Min. thickness for Jag rotors in a Unique?

[cecoen]

I am working on an older Unique with the Jag rear end and what looks like factory Jag inboard brakes.
I went to my trusty Autozone for some rear pads today and asked what were the min thickness for the rotors and the computer came back with ****DO NOT TURN ONLY REPLACE*******
So he didn't have a clue what the specs are. Any one know? Or at least know what model Jag these rear ends are from?
The rotors in this car are not the solid units like what is being used by newer Uniques, they look old and kind of like a vented rotor but it has some sort of ring between the inner and outer surfaces of it? Am I making any sense here?
I do not want to replace the rotors if they don't need it. It looks like getting the rotors off this car is one heck of a task. Thanks in advance.

Chad

[Tony Radford]

Chad, better check with Bill down at Southern and maybe Alan Weaver at Unique. Either/both should be of help.

Tony

[Bob Putnam]

Chad,

The ring is an anti-squeal damper that's used on some Jag models. I haven't seen any aftermarket rotors that use it, and with the right (Lucas/Girling) pads, we don't get squeal anyway.

The Jag manual doesn't give a minimum thickness for the rotor. They simply say
"Replace the rotor if it is not smooth or has ridges." They're not expensive.

[Jon Miller]

I ran into the same problem. There was no minimum thickness listed in the Pep Boy's book so they did not want to turn them. The problem was solved when we turned the rotors over and saw the minimum thickness dimension cast into the inner side of the rotor. It looks like .45, but with them installed I can't get a clear view. It cost me $5 each for the surfacing, you may want to take another look at yours before you replace them.

-Jon

[Cal Metal]

Chad:

Any particular reason you are replacing the rotors? I wouldn't think that are warped. As someone said, there is usually a minimum thickness number cast into the rotor.

[cecoen]

Cal,
I just wanted to be sure they are not dangerous. If they are too thin then they are libel to warp right away anyway. They do look smooth but really glazed over looking. This Unique is about 20 years old with only 2K miles on it and at some point in it's life the brake fluid soaked up a lot of moisture and air so the brakes weren't even being used to their full capacity. One of the front calipers wound up being locked up and the rotors were right at min thickness. So I wanted to be sure the rear calipers were working and the rotors are in good shape. I was worried that when I put it all back together and they are working at full capacity like they are supposed to then the weak parts would show themselves.
I found that the rear pads were about 90% good but really glazed over. I roughed the surface of them back up and re-installed them. But I am worried about the rotor being to shiny for the pads the bed down again. I have heard that the mating process is crucial to good braking?
Thanks Jon, Bob, and Tony for all the help so far!

Chad

[chopper]

I'm going through the same drill with the rear brakes on my car. I talked with a local shop, and he said his literature lists the minimum thickness of the solid rotors as 11.4 mm or 0.450 inches. I'm not familiar with any other type of rotor for the Jag rear brakes, so this may not be very useful in your case.

And, yes, getting the brakes off the Jag rear end is a task. When I called ERA to ask them how the rear calipers came off, they told me "Usually with a lot of swearing." How true, how true. You need a relatively long box-end wrench to break the two bolts loose which hold the caliper to the differential housing, since you have to remove them from the differential side and you can't get any type of air wrench in there (at least, I couldn't). I think all of the rest of the bolts will come off pretty easily with an air hammer if you have one. Once you have the half-shaft loose at the disc, remove the shocks/springs and you can swing the lower arm down to gain enough clearance to remove the brake disc. Hope this helps.

[cecoen]

Thanks Chopper! I thought I was going to have to take it apart entirely to get the calipers and rotors off. I didn't realize that you can just disconect the half shaft from the dif and move it out of the way, too cool. I was stressing about having the hubs pulled off and pressed on. Glad they are not even part of the problem.

Now what about that hole in the hub? The one with the little cap. It appears to be some sort of grease port? Any idea what I am talking about? Should I just stick the end of the grease gun in there and fill 'er up?

Thanks,
Chad

[wilf leek]

Chad - I looked this up in my original Jaguar repair manual.

Firstly, I thought all inboard rear discs were solids - i.e. not the vented type. Anyway, this is the data for the solid discs:

New thickness - 0.5", min thickness 0.45"

For the fronts, new thickness - 0.95", minimum 0.90" (for the vented type discs)

Sorry - translation English to American is discs = rotors! Can't help myself.

Other data:

max permissible thickness variation = 0.0005 (yes, 1/2 a thou)
surface flatness - within 0.0004
max run out - 0.002

If you do fit new discs, ensure the shims between the disc and the diff drive hubs are correct to centre the discs in the calipers. Plus rear end camber is adjusted by the shims between the discs and the driveshafts.

HTH

PS - same workshop manual says "Remove hub bearing grease cap and inject wheel bearing grease until no more will go in. Do not pressurise the grease, or it will be forced past the oil seals. Refit the cap"

[Jon Miller]

It may violate the "Do not pressurise the grease" rule, but this is what I did. I made the aluminum plugs and installed zerk fittings. CWI sells these if you don't have access to a lathe.

[cecoen]

Wilf, AWESOME! Just the info I was looking for! Thank you.
I got the brakes put back together and bled them really well and now I have a firm pedal, finally!

Jon, great idea for the grease caps. I will look into putting a grease fitting in the cap that is there now.
What wheels are you using? The wheel studs in the pic look much larger than the Halibrand wheel studs I have?
I have the MG brakes up front and had to drill and tap holes in the rotors for the studs. To make a long story short, I screwed up one of the new rotors tapping the holes and want to try a larger stud. I think I can salvage the rotor if I can find a larger threaded stud(I tapped the holes one size too big on my rotor). Thanks for all of the info.

Chad

[Jon Miller]

Chad,
I've got Trigo wheels. The studs thread on over the existing lugs to hold the wheel adaptor on.

-Jon

[chopper]

Wilf:

Forgive the ignorance here but, considering that the calipers are fixed and have pistons on both sides. what difference does it make if the discs are perfectly centered in the calipers? I mean, assuming that the disc is reasonably well centered, the pistons will merely adjust to the disc position. I ask this because my calipers had no shims, and the discs were just slightly off center (a couple of thousands, just barely noticeable to the eye). I can see that it would make a difference when you get down to the last few thousandths of the pad material; in that case, one side would wear out completely before the other. However, I've never changed brake pads because the pads wore out, always because the calipers started peeing brake fluid out the seals and contaminated the pads. My old Vette, with stainless steel sleeved calipers, usually required piston seal replacement every two or three years or they would leak. That's what happened this time, too; the caliper seals started to leak and then the pads got fluid on them. I've had several cars with fixed caliper disc brakes and never heard of shimming the calipers to center the discs. Am I grossly misinformed?

Jon Miller:

Forgive THIS ignorance, but who is CWI? Do you have a point of contact and/or phone number? That is a nice installation; seems like a good idea to be able to routinely lubricate the carrier bearings.

[Jon Miller]

CWI (Concours West Industries) is a great source for source for all kinds of suspension stuff. Here is the contact info just in case the link does not work.....

306 Dwight Rd
Castle Rock, WA 98611

phone-(360)274-3373
fax-(360)274-3376
http://www.geocities.com/concourswest/index.html


-Jon

[wilf leek]

Chopper.

A moot point. From a "pure" or empirical engineering standpoint, centering the rotors in the calipers would be the natural thing to do. Stops pistons one side or the other from over-travelling when the pads wear thin. Also, with gross misalignment, the rotors might actually catch somewhere on the caliper.
Lastly, the handbrake caliper arrangement wants to "see" a centralised rotor. Any drag on those handbrake (sorry, emergency brake) pads overheats them and they can detach themselves from their backings. Had that a couple of times before I learnt how to adjust them properly.

In practice, a few thou off is probably good enough. I got mine to within that kind of tolerance, and it did take some shimming.

PS edit - Chopper, I just now realised that you were talking about shimming the calipers - this is not how it works, it is shims between the rotors and the diff drive hubs that are used to centre up the rotors, the calipers just bolt straight on.

Then more shims between the rotors and the driveshaft mating surfaces (i.e. on the other side of the rotor) set the rear end camber.

[excelguru]

This is a very interesting thread! This site rules!

Chad - When did you get a Cobra??!! I must have missed something. Congrats! Chill is having a lunch at his place on the 15th. You should try to make it. You might get a lot of good info. See the thread in the DSCC forum for details.

How often (in miles) should I check the rotor thickness? I have about 2700 miles on it now (that's right, 2700 miles in 4 months).

Also, I seem to remember someone telling me about different brake pads (for the front) that would provide substantially shorter stopping distances (but the pads wore out faster). Mine has the standard pads from Unique. Does anyone know the pads I'm referring to? I think they were Willwoods, if memory serves. The car stops fine, but it could be a little better.

Keith

[cecoen]

Keith, HA, GOTCHA! Actually, it is Chip's unique. I am just tinkering around on it doing some tune up and detail stuff. Talk about wanting one now, after having use of one for a little while I have to get in gear and get one!

I think Chip and I are on for the 15th but I need to email chill to be sure and get some finger bowls reserved for us!

I wouldn't worry about the rotors on your car. Chip's car is almost 20 years old but only has about 2K miles on it but who knows what the condition of the rear end was when it was built? And the brakes weren't working right, so there was some unusual wear on some of the brake parts but not on others. So I wouldn't worry about yours until you need brakes and rear brakes are usually not needed for quite some time after fronts are needed as they do MUCH less work to stop the car.

I just didn't even know where to start to make his car stop better. So I thought I would ask about the rotor thickness while I had it apart.

Keith, will your car lock up under hard breaking? I can't get chip's to lock up yet but I still have to bleed it out some more although it already stops WAY better than it used to. And it still seems to "nose dive" when I get on the brakes hard like maybe the rears aren't doing quite enough yet.

Chad

[excelguru]

I have NEVER been able to lock up the brakes on this car. And that includes a hard braking incident from about 100 against a Toyota Supra Turbo. When I really get on it, the nose will dive a bit and the car gets "squirrely", if that makes sense. I attributed the squirreliness to the high-profile tires, but maybe I was wrong in that assumption.

I almost forgot, I also got on it VERY HARD once to avoid rear-ending a guy who was driving WAY too slow on the interstate. And I do mean HARD. I finally matched his speed when I was 4 inches from his back bumper (no exaggeration). I had to check my shorts after that one (they were okay). The car did not lock up during either incident and I felt that it should have slowed even better (especially when I saw that Minnesota license plate coming at me).

Does it sound like I have the brake bias set too far to the rear? I have no experiences to compare it to.

I have driven in other cars where the brake pedal feels like you're pressing against a brick wall. It feels like the pedal isn't moving at all. In those instances, I've had to practically STAND on the pedal to get the car to stop. I'm not having that problem, my pedal has a nice tactile feel to it.

What do you guys think? Need more info?

Keith

[cecoen]

Sorry Keith, I think I am at the other end of brake balance. I think my rears aren't doing enough. I will however be trying different settings so I will let you know the results.

It sounds like we have similure pedal feel. Mine feels good but just like I can't press it quite far or hard enough to get lock up. Close though.

I know that feeling of not stopping fast enough no matter how hard you press! Been there done that! I hate not being able to controll the amount of force for the brakes. I want them to be able to lock up at will then if I have to stop quickly I can and not worry about weather or not it will stop in time. Man, can't say enough how much I hate that feeling of pressing the brakes as hard as I can but it not stopping enough! Helpless feeling. I must make it stop well, I must make it stop well, I must make it stop well. Wishfull thinking!

Chad

[acaskie]

Keith,

You may want to try Carbotech in Florida.They make excellent replacement pads for Wilwood calipers (ceramic, metallic, a combination, what ever you want) with characteristics determined by your particular preference for wear, operating temperature, etc. and depending on your mix of street/track driving. You just tell them what you want and send them a tracing of your pad. Cost was in the $60 range if I recall correctly. Much improved stopping distance--wear moderate.

the address is: http://www.carbotecheng.com/main.htm