I will be installing new GM replacement rotors soon. I know that once the original rivets are removed, everyone just secures the replacements with the lug nuts. I thought I read a reference a while back in a thread on a more secure way to hold them. Is there?

 

Willem,
If your rotors come loose while you are driving, you're in real trouble!!! For this to happen, your wheel nuts would have to come loose and more than likely one or more of your wheels would have fallen off! Loose rotors would be the LEAST of your problems.

Unless you plan to show your car at NCRS meets, forget the rivets. The only reason they were rivetted to the hubs at the factory was that the hub/rotor assembly was machined as a single unit to save time and money compared to doing them individually.

Your problem now is that when you assemble your new rotor to your old hub you will more than likely have major run-out. By this I mean maybe five to ten thou. Two thou or less is ideal, more than that will result in pulsating of the pedal when braking and the possibility of air sneaking into the calipaers when this is going on.

Now to mounting your rotors. Secure each rotor to the hub with three lug nuts SCREWED ON BACKWARDS. Mount a dial indicator to a fixture like the stub axle or caliper bracket and slowly turn the rotor and check the maximum runout.

If your runout is above two thou, try all five possible mounting positions and swap hubs and rotors (I presume you're doing a pair or two rather than just one) and if you still can't get down, then put brass shims under the rotor on the appropriate stud(s) until you get down to two thou. Of course, the hubs have been wire-brushed and cleaned of all surface rust, dirt, etc. on the rotor mounting flange previously.

Some people might say that two thou is overkill, and it's true that runouts as high as five or even eight thou can be lived with, but IMHO if you're going to this much trouble, why not go the extra bit and ensure that your brakes will be as good or better than when they left the factory?

Regards from Down Under

aussiejohn

 

I'm working on the very same thing.
I've put on new front rotors and the runout is about .007 and .012.

I know its not the rotors. I marked one bolt and no matter which way I put the rotor on, its high when it goes past that bolt.

I thought about shims - It sounds like I would need .005 and .010 shims

Where do you get the shims?

Edit: I found this article which is related: Brake Job Aricle

 

At least one of the guys here has tapped the spindle plate and installed bolts. The bolt heads are quite large in diameter (compared to the threads) and use a hex key to tighten. The head is also very thin, similar to rivet. I know you've seen these types of bolts before, but for the life of me I can't remember what they are called.

 

Flat head allen. I used 3/8"X16X3/4" long on my white 79. The diameter of the allen head is the same diameter of the rivet recess in the rotor but you need to countersink the hole in the rotor. The original rivet hole in the flange is perfect for tapping the threads for the bolts, ya don't even need to drill them any bigger, just run a tap through them. Took maybe 30 minutes to tap all 10 holes and another 10 to countersink the holes in the two rotors with a 1/2" drill. A drill press would be a plus. It just makes for a more complete job.

 

The only problem with that is the rear rotors will only go on one way if you still want access to the parking brake adjustment. Then again the parking brakes are a pain to actually get working properly anyway, so it might not matter...Whoever was in my rear brakes before me thought the same and just ditched all the hardware completely.

 

I did... (crappy cell phone pic)



Used a counter sink bit on the rotor, then tapped the holes in the hub. I feel much safer this way. I used 7/16-24 flat head socket cap screws from McMaster... these bad boys were expensive, and I had to buy a box of 90.

 

Mike,
If you buy rotors that have the rivet holes in them counter sink the holes. I use 3/8-24 x 5/8" flat heads. You have to be sure they are not too long and these work. Also if you get the rive body out without distorting the hole the hole ID is just over the tap size drill for 3/8-24 so all you have to do is tap the hole- no drilling.
I use a 2" grinding wheel to dress the inside of the rotor around the holes as well as the flange surface where the rivets were. The metal is raised at these holes from the rivet install. Any burr, rust, scale will affect the runout. You want the mating surfaces smooth and clean.
I have SS shim stock and a hole punch to make shims up. I've had new rotors out as much as .010" and got them in to under .002" takes a little time and you have to be careful using the indicator but it can be done on the bench with the arm in a vise.

 

Question for you GTR: Have you ever checked the rotor mating surface to see if it is true/flat. Just wondering seem like it would be easier to mill the mating surface flat/true to eliminate the use of shims

 

That won't help if the spindle isn't exactly perpendicular to the rest of the knuckle.

 

Remember, the shim is at the lug hole ... this translates to more
change at the outer edge of the rotor. Thus, to correct a 10 mil
runout, you do NOT use a 10 mil shim .... maybe like 3 mils.

I used aluminum soda can to cut stock from ... and even had to
use some 400 grit wet/dry to thin the stock further.
It's a lot of work ... but worth it in the end. Just make sure you
mark the rotor location and keep shims in place when removing in the future.

All four of mine are in the 1-2mil range.

 

I have found the flange surface out as well. They were rough faced and since the rotors were turned after riveting the finished surface would be correct and perpendicular to the axle/spindle center line. The problems developed when the rotors, mostly rears, were turned off the spindle/axles. The rear rotors usually were chucked in a lathe on the ID of the hot and this in not the best way to do this. The surface in the lathe may have been within a few tenths but once it was mounted on the spindle could be out .005" or more. Add to this the fact dealerships and garages did not have toolroom lathes or trained operators to run them.
Shimming has worked for me when combined with bolting on the rotors. The idea way to do them is either to bolt or rivet them and turn as the factory did or to use an on the car lathe. Both of the options are out of the realm of most so again the shimming/bolting works.
When I set up rotors I check them at the extreme end of the rotor and use a very light touch. In addition the indicator must be dead on and have no movement in it.

 

Well, I shimmed the rotors up using soda pop cans and it seems
to have worked pretty good. (Soda pop cans are about 0.003, btw)

It was fairly easy to get them all within spec. (0.005").
I can see how getting closer would take a bit of time.

Anyways, I put it all together and went for a drive and for the first time,
my vette has no pulsing, and no pull to the left.

I can hit the brakes at 40m/hr and it tracks straight with no hands on the steering wheel.

Of course, I replaced both front rotors and a caliper this time round,
so that may have had something to do with it too!

 

Take a ride and really lay into your brakes on a highway offramp... you'll feel that few thousands. Gary gets his in within 0.002" runout. I got mine in at about 0.003 - 0.004 and I can feel it.

 

Gary, will 3/4" bolts interfere with anything? My local ACE hardware stores have 3/8-16x3/4 bolts.

 

 

Where can the rotors that have the rivet holes be found? Raybestos and Delco do not have them.

 

All the major suppliers (Corvette America, Mid-America, Muskegon Brake, etc.) carry replacement rotors with the factory rivet holes.

 

Sam,Mike

Use 3/8-24 not 16. The rivet holes are just over the tap size drill for 24's so all you have to do is tap them with a plug tap. I use only 3/8-24 x 5/8 flat heads. They do not stick out the flange. Try an industrial supply company like Mcmaster carr.

 

I used the 3/8X16X3/4" long screws without any problems. Tapped the flanges without drilling the holes any bigger. Also a dab of never seize is a good idea.
There was an article in one of the Corvette mags a while back that said they used 1" long bolts. NO way on that!