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#1
10-19-2018, 03:12 PM
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How much is too much disc rotor runout
I just bought some new AC Delco brake rotors and I am measuring .003 runout. Is that too much? I cant find a spec on them and Summit couldn't find a spec on them either.
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#2
10-20-2018, 11:49 AM
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For your entertainment here is the response from AC Delco via Summit
"I was unable to get information on the run out specs for the rotors you purchased since GM considers that confidential and will not give that info to their vendors, including Summit, as well as the general public. As of now, it looks like I have exhausted all of our resources to get that information"
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#3
10-20-2018, 02:23 PM
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There should be some sort of a maximum allowable run-out or wear printed in the shop manual. Everybody that drives knows how a car with warped front rotors feels like when you press on your brakes. On older GM cars from the late 1960s and 1970s; the cars that have vented front disc/rear drum brakes, you can usually turn those front rotors once or twice on a lathe before they get too thin and then they need replaced.
On modern cars or cars with non-vented rotors such as Honda or Toyota with 4-wheel disc brakes they usually just tell you to replace them.
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Peter Serio Owner, Precision Pontiac Last edited by Peter Serio; 10-20-2018 at 02:23 PM. Reason: spelling fix. |
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#4
10-20-2018, 04:02 PM
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Bruce,
As Pete stated a GM shop manual for your particular year will have the run out spec listed. Not sure what Delco/Summit was trying to say, it’s not some corporate secret! I’m guessing the rotors are for the car shown in your profile pic. If so, for reference the GM manual for my 89 TA lists a max. lateral run out 0.005”. This car uses the same tapered roller wheel bearing setup as the early A-bodies so I would say the 0.003” is within limits. If the rotors are for a hub/bearing set up the runout is a little tighter, example: 93 Chevy Lumina, 0.003”; 2003 GMC Envoy, 0.002”. |
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#5
10-21-2018, 12:06 PM
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Thanks guys. The rotors are for a 77 Trans Am. I don't have a shop manual. Ive been asking around and Im getting all kinds of different info. One told me you need a little runout to keep the pads from dragging. Others have told me they need to be straight as possible. Some tell me the spec is .0035 max. Others say .002. I guess I will just try them to see what happens.
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#6
10-21-2018, 02:14 PM
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PA state inspection maximum rotor runout measured on the wear surface is .002. If you can feel it in the brake pedal, or steering wheel it's beyond .002.
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Brad Yost 1973 T/A (SOLD) 2005 GTO 1984 Grand Prix 100% Pontiacs in my driveway!!! What's in your driveway? If you don't take some of the RACETRACK home with you, Ya got cheated    
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#7
10-21-2018, 06:53 PM
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Quote:
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#8
10-22-2018, 02:11 PM
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Just saw this on Summit. The Raybestos Rotors for this application say they are within .004 runout on the last paragraph of the overview. https://www.summitracing.com/parts/a...model/firebird
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