HI GUYS- I tested the runout of my rear rotors and the dial came up. to 10 do I need shims and what size and where do I place them thanks bob

 

010 is not good. You need to make sure there are no burrs on either the rotor or flange. All mating surfaces need to be clean. Do not machine the rotors. Shimming can be done and if done correctly can usually bring in the runout under 003"

 

thats the problem how do I fix this thanks

 

You need to put shims between the rotor and the axle flange
You can buy shims, buy tapered shims or make them out of pop/beer cans
https://www.corvetteforum.com/forums...out-shims.html

Tapered shims


diy shims


M

 

If you shim without using the tapered full discs, don't just put a shim on one stud and call it a day, put thinner shims on the next ones so you're not "bending" the rotor between the shim and the opposite corner

Rotor shim compensations

Shim on a stud - Shim on the next 2 studs
.010 .0038
.006 .0023
.005 .0019
.003 .0011

Shim on/between 2 studs - Shim on the next 2 studs
.010 .0062
.006 .0037
.005 .0031
.003 .0019

M

 

The full hub tapered shims are the way it should be done. I realize sourcing those shims has been awful hard recently. When I went through this last year there was a 90 lead time on shims. Maybe it’s not so bad now, but it wouldn’t surprise me if it’s still a PITA.

Here is what I would do. I realize this won’t be right to run but I would want to figure out is the runout in the hub or rotor. I would rotate the rotor on the hub 150 degrees or so and see if the runout goes away (split difference of runout between hub and rotor), moves with the rotor (all the runout is in the rotor) or stays the same following the hub (all in the hub).

If it’s just in the rotors you could get a local shop to cut the rotor slightly to true it up on a regular brake lathe. If it’s in the hub or some combination there of and you can’t get it shimmed right you would need to see if you can source a place that has the old on vehicle brake lathes that would true up the rotor on the car.

Couple of ways you can go about this.

In my case the initial runout (over .010 on the hub itself) was a funky worn bearing spacer. Once I fixed that I found there was still some runout and the remaining was in the rotor. Local garage turned and straightened the rest of the runout issue out.

Edit:
This is the type of shim that is best to use Shim but as I briefly looked for this it appears it’s out of stock at both rock auto and Summit which means it’s likely not any better now than it was last year for me.

 

I never used the "1/2" moon shims, I use ss shim stock on whatever stud requires it - that could be 1, 2 and rarely 3 studs. They won't warp the rotor and I have done literally 100's of them- all under 003".

The deburring part is important as times you won't see a slightly raised area around a hole, but it will be enough to create excessive runout.

We can tell you how to do it but you have to do it, so you have to look close at the mating surfaces and get an equal torque on holding the rotor on - using lug nuts or bolting the rotor to the axle flange.

 

Go to McMaster Carr...order .001" .002" SS316 shim sheet stock.
Make PIE slices with two holes for the lug nuts in an 'assortment'.
MAKE SURE that you know how to properly run AND READ A DIAL INDICATOR.
Affix dial indicator to the trailing arm. Measurement head 1/2" inch in from the outer edge of rotor (shim to worst case scenario, not best case scenario.)
Mix and match the pie slices. .010 can probably be negated with LESS THAN .002" at the hub..
Spread the load out over 1/3rd of the hub.
Use large washers and standard chevy 'steelie' lug nuts to TORQUE the rotors in 5 places to assure accuracy.
REMEMBER! You are also dial indicating the play in the wheel bearings AT THE SAME TIME! (loose crappy wheel bearings...GIVE UP....you'll never get there!

Incentive...one of mine was out, despite my best lathe turning efforts .010", result after about 45 mins of @#$%^ around (above) .0005" (half a ten thousanth)

Chevy SPEC is .004" at the edge of the rotor....for me that's too much....I won't go over .002"

Good luck and TAKE YOUR TIME...be ****!

Unkahal

 

Again, I rarely have to turn a chip on an axle flange or rotor. Shims don't have to be huge and those old NAPA ones are not needed. Imperfections cause most of the issues, especially if they have been worked on before.
The attached video is a finished arm I built- 001" runout max- no machine work needed, no 1/2 moon shims, rotors are NAPA new premiums about $60 each I matched to the axles. As they say in bodywork- it's all in the prep. That is where I would be looking but now I am done with this thread.

 

Like has been said. First make sure the surfaces on the hub and rotor are super clean. BUT, after you check the runout of the rotor, remove the rotor and turn it 1 lug and check the runout again, do this for all 5 positions. Chances are one of the positions will be good, you are done. If not, then you have to check if it's the hub or the rotor. If it is just the rotor, it can be turned. If it is the hub, the fronts, you can do new bearings and races pretty easy and see if it's better. The rear hubs would be much more difficult. Now, if all this fails you can try shims as a last resort. My experience, it didn't make any difference.