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Composite brake rotors.

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I found out the hard way that my 96 Regal requires composite brake rotors. Don’t know if it is because of the thick factory aluminum rims or if the car came with short wheel studs.

 I ordered a set of Full Cast rotors from the rock and because of the thicker flange I could only catch 3 lug nut threads.  Three threads are not acceptable so I finally talked the rock in to taking the Full Cast rotors back.


Went to the Dealership and ordered two rotors that were supposed to be composite but both turned out to be Full Cast. Ended up finding two correct composite GM rotors on ebay. The two NOS rotors I purchased are over 20 years old but were my only option.


Has any one run across this before?




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These cars are way way too mass market to require anything more exotic then cast.

You have stock wheels right?

It almost sounds like someone previously broke the studs and replaced them with short ones - this was done because you don't have to loosen the hub assembly to install them. It works with steel wheels, not so much with aluminium.

I'd examine the length of your studs and see what you can get your hands on. I probably have a few of the proper length out in the garage if you need an approximate measurement of how long they should be. Either way once you loosen the 4 hub bolts on the hub it will allow you the clearance to slide the proper ones in.

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 Not sure you understand that most w’s came with composite rotors that have steel mounting flanges like in my picture above. Every GM car I have owned came from the factory with composite rotors.

My 96 Regal is all stock with factory aluminum rims and nothing has ever been changed out. With the factory composite rotors each lug nut catches 6 full threads.

Edited by ron350
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Composite brake rotors have been around since the early 1980s. When they were introduced, everyone was worried about soaring fuel prices (just like today), and auto makers were scrambling for ways to take weight out of their vehicles so they could meet government Corporate Average Fuel Economy (CAFE) standards. So one of the changes that was made was to replace heavy cast iron rotors with lightweight composite brake rotors.


This type of rotor derived its name from the fact that it combined a stamped steel center hat with a cast iron rotor. Thus, it was a composite of two different materials. The new design proved to be about 20 percent lighter than a conventional one-piece cast rotor and saved up to a couple of pounds per rotor.

The composite design also allowed the rotor disc on some applications to be cast out of a special "dampened" gray cast iron for added noise suppression (dampened cost iron is not structurally suited for use in a one-piece cast rotor).

Some of these rotors also featured redesigned cooling fins for better heat management. Some were also directional for use on either the left or right side of the vehicle. Directional rotors are not interchangeable side-to-side because reversing the direction of rotation changes the cooling characteristics of the rotor.


Some of the earliest applications for composite rotors were the 1982 Lincoln Continental, 1984 Ford Mustang SVO, 1987 Ford Thunderbird Turbo Coupe, and 1988 Ford Taurus and General Motors front-wheel drive "W" body cars (Buick Regal, Oldsmobile Cutlass Supreme and Pontiac Grand Prix). Since then, the number of vehicle applications has continued to grow.


As the vehicles with composite rotors accumulated mileage and came out of warranty, the aftermarket discovered that composite rotors required some special service procedures. Because the stamped steel center hat is not as thick nor or rigid as that in a one-piece casting, the center hat on a composite rotor must be fully supported with special adapters or oversized bell caps when the rotor is resurfaced on a brake lathe. The other alternative is to bolt the rotor to the hub (with the lug nuts reversed to provide better support and to prevent deforming the center hat) and to then resurface the rotor with an on-car lathe.

If a composite rotor is not supported properly, it can wobble and flex creating runout and surface finish problems. Both sides of the center hat must also be clean and rust-free for the rotor to turn true.


Some of the early composite rotors experienced corrosion problems and were recalled for replacement. Ford switched some of its models back to conventional one-piece cast rotors for awhile, and issued a service bulletin (#91-8-9) saying it was okay to replace composite rotors on the 1986 to 91 Taurus and 1988-91 Continental with one-piece cast rotors (P/N F10Y-1125-8). The corrosion problem is mostly history now because the OEMs now apply a protective coating to the stamped steel center of the rotor to inhibit rust. Aftermarket composite rotors typically use stainless steel for the center section to prevent rust, and the circumference of the center section is also dovetailed (notched) for increased strength where it mates with the rotor.

Vibration problems were also blamed on the design of the composite rotor. But the underlaying cause more often than not turned out to be improperly torqued lug nuts. Any rotor can warp if the loading on the lug nuts is not even. That is why an impact wrench should never be used to tighten lug nuts, unless it is used with a torque-limiting socket. Otherwise, a properly calibrated torque wrench should be used to tighten the lug nuts in a star or cross-pattern sequence.


When worn or damaged composite rotors need to be replaced, be careful not to intermix rotor types side-to-side. Rotors should always be the same type on both sides. Replacing a composite rotor on only one side with a cast rotor may create a brake pull. So do not replace a composite rotor on one side of a vehicle with a cast rotor unless the rotors on both sides are being replaced.

Cast replacement rotors for vehicles that were originally equipped with composite rotors are available from various aftermarket suppliers. But other aftermarket suppliers recommend against substituting one type of rotor for another because the cross-section of the center hat on a cast rotor is significantly thicker than the stamped steel center hat on a composite rotor.

The difference may range from 1/8 to 1/4 inch or more depending on the application. This may not sound like much, but it does reposition the wheel slightly further out on the hub. This adds positive steering offset and alters the scrub radius of the steering geometry. The amount of change is not great, but neither is the amount of scrub radius that is designed into many vehicles. Scrub radius affects steering feel, steering effort and steering feedback. It also plays a role in the way braking and engine torque affect steering.

The change created by replacing a composite rotor with a one-piece cast rotor may be enough to alter the scrub radius from negative (which is the case on many front-wheel drive cars) to zero or positive offset. This, in turn, may create a noticeable difference in the way the steering feels and reacts, especially on cars with rack & pinion steering that are especially sensitive to steering feedback.

There is also a concern that substituting a thicker cast rotor reduces the overall length of the lug studs, which reduces the number of threads available for the lug nuts to retain the wheels (especially on thicker alloy wheels).

Suppliers who subscribe to the "replace same with same" philosophy say you are apt to encounter far fewer problems when you install replacement parts that are the same design and function as the original. Those who do not subscribe to this philosophy say there is often room for improvement over the OEM design.




Good Luck!

Edited by Cutlass350
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It does sound like there's something incorrect with your wheel studs. I have always tried to get factory composite rotors on mine, but the vast majority of first gens at this point in their life have plain cast rotors on them and there's no issue with thread engagement.

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1.  I used to buy "all cast" rotors for my '92 Euro 3.4.  In the end, it didn't seem like those rotors made any real difference in rotor warpage.  I don't remember if I've got "all cast" or composite rotors on the vehicle now.  Never had a problem with stud length and OEM wheels/lug nuts.

2.  Look at most any GM brake drum from the last fifty or sixty years.  They're "composite" too--formed-steel center, cast-iron friction surface.  No one complained about them.  Buick used to have aluminum drums with an iron friction surface.

Edited by Schurkey
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OK here is a picture of the rim and the studs. The studs measure 1.300” off the face of the hub. The studs have a smooth blank section on the end of each stud with no threads that measure 0.300” so that only leaves 1 inch of threads on each stud. All 20 studs are exactly the same.



Regal Brakes 025A.JPG

Regal Brakes 028B.JPG

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I just compared your studs to a factory one off a '94 Regal. They appear to be identical length, with the exception of the '94 stud not having the extra blank space at the end. There's only about 1 1/8'' worth of threads on the stud(measuring very top thread to very bottom). Have you compared the thickness of your factory composite rotor to the other ones you've tried?

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Yes all of the steel center flanges on the composite rotors measure 0.100”or 1/10 inch.

The full cast Power Stop rotors measured 0.250” or ¼ inch thick.

 Looking at the lug nuts seams like the factory only wanted 6 (six) threads of engagement on each stud. Each lug nut only has 7 (seven) threads in each nut and the back section is hollow so that the nuts can’t be threaded on aluminum rims backwards.

Every thing will be fine now that I found the NOS composite rotors pictured above.


Regal Brakes 032A.JPG

Edited by ron350
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Even though you've solved your problem, I've added a picture of the used(probably 220,000 miles) factory stud I removed. Maybe it'll help someone on a future search...



Edited by mfewtrail
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Guess I learn something new every day. I had no idea.

Granted it all seems to be academic at this point since I don't see any listings for composite style ones now.

I will say that in all the years I've been turning wrenches on these cars I've ever had a rotor fitment issue like what you describe, that is still really weird to me.

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  • 3 weeks later...
On 11/16/2020 at 7:54 PM, ron350 said:

Regal Brakes 025A.JPG

For what it's worth now a few weeks later, I've owned 3 W body cars and this doesn't look out of the ordinary to me.

As far as brake rotors, I bought an EBC front kit for my '93 Regal of their "OE" rotor line with "red" (their mid level I believe) pads. Have been very happy with it. Not sure as to whether the rotors utilized composites or not but believe they are the best brakes I've bought. I had to wait some time for them which was fine as I actually believe they were made to order. They are made in England; rotors were finished in USA (rust prevention).

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