation. They're frequently .005"-.007" off. Re-machining a rotor by itself will not fix this.

Because the spindle face is not perpindicular with the spindle itself would machining the spindle face flat cure the run out problem?

I must have got lucky when I did my brakes after receiving my '69 'cause with out knowing about the problems with run out,I assembled them without checking if the rotor was true. When I took it for a ride I paid attention to to how the pedal felt,as far as pulsing,and felt none.
I have had no problems with air getting into the system and the pedal is still very firm. Must have been my lucky day.

Absolutely- all other things being equal. :thumbs:

GM never sold rotors by themselves, always with a hub or spindle attached and finish ground to spec., the same as production line assemblies. Separate rotors are an invention of the aftermarket AFAIK

where's the run-out.
 

Mike, I respect your position but in my experience and you may have done more of this than I have, I've done a few, Except for the bent spindle which i found on my own "Old Red Chev" and stated in a previous thread, i have never encountered a spindle face off enough to have to remachine it. I believe you are saying that GM turned the rotors after they are on the car, or at least after the rotors were rivited to the spindles. I don't know that, but I do know that I don't know any shop in my area, going all the way back into the 1960's that turned new rotors on Vettes after the rotor was installed. If I have a spindle flange off that bad, I would replace it or turn it on a lathe before I would mess around with shims.

When checking runout you hold a prybar against the rotor as you turn it, so bearing play is not a factor.

I have found runout in the spindle face of 5 thou on both of the rears that I last worked on. We set them up in the lathe and machined the face. It was not easy to do. You can't just clamp it in your 3 jaw chuck and assume it is straight. What we did was chucked the splined portion in the lathe and the other end in the live centre then set the dial gauge on the bearing surface and played with positioning until we had 0 on the bearing mount area.

At the end of the day, not worth it. The manufactured tapered shims are the way to go. Very simple to use. Mark your high spot on the rotor and position the thin side of the shim there. They come in 3, 6 and 9 which will put you within a thou or two.

I'm a little uncomfortable with the individual shim thing. You loose the full face contact which I feel adds to the integrity of the part. Don't have any neg experience, or heard of any, just a feeling I suppose.

Steve g

GM machined the rotor after it was riveted to the spindle or hub prior to assembly on the car. Do a search on Gary Ramadie's old posts, you'll see the dozens he's found that were .005" to .007" off.

The concept of separate rotor and spindle/axle assemblies was very rare back in the 60s and 70s, now almost every car comes that way.
Axle flanges can bend slightly from use (potholes, curbs etc) to throw them off enough for runout. The reason why it wasn't a problem back when, was because the rotor and spindle was a one piece unit on most cars and an ordinary brake lathe used the bearing races to turn the rotors. Very simple. Same holds true for the C3 front rotor and spindle, riveted or bolted together and turned on a regular brake lathe and everything will be fine. If you look at the front spindles vs the rear axles, you will see that the backside of the front spindles ramps up and adds a lot of strength whereas the rear axles are almost a straight flange front and back.

The rears were not an issue with most cars back then, they had drums.
The forged axles were machined all at once, so the flange face and bearing surfaces were all concentric and perpendicular. Eaton, who was an oem GM axle manufacturer (don't know if they did covette axles) held tolerances of .0005" (ten thousands) on the axles. Pretty remarkable considering they would hog off almost 3/8" at once.
Then the rotors were riveted to the axle and the rotors were finish turned.
The riveting could well have distorted the flange a bit at those tight tolerances and the rotor faces were still a rough casting anyway.

The best and most economical way by far to repair any rear runout if you have good bearings, is to use a good on car lathe. Lucky to have several around here within a couple miles. If none available, then shims.

That's a rough job on a shop lathe at best. A few smaller shops offer this service and I really wonder how good the results are given the equipment available. Definitely not a money maker, but I like it better than shims.
Originally they were turned using a face driver instead of a chuck, all in one operation so the axle came out true by default.
If one were to do a lot of these, about the only way would be to use a precision collet that grabs both bearing surfaces and truing the flange face would be a 3 minute deal from opening the box to sealing it.
A friend of mine used to buy blank collets (expensive) and machine them for each job on his cnc turning center. He also had jobs machining new pistons. As an example of his tolerances, he would let the spindle bearings run a half hour each morning to warm up and gain consistency.