1979 Corvette. I am replacing the front calipers (New Delco) and rotors and pads (Powerstop z23). Disc runout has measured +.015. I reset the the disc to nearly opposite the 0 degree mark and the 0 degree mark is on the same place as before on the hub. So I don’t think it is the rotor. If I explained this correctly where is the problem? Is it the hub bushings, the hub, both or something else? I defer to the forum geniuses. Thanks!

 

In the beginning, At the factory. The rotors were riveted to the hubs and then machined. There is the problem.
2 fixes. One. shims behind rotor. two. mobile disc grinding service, where they machine the rotors on the car.

 

Wow! Australia! Thanks 4-vette! I looked for shims on the web but they aren’t available. It seems a shame to have new good rotors machined to fix this. New hubs and / bushings is not an option? Thanks from Jacksonville FL. Doug

 

shimstock and some shears/snips
can be found a hardware stores.

 

Thanks Cal and GTR! I take the shim goes over the studs?

 

You actually can cut front rotors on a standard Brake Lathe with the hubs secured to the rotor via the studs....just make sure you mark where to index. I haven't seen this done in years though as Brake Lathes are going away and most just shim now anyway......

Jebby

 

I have to disagree with machining the rotors on the spindle. Think about it. If you do that you will end up with a dish.

 

If your using the o-ring style caliper pistons how much run out is acceptable. The same or can they tolerate more run out.

 

How?

Jebby

 

There is also a special tool set up to turn the rotors while mounted on the car.
But, over all the thing to do is to try indexing the rotors for the least amount of run out, then shim.
Some people use beer can shims, but I prefer the tapered shims available at Napa, etc.

 

Thanks Sayfoo! For some reason the tapered shims are no longer available. Everyone is out of stock. My impeccable timing strikes again. But beer can shims sound right up my alley. I will resist the temptation for now.

 

Thanks GTR, I really don’t want to cut the brand new rotors. And I think there is too much to cut. Shimming will be the way to go.

 

Yeah, the rotors almost have to be machined on the car. Lot of shops can do that. Get them as close to 'true' ahead of time, as you can. I would AVOID machining the hubs or rotors on a standard bench-top brake lathe. I've had terrible results on multiple occasions where I've done that. The brake lathe spins the hub and rotor on it's spindle, but does NOT spin the hub on the hub's bearings. You're at the whim of the technician who like to walk away and come back and give them to you. I've done it. Waste of time/money ...despite what the shop will claim (been there). The hub/rotor really should spin on it's bearings to get it 'true' and that's kind of the tricky part.

Or you could fab a simple jig like I did and machine the hub faces 100% 'true' (pic below). And any 'good' rotor will run true thereafter.

 

Thanks Mark, Unfortunately the pic didn't show up on my phone. I will check if it did on my tablet later tonight. I look forward to seeing it as I have thought about that. I am guessing it would be a balance issue if I used a hand grinder and just did the best I could? The shim will be fairly large to make up the .015 from .000. Sheesh!

 

Ha ha ...try the picture again. I had to remove and put up the 'right' picture.

With the jig, the drill in the background rotates the hub (drill peg goes into that 3/4" (20mm) wide bar below the hub), and a piece of heater hose on that peg provides grip. Obviously there are a number of ways to skin a cat here. I would have used an electric grinder, but my air grinder bearing is super tight and I thought a little easier to mount. But yeah, otherwise and electric grinder was my first choice. I thought it would actually work better -- a guy could lock the motor 'on' and adjust the speed with a router speed controller. I only had 2 hubs to machine so I did what I did.

I previously took a hub to a machine shop (bench brake lathe). They A) removed too much material, B) there was a 'ridge' from their cutting bit skipping over the holes, and C) ultimately it had as much run-out as when I took it in! I had to buy another (used) hub off ebay for $75. My rig I can grind off as little as necessary and get it 100% right. The finished surface is very nice. Lot nicer than the bench brake lathe was.

This wouldn't necessarily work for the rear hubs ...a guy would need to come up with a different system for them, or make a jig that would work both ways. Rear spindles would be a lot easier for a shop to chuck in a lathe and run a couple passes, and get true.

.

 

Mark That is ingenious!

 

Mark, I am sure it is my phone but still no photos. I can check my tablet when home around midnight. I look forward to seeing it. Thanks for your input!

 

I have seen the picture! Now if you could fix my phone to display this forums images that would also be helpful. That is some great work Mark! It is a little out of my league or maybe a lot out of my league. I am apparently a junior member for good reason. We will see what I can do with your ideas. I may just tackle it by hand with a die grinder or resort to some kind of shimming.

 

They sell new hubs which are supposed to be machined true.
By the time you figure yer (my) time scheming and making a jig to true up hubs, it might be better time/money spent buying new hubs that do run true.

I wouldn't bother trying to true them up free-hand with a die grinder. You'll ruin hubs and have to buy another set. But you could attempt to make a jig out a couple pieces of really straight sliding wood (threaded rod, etc) ...and ingenuity. I considered that. It would work. I would use a 4 1/2" angle grinder though ...and a router speed controller. It's easiest to remove the car's spindle and use that as a work base.

Honestly you might be far ahead if you just sold your hubs (say, $100 for both) and put another $300 towards new ones. By the time you figure all the time/money and screwing around ..you could get some new ones that will work perfect and be done with it. It's an idea. I had everything on-hand to make my little jig, and it was kind of a 'thought experiment' too. But it still took some time, more than I was planning. I'm not very smart with my time as you can see .... LOL! Sometimes ya got to evaluate when it's the right time to punt or go for it on 4th and 15 (hey, depends on which QB ya have, right?)....

https://www.ebay.com/itm/383477161951

 

Mark, All good suggestions. I have thought of replacing the hubs too but I didn’t know if that would fix the problem. Is it the hubs that aren’t true or is the spindle or both? Also are remanufactured hubs just as good as new or would that be inheriting another runout issue? Any thoughts are appreciated.