72455

Hey all I saw this post in the classifieds https://classicoldsmobile.com/forums...***-442-a.html
and noticed that the rotors are grooved. Why?

Thanks,
Dave

don71

I would guess to allow not only water but small debris to leave the pad and rotor surface for more effective braking.

Allan R

Because they're NOT stock rotors. They're aftermarket.

72455

I wondered...because when I changed rotors and brakes on my 72 Supreme my rotors were grooved exactly like these. I thought there was something wrong with them so I tossed them.

oddball

I thought some of the pre-70 rotors came from the factory with that groove?
Just speculation, but my guess is that groove is a wear indicator. It'll show the minimum thickness for the rotor - it's trash once the groove isn't visible. Good for folks that use "lifetime" pads that wear the rotors instead of wearing the pads. Those were quite vogue a few years ago.

jaunty75

Alan, do you want to hang your hat on this?

I looked at the brake sections in the '70, '71, '72, '73, '75, and '78 chassis service manuals, and they ALL show one or two grooves in the disk rotor whenever they show it in a figure. Below is a figure out of the '70 manual, and those things being pointed at by the red arrows sure look like grooves.






Here's a figure out of the '78 manual. One groove right in the middle. I didn't put an arrow because it's obvious. The same exact figure appears in the '75 manual.




Any aftermarket rotors I look at for the early '70s Cutlasses at rockauto, and there are several brands shown, do not have grooves. I'm wondering if they were put in originally as a heat dissipation thing? I'm not sure that that makes sense, though, as one groove wouldn't dissipate much heat. Maybe helped channel brake dust out of the way?

In any event, it certainly looks like there WERE grooves on these rotors originally.

jaunty75

Didn't think of this! Good guess. I'll bet you're right. I wondered what "discard dimension location" means in the second image I posted above. Does that have anything to do with the groove?

cdrod

My first car in high school (1980) was a '71 Cutlass Supreme that had the original factory rotors with this groove. I assumed that was a wear indicator.

Rocketbrian

Agreed, the groove was definitely original equipment. Non grooved are aftermarket. Inline tube are now offering the grooved versions at premium price. They refer to it as having the correct wear groove.

tru-blue 442

My 70 442 has the grooved discs as well.

11971four4two

Was there not a bolt on hub
TWO 2 Piece from the factory???
3 of my cars with GM OEM rotors have a groove in them....