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Posted by: rjames Apr 19 2010, 12:31 PM

Slight warpage on both my new rear rotors. From an old thread I found (that I can't again find now) the conclusion was that new rotors do not need to be turned. However, I notice that after installing the new rear rotors there is slight warpage on both of them. I estimate the warpage to be roughly .004".
Is this a big deal? Should I just run them and hope it isn't noticable when braking?

I'm getting ready to bleed the brakes after installing PMB calilpers, new pads, rotors and lines, but I don't want to waste time trying to perfect the feel of the brakes if I'm just going to have to remove the rotors and get them turned anyway.

Posted by: Eric_Shea Apr 19 2010, 12:36 PM

Did you check the hubs for runout? You may have that much before you even install the rotors.

I'm not in front of my manuals but, there's an acceptable runout spec. .004" seems OK from where I'm sitting but, not a real answer right now.

Posted by: dr914@autoatlanta.com Apr 19 2010, 12:44 PM

yes definitely check the hubs, maybe not the runout but the mating surface. May want to sand the rust off first and make sure it is firmly mated with the two rotor screws

Posted by: ericread Apr 19 2010, 12:51 PM

Doesn't Eric Shea sell the pre-warped brake pads so they'll fit properly?

Posted by: rjames Apr 19 2010, 12:56 PM

I did not check the hubs for runout- (how does one do that, btw?) but did sand them both down to nice shiny bare metal before mounting the rotors. That being said, there wasn't much rust to remove.

Posted by: rjames Apr 19 2010, 12:56 PM

I also heard that he will run alongside your car and grab the wheel with his bare hands to stop the care from moving if need be.

Posted by: Phoenix 914-6GT Apr 19 2010, 01:03 PM

I have had rotors come to the shop needing to be turned but Zims should not need to be turned. They "should" be true.

Posted by: underthetire Apr 19 2010, 01:39 PM

considering the .006 venting clearance, sounds excessive to me.

Posted by: Eric_Shea Apr 19 2010, 04:14 PM

You always adjust the venting clearance on the high side of the runout. There will always be runout. .004" is a thick piece of paper.

Now... the running along side the car thing is an Internet myth. I did it "once" but I doubt it could be repeated. Plus... it was a very good customer.

Reminds me of this:

Posted by: SirAndy Apr 19 2010, 04:31 PM

Anyone remember the video of the guys that were going to do burnouts to get rid of their old tires and burnt up the clutch instead?

Gawd, there's some funny stuff out there ...

Posted by: ConeDodger Apr 19 2010, 04:40 PM

Yup! Late Corvette. Must have had the traction control still on... Idjut!

Posted by: rjames Apr 19 2010, 07:01 PM

I just have a feeling that thick piece of paper or not, I'll feel it in the pedal. Weird thing is that after I adjusted the venting clearance I could have sworn I spun the rotors and there was no rubbing on the pads- for either side of the car. 3 days later I have rubbing on both sides of the car. On the passenger side both the inner and outer pads rub at different points on the rotor! Fuch.

Maybe I should remove the rotors and check the hubs, or at least rotate the rotors 180 degrees on the hubs to see if it makes a difference.
If the hubs are off are there shims that can be used to correct this?

Posted by: PRS914-6 Apr 19 2010, 07:06 PM

The normal spec for new or machined rotors is typically less than .002. That said lots get installed with more. I do .003 or less. That maximizes braking while minimizing shudder while braking and soft pedals.

A dial indicator on a hub is appropriate and if bad.....check is to see if the wheel bearings were installed improperly on the fronts if it happens. See the last comments I made on http://www.914world.com/bbs2/index.php?showtopic=90121

Posted by: rjames Apr 19 2010, 10:23 PM

That's what I was afraid of. My guestimate as far as how much warpage there is based on a feeler gauge, not by using a dial indicator (don't have one). I set the rear clearance to .004, and then in a spot on the rotor the pad rubs. That tells me that it's roughly warped by .004", or the hub is not true.

Additionally, it is the rear rotors I'm talking about. Not the fronts- at least as far as I know. I haven't checked them yet.

So can shims be used if need be on the rears? If the hub is bad... aw hell. The work day was bad enough- I don't think I will want to hear the answer to this.

Posted by: PRS914-6 Apr 19 2010, 10:50 PM

Skip the shims. Bad idea. Your idea to rotate the rotors 180 is worth checking. Are you using a dial indicator?

I like to knock the studs out and file the dings out of the flanges but that may not be possible assembled. Years worth of tire monkeys using impact wrenches can warp the face of the flange.

Posted by: ME733 Apr 20 2010, 07:39 AM

.........IMO.....It,s a waste of time AND money to turn brand new rotors...I know friends who did this......IF a rotor has a minor amount of distorion...003ths inch seems to be a industry standard(for runout)....just use it for awhile, get it very hot many times, over many miles. ensure you have "heat sunk".."heat sink" the rotors, ..BEFORE you have them turned...that way all the warpage has taken place... as a note of intrest: no one I know had any success with puting them in an oven to speed up the process.(heat sinking)..AND the O.E.M. rotors are always made of better material, and warp less than any aftermarket rotor....I.M.O.

Posted by: underthetire Apr 20 2010, 08:33 AM

Very thick paper! It's actually more like 2 sheets. In my trade, .004 is a HUGE amount. Thats why I can't do wood working. Just too sloppy for me.

Posted by: Eric_Shea Apr 20 2010, 12:24 PM

I'm finally at the factory manuals:

Lateral "Wobble" (find it funny but appropriate they call it wobble) = .0008" or 0.2mm

Couple of things I would check:

1. Are the sub axles torqued to spec.? Your measurements seemed to change after driving the car is why I ask. Something is moving.

2. Can you get a dial gauge on the hub? That would be best as you could eliminate it from the equation.

3. Dial gauge on the rotor (if the hub is in spec.)

Posted by: PRS914-6 Apr 20 2010, 12:37 PM

Eric, you must mean .008 or .2mm/.0008 or .02mm

If its .008 I would suspect that is a "in service maximum" and not new installation. You would feel .008 in the pedal and the steering wheel when the brakes were applied.

.0008 tolerance would seem impossible to obtain from the average mechanic, production machines and old cars....

Posted by: Eric_Shea Apr 20 2010, 03:15 PM

Check the factory manual... I was just parroting from that.

I thought I wrote it down correctly. I definately could have written it down wrong. With the 50's approaching I'm moving to the 2.00 mag readers.

I agree... that's a thick hair (or "a very thick hair!") But what it is telling me is, they don't like to see .004"

I'm away from the office computer that has the manuals on it. Someone here has to have them -or- I'll weigh in with confirmation later.

Posted by: PRS914-6 Apr 20 2010, 04:01 PM

OK, I broke down and drug out the manual. It's .008/.2mm so Eric ya gotta take a zero out of your number.

The spec is not in the "wear limit" column but in the specification column.

Will .008 work? Yes Would I assemble at .008?...No But that's me and I'm anal.

Posted by: Eric_Shea Apr 20 2010, 04:03 PM

I'm just use to adding zero's. Now if I can figure out a way to do it with every paycheck.

Posted by: underthetire Apr 20 2010, 04:19 PM

My top hair is .0018
Bottom hair-literally, is .0026

Posted by: Eric_Shea Apr 20 2010, 06:10 PM

Now I see what you were talking about Paul...

When I looked up at the screen again I copied the thickness tolerance. Came back to my office computer to have the PDF staring at me. At least I got one right!



So... bottom line; it's within factory tolerance. Let us know how it turns out.

Posted by: KELTY360 Apr 20 2010, 06:21 PM

What color hair?

Posted by: underthetire Apr 20 2010, 06:54 PM

Mostly white and grey now !

Posted by: charliew Apr 22 2010, 02:08 PM

While your at it check the paper thickness, on normal 20lb copier paper it's always been .003. Also won't worn axle bearings let the rotor and hub wobble?

Posted by: underthetire Apr 22 2010, 02:27 PM

.0025 on one sheet recycled type, and .004 on our heavyweight bright white. Thats just with a micrometer though, so could be +/- .0001.

Posted by: Eric_Shea Apr 22 2010, 03:45 PM

Is that "heavy" weight bright white or "very heavy" weight bright white?