Options for Cobra rear rotors on the stock hubs

Kenz

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Quick question, when doing the Cobra rear cobra rotors on the Cougar.

The two options I have found for mounting the Cobra rotors is to completely re drill a seperate MN12 pattern, or to elongate the existing pattern.

Has anyone simply drilled out the existing pattern to a larger opening? I have a drill press and thought it might be easier to get a large bit and just open them up more. Would there be any disadvantage to doing this? It seems like it would be easier option. Also I will be using the hub adapters from the caliper relocation kit.
 
The rotors should be hubcentric, so elongating or enlarging the holes will have the same effect.
 
Its been a minute since I had the rear rotors/drums off of stock pattern hubs but I know for a fact in the front the hub has a step where a Mustang rotor is hubcentric to a MN12 hub despite the actual hub going into the wheel bore being smaller for the 5x108 wheels. If that's the case you can just oblong the holes in the Mustang rotor inward, which I have done to the fronts
 
Scratch that, it just came back to me when I first did the hub swap way back when I took a pair of MN12 rear rotors to get the hub center machined to the larger Mustang hub bore so the rear hubs definitely don't have that step the fronts do. You'll definitely need a hubcentric ring to use the Cobra rotors
 
Fast Ed includes the rings with the reloc brackets, if you ask.
 
I have the hubcentric rings already so sounds like I'm set. I'll be drilling out the holes in a few days. Will work my way up in size slowly and test it to get a tight fit. Might save others some time to know exactly what size bit to use.
 
IIrc, ours are 108mm, mustang is 114. so egging them the diameter of a 6mm screw is what you want. A drill press with an end mill the dia of a wheel lug would be perfect.
 
IIrc, ours are 108mm, mustang is 114. so egging them the diameter of a 6mm screw is what you want. A drill press with an end mill the dia of a wheel lug would be perfect.

I’ve done it with a large coarse round file, gets it done quicker than you’d think
 
You can use your stock rotor to lay out the second pattern on the Cobra rotor.

Good idea, I willdo that when I get the car apart. I actually used one of the Cougars original wheels, and my "clearance rack" Tractor Supply digital caliper.... it was a bit cumbersome but it got me a good enough measurement to order a few drill bits.
 
I was lucky enough to have a friend who went to a machinist he knows here and he made me a jig out of 1/2" steel plate with both patterns. We just bolted it to the new rotors and drilled them on a drill press.
 
I have the brackets to move out the rear rotors for use with a corvette 12" rotor. Any idea what year rotor and what's the diameter of this Cobra rotor?

Mark
 
I have the brackets to move out the rear rotors for use with a corvette 12" rotor. Any idea what year rotor and what's the diameter of this Cobra rotor?

Mark
The Cobra rotors are 330mm vented, just shy of 13"

296mm or 11.65"

I don't know about the Corvette rotors. If it helps though you can click on the "Info" next to the part numbers at Rockauto.com and it will give you details of most parts. It's not always accurate, it says they are solid not vented. This is from the Cobra rotors I am using...


1710871695452.png
 
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the Cobra rear rotors are 11.65", no moving them out and effectively altering track width, you just need relocation brackets to move out the calipers an extra inch
 
I looked up the wrong rotors on my previous post. Had to double check I bought the correct ones :poop:
 
This effectively makes running a 15" rim impossible, correct?
 
This effectively makes running a 15" rim impossible, correct?

Never tried it but I believe so. There’s virtually no perceptible benefit in terms of braking performance over the factory rear discs, so unless you’re hub swapped and there’s no other good rotor choices or you’ve got larger diameter wheels and don’t want the brakes to look puny it’s not really worth the trouble
 
There was a time almost 15 years ago when I had to put on my old stock 15" salad shooter as a spare when one of my 17" tires busted in the garage or the driveway. I don't remember why I didn't use the 17" spare in my trunk, but the stock 15" cleared the relocated rear caliper by less than the thickness of a business card.
 
Never tried it but I believe so. There’s virtually no perceptible benefit in terms of braking performance over the factory rear discs, so unless you’re hub swapped and there’s no other good rotor choices or you’ve got larger diameter wheels and don’t want the brakes to look puny it’s not really worth the trouble

Not expecting an improvement for the brake system at a whole. My goal is to hopefully improve the parking brake. It has always been much weaker compared to the Tbird I used to own. The hope is that with new calipers, better pads (semi metallic), and bigger rotors it has more holding force. The way it is now I have to really crank on it to hold the car on a hill. It has always been that way, even before I switched to the handbrake.

Also doing it for looks, the factory rear rotors look puny behind even 17's. I bought coated non drilled rotors so the unused portion of the disks should stay clean and not rust.
 
Not expecting an improvement for the brake system at a whole. My goal is to hopefully improve the parking brake. It has always been much weaker compared to the Tbird I used to own. The hope is that with new calipers, better pads (semi metallic), and bigger rotors it has more holding force. The way it is now I have to really crank on it to hold the car on a hill. It has always been that way, even before I switched to the handbrake.

Also doing it for looks, the factory rear rotors look puny behind even 17's. I bought coated non drilled rotors so the unused portion of the disks should stay clean and not rust.

If your parking brake is already weak now it isn't going to get any better from a rotor diameter change, anything to improve parking brake performance is going to be in physical adjustment of the cables, pad quality and if they were properly bedded to the rotors. If you're parked on a hill and the car is creeping down with the parking brake set it'll still do that regardless of the rotor choice


FYI a dirt cheap and equally effective workaround to coated rotors you can use header paint on that center portion of a new set of uncoated rotors and fins too, just mask a very slight overlap onto the swept area and the pads will quickly "machine" the excess paint off. Mine have held up for almost 6 years

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I've used caliper paint on my rotors for years and it also holds up very well, just make sure to prep everything really well or after a few years paint will start to flake off. I always use a razor blade to clean off extra paint from the contact surface, I ruined a set of pads once by using the pads to clean off the extra paint.
 
Will the old bracket extension work with the cobra rotors? I had never installed the brackets I got years ago from someone on the TCCOA. I remember they used corvette rear rotors but I'm not sure which ones and I did search the old site to no avail. I got these brackets in the late 90s.

Mark
 
Will the old bracket extension work with the cobra rotors? I had never installed the brackets I got years ago from someone on the TCCOA. I remember they used corvette rear rotors but I'm not sure which ones and I did search the old site to no avail. I got these brackets in the late 90s.

Mark

I imagine the Corvette rotor might just be a mod lost to the ages with better alternatives coming to light, I actually never heard of it until now, I remember late C4 Corvette front Calipers being a thing since they’re the same as 13” Cobras but nothing about the rears.

If the brackets you have are designed for a true 12” rotor they might place the calipers in such a way on the Cobra 11.65” rotors where the pad isn’t fully contacting the rotor. In terms of inboard/outboard spacing no clue
 
I'm thinking it has to be the C4 vette given the time frame. As I recall and this was a long time ago the corvette 12" rotors only had to be predrilled to work. My front brakes are AP Racing four pot calipers and I'm running 13" rotors. I was the only guy that had 13" rotors and 16" wheels. There was maybe a matchbook cover of clearance so no much.

Mark
 
I'm thinking it has to be the C4 vette given the time frame. As I recall and this was a long time ago the corvette 12" rotors only had to be predrilled to work. My front brakes are AP Racing four pot calipers and I'm running 13" rotors. I was the only guy that had 13" rotors and 16" wheels. There was maybe a matchbook cover of clearance so no much.

Mark
That’s right I remembered that about your car. I soon went with the 17”+ wheel crowd but when I was first familiarizing myself with the forum and platform I really just wanted to stay in the 16” wheel range because I thought they looked better, seeing your car with 13” brakes and 16s was one of those mental notes I still seem to have archived :tongue:
 
Will the old bracket extension work with the cobra rotors? I had never installed the brackets I got years ago from someone on the TCCOA. I remember they used corvette rear rotors but I'm not sure which ones and I did search the old site to no avail. I got these brackets in the late 90s.

Mark
Here are some measurements of the brackets for the Cobra rear rotors.

PXL_20240320_194425086.jpgPXL_20240320_194556621.jpgPXL_20240320_194730335.jpg
 
Think I might need to slide my nephew's 15*9 Welds over my brakes when I get them done. Would be great if I could run a 295 drag radial on a 15.
 
Was Dave xr7-dave? Was fast ed yet another person? I seen to remember them both, but they are the same In my scrambled memories.
 
XR7 Dave, Dave Dalke, Super Coupes Unlimited Inc.

fast Ed is in Canada and is/was a member of the SCCoO(ntario) chapter of SCCoA.
 
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