Rotors for 1970

[Steve G.]

I am looking to replace rotors on my '70. I found some for sale - see pictures attached. One of the front rotors has 6 holes in the circumference of the hub (not sure if that's the right term for the 'top hat' section). The other has no holes, which is how the originals are on my car. Does anyone know if the holes were correct for 70? Is there any other way to determine if the rotors are correct or not?

What is best way to restore rusted rotors - bead blasting or rust remover chemical dip? thanks, Steve

[Tom L.]

Re: Rotors for 1970

sThe holes are for the rivets to the hub. They all came with rivets. You can buy replacement rotors with or without the holes. I converted the rivets to screws and shimmed the rotors to get as close to zero run out as possible.

Regarding clean up. The rotors are cheap enough to just replace. Rust on the backside of the rotor at the hub mating surface will cause excessive run out.

[Michael W.]

Re: Rotors for 1970

The rotor with the holes in the sides of the top hat (2nd photo) is not original to any Corvette. The rotor in the 3rd pic is for rear wheels only. The two 'extra' holes are for adjusting the parking brake.

The rotor in the first pic appears to be a nice example of a front rotor and hub. A quick bead blast would bring the surface appearance up to scratch.

What's wrong with your present rotors?

[Tom L.]

Re: Rotors for 1970

Steve,

I misunderstood your question. I didn't expand the picture until I read Mike's post. I was talking about the rivet holes, not the ones on the side.

I've never seen holes on the side like that. What's perplexing is that the rivets look original, which suggests that it's not a replacement rotor. Perhaps the holes are for balancing of some sort.

[Bill C.]

Re: Rotors for 1970

Steve,

Before you buy those have the current owner take them to a brake shop and have them checked for thickness and run-out.
If they are to close to the discard value, then they are worthless.

I would never put a set of used on a car without turning them 1st. To much run-out and you will get a bad pulsation in the pedal - which may lead to brake softness over time. If they are turned and are at or near discard, they can warp under heavy braking.

You can get the front rotor assemblies new (with the rotor rivited to the hub). I have seen them priced around $200 per side. As for the rear rotors, and reproduction/auto-store set would work.

The rivits really serve no functionality purpose. They were used durring assembly to keep the parts held together while the frame was being assembled. The rims and lug nut torgue do the job just fine.

[Mike G.]

Re: Rotors for 1970

i just used repops from lonestar for mine. they looked like the originals.

[Tom L.]

Re: Rotors for 1970

Not to disagree with Bill, but the rivets actually do serve a purpose. Most disc brake systems employ a floating caliper that slides on pins or some other mechanism. The floating design allows the caliper to move with the wobble (excessive run out) of the rotor.

Our cars have a fixed caliper system, so any wobble in the rotors will result in pulsations in the pedal. Excessive wobble will result in air being sucked in through the caliper's piston seals, thereby softening your pedal.

To address this problem, there is a very tight run out tolerance for the rotors. I've read that the rotors were originally surfaced following riveting to the hub/spindle to compensate for the vagaries of the hub/spindle machining. The service manual also mentions resurfacing the rotors while on the car for the same reason.

The problem with drilling the rivets out and allowing the lug nut tension to hold the rotors in place is that you get rust and crud in between the spindle/hub and the back of the rotor, which will cause run out that is significantly higher that the spec, which from memory I think is somewhere around .005". Even the way you torque the wheels can impact the run out.

By way of illustration, my new rotors exceeded the run out spec right out of the box once they were mated to the spindles/hubs. If you don't have access to a machine that can surface the rotors on the car, the next best thing is to shim them out until the run out is within spec with the rotor fixed to the spindle/hub. For ease of maintenance, I elected to tap the rivet holes and screw the rotors to the hubs/spindles rather than replacing rivets. With a little monkeying around, I was able to get the run out to within .001".

Gary Ramadei (ph) did a fantastic paper on this a while back in which he described exactly how to do it and what hardware is needed. I may have it in .pdf if you need it.

[Bill C.]

Re: Rotors for 1970

I used to work at a brake shop for a good 3 years while in college.

Tom is correct that even new, rotors will have run-out out of spec.
We always turned new rotors on the machine - period.
Cars with front wheel drive, we used a machine that bolted where the caliper was. Put the car in gear and the rotor would be resurfaced (machine has two counter-rotating pad with 80grit paper on them.

They called thus thing a "grizzly grinder" ---


The best way to get new rotors is with the hub assembly. They are usually machined as a unit and the run-out is really darn close.

I was fortunate not to have to shim the rotors on my car. I did re-use my hubs with NOS GM rotors. I checked the run-out and it was okay.

I guess the most important thing is not to blindly purchase rotor or rotor/hub assemblies without having them checked 1st.

[Steve G.]

Re: Rotors for 1970

Thanks guys. Based on responses, I will stay away from the front rotor with the holes in it. I will buy the other front rotor and beadblast and paint it. Will also have it turned (slightly). On the rears, I was planning on going to Bairs to rebuild my trailing arms anyway. I believe I can buy reproduction rears and have them put it on and check runout. Does anyone think I'd be better off buying these rears (measurements on them are 1.25" and 1.23") and giving them to Bairs to install?

Michael, I am replacing mine because I am doing a restoration (hopefully to NCRS specs) and mine were too thin, beyond the 1.215" on both front and rears. Is the spec same for both?

Tom, yes, please do send me the pdf on how to reduce runout! Does that address front and rears? thanks, Steve

[Michael W.]

Re: Rotors for 1970

Michael, I am replacing mine because I am doing a restoration (hopefully to NCRS specs) and mine were too thin, beyond the 1.215" on both front and rears. Is the spec same for both?

Tom, yes, please do send me the pdf on how to reduce runout! Does that address front and rears? thanks, Steve

I think that spec addresses minimum thickness for resurfacing, not in-service limits. If the surface of your present rotor is still acceptable, I'd be reluctant to start messing with mix-and-match rotors and hubs. Keeping them intact as a matched set (as GM intended) will avoid the need to shim or worry about runout.

If the rotors are just not reusable for whatever reason, have the new rotor and hub/spindle machined as a set. Having them re-riveted or screwed together is unnecessary but does no harm.

[Tom L.]

Re: Rotors for 1970

Steve,

Here's a link to Gary's paper.

http://www.lbfun.com/warehouse/tech_...unout_Dial.pdf

[Steve G.]

Re: Rotors for 1970

Thanks. That was an excellent article! I assume the shims go between the hub and the rotor at flat head screws. And flat heads between hi and lo areas would need to be shimmed with fractional values so the rotor won't bend. Is this the same procedure the factory used?

How do you measure end play?

[Terry M.]

Re: Rotors for 1970

Thanks. That was an excellent article! I assume the shims go between the hub and the rotor at flat head screws. And flat heads between hi and lo areas would need to be shimmed with fractional values so the rotor won't bend. Is this the same procedure the factory used?

How do you measure end play?

The factory didn't use shims. The rotors were machined when mounted to the hub, thus any inaccuracy between the hub and rotor was machined out.

Modern cars are not machined that way, and usually minor wobble isn't an issue because the caliper is designed to "float" and be self aligning. On occasion the tolerance is so far out that shimming is required to stop pulsation. A kit is available with tapered shims to allow shimming of out of tolerance rotors and still maintain full contact between the rotor and hub. Such kits contain various shims in different bolt patterns for a fit to many makes of cars. They are quite expensive due to the large variety of shims in them. I have tried to get shims for just one application without success.

Your FSM (Factory Service Manual) has an excellent section, with pictures, of how to measure end play and run-out.

[Joe L.]

Re: Rotors for 1970

Not to disagree with Bill, but the rivets actually do serve a purpose. Most disc brake systems employ a floating caliper that slides on pins or some other mechanism. The floating design allows the caliper to move with the wobble (excessive run out) of the rotor.

Our cars have a fixed caliper system, so any wobble in the rotors will result in pulsations in the pedal. Excessive wobble will result in air being sucked in through the caliper's piston seals, thereby softening your pedal.

To address this problem, there is a very tight run out tolerance for the rotors. I've read that the rotors were originally surfaced following riveting to the hub/spindle to compensate for the vagaries of the hub/spindle machining. The service manual also mentions resurfacing the rotors while on the car for the same reason.

The problem with drilling the rivets out and allowing the lug nut tension to hold the rotors in place is that you get rust and crud in between the spindle/hub and the back of the rotor, which will cause run out that is significantly higher that the spec, which from memory I think is somewhere around .005". Even the way you torque the wheels can impact the run out.

By way of illustration, my new rotors exceeded the run out spec right out of the box once they were mated to the spindles/hubs. If you don't have access to a machine that can surface the rotors on the car, the next best thing is to shim them out until the run out is within spec with the rotor fixed to the spindle/hub. For ease of maintenance, I elected to tap the rivet holes and screw the rotors to the hubs/spindles rather than replacing rivets. With a little monkeying around, I was able to get the run out to within .001".

Gary Ramadei (ph) did a fantastic paper on this a while back in which he described exactly how to do it and what hardware is needed. I may have it in .pdf if you need it.

Tom------


You've hit on the reason the rotors were originally riveted to the spindles (rear) or hubs (front). Having them riveted together "stabilizes" the runout once the rotors are machined as an assembly with the spindle or hub. You'd be surprised how much variation in runout can occur just with the limited rotational movement possible with no rivets and just the lug bolts locating the rotor and hub/spindle. Of course, once the wheel is installed and the lug nuts tightened, the rotor and hub/spindle are "locked". But, just what "relative position" the rotor and hub/spindle are at when this occurs is uncontrollable. The rivets control it.

It is not generally possible or, even, advisable to machine the rotors with riveted or otherwise clamped-together rotor and hub/spindle. This is because in doing so, the thickness of the rotor is reduced. This is to be avoided. If substantial runout exists prior to machining, the rotor might have to be significantly reduced in thickness.

The factory didn't have to contend with this problem. That's because when they riveted the rotor and hub/spindle together, the rotor was UNFINISHED. So, they cut the rotor to finished thickness AND in-spec TIR in one machining operation. Unfinished rotors are not available in SERVICE.

A better way to correct runout is to use shimming and, especially, the tapered shims available from "Brake Align". Doing so avoids the need to cut the rotor, at all.

Another way of accomplishing it is to machine the spindle or hub to eliminate runout on the rotor contact surface. At this point, very little material removal is required even if the runout is substantial. Then, use a NEW rotor (NOT an original used rotor). New SERVICE rotors (without attached hubs or spindles) are usually machined to very close TIR tolerances between the hub/spindle contact surface and the disc brake rotor surface. So, when the corrected hub or spindle is mated to the NEW rotor, runout will usually be within TIR spec. If it's not, re-indexing the rotor one lug position ( 5 positions are available) at a time will usually get it well within spec. Then, use rivets or countersunk, flathead screws to "nail" the rotor to the spindle or hub. Done.

I think the reason that your new rotors produced unsatisfactory runout "out-of-the-box" was NOT the fault of the rotors. It was MUCH more likely the fault of the hubs and spindles. I have found original GM hubs and spindles to be WAY off. They apparently did not hold real tight tolerances on these parts since they knew that any runout would be corrected when the UNFINISHED rotors were attached and machined as an assembly with the rotor or hub. Even GM SERVICE spindles are often WAY off. I've had some that were up to .015" off as measured on the spindle face. This translates to TWICE that on the outer edge of the rotor surface when the spindle is installed on an otherwise PERFECT rotor.

[Joe L.]

Re: Rotors for 1970

I am looking to replace rotors on my '70. I found some for sale - see pictures attached. One of the front rotors has 6 holes in the circumference of the hub (not sure if that's the right term for the 'top hat' section). The other has no holes, which is how the originals are on my car. Does anyone know if the holes were correct for 70? Is there any other way to determine if the rotors are correct or not?

What is best way to restore rusted rotors - bead blasting or rust remover chemical dip? thanks, Steve

Steve-----


I've seen rotors with as many as 2 of these holes in the sides of the "hat", and I've always assumed they were used in balancing the assembly. I've never seen one with more than 2 holes until now. However, personally, I have no doubt that the holes in the rotor you have pictured are original to it. For one thing, there would be no real reason for anyone to drill these holes, especially in a rotor still riveted to a front hub. So, I think that this rotor proves that at least one assembly required more than 2 balance holes.

Regardless, I would be leery of acquiring such an assembly. Why, if it's original? Well, any assembly that required this much material removal to achieve proper balance would concern me. So, why would I want to buy anything that concerns me, whether that concern is justified, or not?