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Art DeKneef
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I thought I saw this somewhere but can't find it now. I thought I saw the minimum and maximum.Does anyone know what the acceptable thickness of the front rotors are?

Lash Russell
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Hi Art,

  I could be wrong but I believe that the rotors are not recommended for re-turning, not sure why.  Having said this I have had my rotors turned on different JH's I've had and it worked out fine.  What I did was have the shop take off the minimum amount possible.  In all of the cases where I've done this, the rotors needed surfaces cleaned up only as opposed to turning to get warped rotors straight which could require allot of meat removed.

Hope this helps,

Lash

Greg Fletcher
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Lash is right. The rotors are very thin to start with, it's not recommended by the manufacturer to turn them. However, like many thrifty JH owners, I too have also had a local machine shop turn a set for me some years ago, only taking off a small amount. Keep in mind that some shops these days will not want to do it since they don't know what the "factory minimum thickness" is. I've never seen that myself, not sure if it exists. You'll need to find a shop that will work with you.

Art DeKneef
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Thanks both for the info. The rotors don't look bad except for the surface rust that is present. I was looking to just clean them up so to speak. I'll talk to the guy at the shop and see what he thinks.

JodyKerr
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Art,

Some will hear what I say and scream bloody murder. But if it's just a little bit of surface rust, knock it back with fine steel wool. Once the brakes are used a few times they'll finish the cleaning job. Any car that doesn't get used often picks up a little rust on the rotors. The next time you drive it, the rust gets knocked off.

Jody

Dan Eiland
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Hi Art,

According to my shop manual it says the permissable run-out of the disc is 0.004 inch or 0.1 mm. Found this on page 10 in my shop manual under Brakes.

Dan Eiland

colinw59
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I had a slight shimmy on braking about 50 or so on 15851. I had both rotors turned, removing the minimum material. Just got back from a 500 mile road trip and the brakes are great. Let's see how long they stay true.

Greg Fletcher
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Too much runout will create wheel wobble and pulsing during braking- that .004 sounds pretty high. I would think below .002 would be a better target to shoot for. That number does nothing toward giving us a minimum thickness for the rotor of course. I've never had had problems getting JH rotors turned, but all thin rotors have the same theoretical problems, you increase the risk of heat fade, cracking and failure. That is why most rotors have a "discard thickness" or "minimum machine to" specification on the casting.

Art DeKneef
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I thought that also. But after looking at 4 rotors, I see no such numbers anywhere.

Best I can measure the rotors I have show thickness of:

76 GT 51,000 miles - 9.14 mm and 9.03mm

74 JH 34,000 miles - both are 9.40mm

Since I have no history on either of these cars as they were parts cars, I could speculate that the GT had the rotors turned at some point.

Art DeKneef
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Here's an update on the rotor thickness for when you might need it.

I was at Delta Motorsports today and some how I remembered to ask Jim if he knew what the thickness of the rotors should be. He said he got the information from where the rotors came from, Vauxhall.

According to the shop manual he had:

.378" or 9.6 mm when new

.343" or 8.7 mm minimum permissible

At least we have some numbers to roughly go by now.

pbahr
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I can confirm that the thickness that Art presents are accurate.

The key to haveing the rotors machined is to go to a shop that knows what they are doing.  I've had a couple of rotors screwed up by jerks who don't know what they are doing.

For road use, I don't see any problems with machining down to the allowable minimum. 

If your rotors are really that bad, just buy a new set..........

YELODOG

pearst
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Your comments on the condition of my rotors would be appreciated. Too far gone to have cleaned up? If so, where to buy new ones?

Attachment: rotors.jpg (Downloaded 137 times)

pbahr
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Those rotors look fine to me.

Do you have any problems braking?  If you do, have them turned down by a reputable shop (if you can find one).

YELODOG

pearst
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Somewhat soft pedal with lots of fade.

JodyKerr
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Sounds more like a Master Cylinder/rubber brake line issue.

Jensen Healey
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If the MC, rear brake cylinders and the brake servo are OK, get some Hawk 118 pads and braided brake hoses. Flush the system with Dot 4 fluid. 

pearst
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Thanks fellas!  New pads and SS lines are already on the way.  Still considering whether I resurface those nasty looking rotors or not.  Will inspect master cylinder tonight.

jomac
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10mm thick design , min thick 8mm



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