Rear brake rotor run out

[Timothy B.]

Checking the rear rotor run out on my 67 because of a leaking caliper and squeak while driving. The attaching rivets were removed years ago when I had the bearings serviced and my rotors had some rust pits. I took the rotors to the machine shop and had them shot peened to remove rust then kiss cut approx .005 each surface, they are still approx. 1.230 also had them countersink the rivets holes for flat head screws.

Installed them back on same side as removed with the screws, the drivers side according to my dial indicator mics at .0025 run out but the passenger side mics at .014 1/2" from rotor edge, all surfaces clean without burrs. I took a aluminum pie plate approx .012 and made a shim and installed it on the lowest spot. This spot was over one of the flat head screws and not over a wheel lug, after tightening the screws runout is approx the same as other side at .0025.

My question is about the shim thickness being to thick at .012 causing to much gap around the rotor and is the run out going to change after I tighten the lug nuts to 65 70 ft.lbs. My thoughts this morning are with measuring .014 run out at the rotor edge I should have only had to use .006 or .007 shim at the screw hole instead of the .012 pie plate shim.

All comments are appreciated, I am feeling like I need to go back and take another look, I removed and installed the dial indicator a few times and the .0025 was consistent. Down the road I plan to install new rotors because of the rust pits near the outside edges of these original rotors but I needed to correct the squeak and leaking caliper in the meantime.

PS, Mrs. Smith pie was very good but belly very full this morning.

[Gary R.]

Re: Rear brake rotor run out

Hi Tim
0025 is fine. I typically shim on the studs not the flat heads. I use SS shim stock so rust or material spreading is not a issue. 012 is more then I typically see but not unheard of. Be sure to kiss grind the axle flange where the rivets were since they will raise the metal a little and inside the hat at all the holes.

As long as the rotor is tight and the runout under 003" you are good to go. If you replace the rotors in the future do the same thing. I always tap the rivet holes and bolt them on. I can dial them in to under 003 everytime and never need to turn a chip. The advantage of bolting them on is the rotor is locked and not going to change with wheel changes and access is easy compared to riveting back on. I have 100's out there like this and never had a problem.

[Michael W.]

Re: Rear brake rotor run out

C

My question is about the shim thickness being to thick at .012 causing to much gap around the rotor and is the run out going to change after I tighten the lug nuts to 65 70 ft.lbs.

Flip the lug nuts around and torque them to the rotor without the wheel in place. Measure the runout.

[Richard G.]

Re: Rear brake rotor run out

Tim
I would want to know if the run-out is in the rotor or at the axle. Or more likely the combination of the two run-outs.
Measure the axle at the peripheral and report back. It will be less because of the diameter difference between the rotor and the axle hub.
You may find a run-out sweet spot by checking all five clocking combinations between the rotor and the axle.
If the shop machined the rotor perfectly the run-out is in the axle. You could put this issue to bed by pulling the axle and having the face machined perpendicular.

Gary: Wouldn't this situation require taper shimming? Smaller shims between each successive bolt?

Rick

[Timothy B.]

Re: Rear brake rotor run out

Thanks Gary and Mike,

I want to add that when I used that .012 shim the low spot moved to the screw on the left after tightening the screws if that makes sense. That area was originally about .0015 high before the shim. I may switch to beer and see if something like a .010 shim helps.

This rotor on the other side (drivers) dialed in at approx .0025 without any shims so I will leave well enough alone but I wonder how it would be on the problem side.

Good idea on the lug nuts Mike I will double check that also.

Richard, I will probably go back and check the spindle flange and my work but at this time I don't want to pull the spindle although as we know that's the correct way but in service if a small shim is safe to use that is good enough for me.

Gary's method with the flat heads is nice and neat plus you can access the parking brake if ever needed. Gary, if you read this where is the best place to buy these rotors today.

[Michael W.]

Re: Rear brake rotor run out

You may find a run-out sweet spot by checking all five clocking combinations between the rotor and the axle.

Rear rotors will only fit correctly in one position and have the parking brake adjustment hole available.

99% chance that the hub face is not perpendicular. GM made little effort to ensure this as the rotor friction surface was to be finish machined after the hub/spindle was permanently mated to the rotor. It is always a mistake to have a rotor machined separately from it's mating part. It must always be done as an assembly.

[Richard G.]

Re: Rear brake rotor run out

Yes I understand a good fix that functions it where you want to be.

If you used a singe shim it may account for the low spot moving as the hub cocked to one side. At the very least you might try shimming the low spot at two bolts, to reduce any rocking that may occur.

Gary's fix works without issues, so I would recommend it. Without the cap screws I can envision where the torque sequence could easily have an adverse affect on the run-out. I would add the flat head cap screws if possible. I also forgot about the disc brake cars having only one clock position. Another options may be to have the brake disc machined on the car. However with that much run-out you could be inviting other issues, such as unbalance by doing so.
Seems to me shimming and securing the hub is the best option per Gary's recommendation.

I just took a skim cut, on my lathe, on one the axle's face, to be absolutely sure it was flat and perpendicular. I frankly was amazed it was so close, less than .0015 run-out. First cut only removed the rust layer. I have one left to do. Possibly I will video the process? I don't see many videos posted on this forum and have never done one myself.

[Joe L.]

Re: Rear brake rotor run out

Tim
I would want to know if the run-out is in the rotor or at the axle. Or more likely the combination of the two run-outs.
Measure the axle at the peripheral and report back. It will be less because of the diameter difference between the rotor and the axle hub.
You may find a run-out sweet spot by checking all five clocking combinations between the rotor and the axle.
If the shop machined the rotor perfectly the run-out is in the axle. You could put this issue to bed by pulling the axle and having the face machined perpendicular.

Gary: Wouldn't this situation require taper shimming? Smaller shims between each successive bolt?

Rick

Rick------


Tapered shims are available from Raybestos. They are available in sizes to correct 0.003" and 0.006" runout and, possibly, others.

[Richard G.]

Re: Rear brake rotor run out

I had no ides these existed.
A link to the video that shows the application procedures.

https://www.youtube.com/watch?v=SSxCNbjKi-0

Thanks Joe.
How cool is that!
This would be a perfect fix. I was thinking you had to use individual shims not a factory system.
Max run-out for the Raybestos plates is .009. They caution to not use more than one plate per wheel.
Still you may be able to reduce the runout to .003 much better than what you have now.
Rick

[Joe L.]

Re: Rear brake rotor run out

I had no ides these existed.
A link to the video that shows the application procedures.

https://www.youtube.com/watch?v=SSxCNbjKi-0

Thanks Joe.
How cool is that!
This would be a perfect fix. I was thinking you had to use individual shims not a factory system.
Max run-out for the Raybestos plates is .009. They caution to not use more than one plate per wheel.
Still you may be able to reduce the runout to .003 much better than what you have now.
Rick

Rick-------


Many years ago these tapered shims were exclusively available from Stainless Steel Brakes Corporation in Clarence, NY. Then, from what I was told or otherwise learned, there was some sort of patent dispute between the inventor of the shims and Stainless Steel Brakes which resulted in Stainless Steel Brakes Corporation no longer being able to manufacture or sell the shims. They were subsequently off the market for quite some time. Later, apparently Brake Align came into being and, apparently acquired the patent rights. These are the shims now also marketed by Raybestos.

[Gary R.]

Re: Rear brake rotor run out

Hi guys,
To answer your questions on what I do.

For new rotors, for a street car - not a road racer, I just use NAPA premium rotors. They don't have the rivet holes and I don't what the rivet holes. I transfer the bolt hole pattern from the axle then drill, double countersink, and deburr the rotors. I tap the axle rivet holes 3/8-24 and use flat heads 5/8" long. I set them up when rebuilding arms so the arm is on the bench. I check the runout at the farthest point from center on the rotor. Very slowly rotate the rotor and mark the reading at each stud. I shim from the lowest one. I use SS shim stock that come in 001" increments. I remember those tapered shims from SSBC about 35 years ago. I never needed to use them. Most of the time one stud has to be shimmed but sometimes two. The SS won't rust or compress like other stock might.

[Jim T.]

Re: Rear brake rotor run out

I solved my rear brake rotor runout on my 1970 about 30 years ago. The Chevrolet under my 1970's factory warranty removed my riveted disc rotors from the spindles on both sides due to my complaints about noise from the rear. They replaced the spindles and bearings, this was before 1975.
I solved the rotor runout and air being pumped into the brake calipers by removing the spindles and brake disc rotors and sending them to BAIR's to be fixed. No problems with air in the calipers since this was done and the disc brake rotors are riveted to the spindles again like the way they were from the factory.

[Joe L.]

Re: Rear brake rotor run out

I solved my rear brake rotor runout on my 1970 about 30 years ago. The Chevrolet under my 1970's factory warranty removed my riveted calipers from the spindles on both sides due to my complaints about noise from the rear. They replaced the spindles and bearings, this was before 1975.
I solved the rotor runout and air being pumped into the brake calipers by removing the spindles and brake discs and sending them to BAIR's to be fixed. No problems with air in the calipers since this was done.

Jim------


Going the route you did is definitely one way to solve the problem. However, there is usually a down side to it, especially if original rotors are used. Here's why: when GM originally manufactured the spindle/rotor assembly they had the benefit of starting with unfinished rotors. So, when the assembly was finish-machined on the spindle centers or bearing surfaces, the rotor was machined to new rotor thickness spec of 1.250".

Unfinished rotors have never been available in SERVICE. So, when field machining is performed to correct rotor runout, usually the spindle face is first machined to correct runout (which is often considerable on the spindle face). Then, the rotor is attached and checked for runout. Especially if the original rotor is used, there will almost always be runout in excess of specs. So, the rotor friction surfaces are then machined on the spindle centers or bearing surfaces. In the process, material is, of course, removed from the friction surfaces. Only 0.035" can be removed before the rotor is scrap. In any event, the rotor ends up at less than new thickness. That's OK but it's not my preference. I'm not a fan of "cut" rotors.