[Terry M. Hunt]

Here are some pics of the front rotors on my 71 GTO. I bought the car new and these are the original rotors with 110,000 miles on them. The passenger side rotor has a groove around it just beyond the hub. I’m not positive, but I seem to recall this groove was always there. I even seem to recall seeing the same groove on the driver side rotor as well, but not positive.I never quite understood what this groove was. I speculated that it was some kind of wear indicator that would disappear as the rotor wore down and approached minimum thickness. If this is correct, I’m now puzzled that the driver side rotor has no groove. Seems odd that one side would wear so much more than the other. If this groove has nothing to do with indicating wear, was it just the way these original rotors were made? It seems odd that the passenger side could have been damaged/scored in some way that would produce this groove that looks like it was machined into the rotor. I have found a couple of what are described as NOS rotors for the car on line and the photos of them show no groove. I’m getting a very dry scraping sound when braking. A quick look at the pads through the caliper seem to show plenty of material left. I will be tearing the thing apart to complete a diagnosis, but would love to hear any theories on this groove in the rotor first.

[67drake]

I assume those are old rotors? I haven’t seen a grooved one in years myself. I believe they were machined like that to stabilize the pad as it wore in for less chatter.

[70gtojosh]

I have not been looking at or working on these cars for 40 or 50 years but I have been for 20. Never seen a groove that deep. Some kind of manufacturing defect? Someone tried to machine them and their equipment went haywire and they put it back to service anyway? A rock got jammed in there and tore the rotor up? I don't think you could machine it back flat again and still be within the useable thickness. I think that would cause a noise at some point when the pad wears into the groove. I would be looking for a different set of rotors get a new finish on them, new pads and rock n roll.

[1965gp]

I seem to remember my 70 GTO having grooves in the rotors. I believed they were original as the car only had 65k on it and was pulled off the road in 81. I thought they were a wear indicator but I don’t think they were as wide as the groove in yours. I think about 1/16th on an inch?

[Sirrotica]

I have been working on them for 50 years, it's not a defect, it was engineered into the rotor design from day one by GM.

One answer I was told previously, is it was designed to carry away the dust particles from the wear of the disc pads. Another was it was designed to cancel brake squeal. One other I have heard was to release the gases from the resin used to bond the pad material together as it gets hot during use. The final one I had heard was to remove water after the vehicle had been driven through deep water, such as a puddle. Is that the truth, or someones imagination? I can't give you any more than those explanations.

If it was for noise cancellation it didn't work very well because they still squealed with the groove. The best way was to glue the backside of the pad to the caliper with anti squeal glue, or RTV, both, worked to minimize, or stop squeal.

Back when these cars were new, or just used cars, they all had that groove machined into them, as did the replacement rotors. In 1970 (senior in high school) I was doing my Vo Tech, Co op training at a Pontiac dealer in my hometown. They all had the grooves in the rotors when delivered from GM.

I haven't changed one of these rotors for decades, so I have zero knowledge if replacement new rotors have the groove, or not.

[64-3Deuces]

Originally Posted by Sirrotica

I have been working on them for 50 years, it's not a defect, it was engineered into the rotor design from day one by GM.

One answer I was told previously, is it was designed to carry away the dust particles from the wear of the disc pads. Another was it was designed to cancel brake squeal. One other I have heard was to release the gases from the resin used to bond the pad material together as it gets hot during use. The final one I had heard was to remove water after the vehicle had been driven through deep water, such as a puddle. Is that the truth, or someones imagination? I can't give you any more than those explanations.

If it was for noise cancellation it didn't work very well because they still squealed with the groove. The best way was to glue the backside of the pad to the caliper with anti squeal glue, or RTV, both, worked to minimize, or stop squeal.

Back when these cars were new, or just used cars, they all had that groove machined into them, as did the replacement rotors. In 1970 (senior in high school) I was doing my Vo Tech, Co op training at a Pontiac dealer in my hometown. They all had the grooves in the rotors when delivered from GM.

I haven't changed one of these rotors for decades, so I have zero knowledge if replacement new rotors have the groove, or not.

Yes, that was a GM design in the early 70’s thereabouts. I’ve seen NOS GM rotors from that era listed for sale that had the groove—see pics -71 Chevy Impala/Caprice. Not sure when the groove was discontinued and I don’t believe later replacement rotors have one. With the exception of reducing brake squeal, the reasons you stated for the groove make sense, the most plausible being the release of brake pad gasses. When the pads get hot the pad binders release gasses that prevent full contact of the pad against the rotor. That’s one of the reasons why high performance rotors are drilled and/or slotted.

As to the OP, there is a totally different wear pattern on the driver’s side rotor. That shiny ring closest to the hub appears to be where the groove was. Mic both rotors and compare the wear difference between the two.

[Held for Ransom]

http://www.forums.maxperformanceinc....d.php?t=677580

Might want to search for the assembly date stamp to see if original. They still had the groove in '74.

[scott70]

my 70 had the grooved rotors when I bought it in 86'. Havent looked recently but there was a vendor that had aftermarket rotors with the groove made into it.

[70gtojosh]

Ya I don't recall ever seeing rotors like that. Thanks everyone for "learning " me on that.