I am having the calipers on my 76 rebuilt by Vette Brakes to NCRS specs. I've measured the thickness of the original rotors and find 1.250" on one and 1.181" on the other. The surface of the rotor is very good with no grooving. We all know 1.215 is the minimum wear limit, stamped on the rotor and called out in the GM service manual. Is there any tolerance here? Should I immediately discard these rotors for new because of the .034 difference? I have not checked runout at this point. Opinions please.... Thanks.
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Re: C-3 disc brake rotor minimums
I think you answered your own question, obviously you need at least ONE new rotor. It's your LIFE at stake here, and maybe mine if you live near me. I think I'd assume minimum means MINIMUM. -
Re: C-3 disc brake rotor minimums
The 1.215" is the minimum machining limit and not the minimum wear limit IIRC. The difference being that a shop can machine a rotor to 1.215 if need be, and then return it to service. Through normal wear, the rotor will eventually end up at a thickness of less than 1.125"- as is yours presently at 1.181". You're only 2.7% below the minimum machining limit. Had you not measured the rotor, you'd have no way of knowing what dimension it is at, and be no different than the other 99.999% of Corvette owners.
I can't quite agree with the statements that your life is at stake and don't believe that horrible things will happen. Usually a rotor below it's normal minimum thickness will be more subject to warping and loss of parallelism of the surfaces. This will be noted while braking as a pulsing pedal.
If your run out is good, I'd leave well enough alone.Comment
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Mikey has it right.again *NM*
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Re: C-3 disc brake rotor minimums
Especially if the thin rotor is on the rear, which only has to dissipate about one-third of the car's kinetic energy, I don't think it's a big deal, especially if you don't plan on severe service.
The C2/C3 brake system has HUGE thermal capacity. As stated, the rotor might be more subject to warping (or cracking) if you use the brakes hard, but you certainly aren't risking your life. The minimum rotor thickness requirment is a FMVSS, and I suspect GM and other manufacturers are very conservative, when establishing the minimim.
DukeComment
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Re: C-3 disc brake rotor minimums
If you're going through all the trouble to get your original calipers rebuilt to NCRS standards, why not just do the job right one time. When I do my own work, I usually save enough on labor charges that I can easily afford to replace those "borderline" parts just so I won't have to bother doing the job again later. Replacement rotors on e-bay sell for about $50 to $60 a pair with shipping included and even have the rivit holes if you ask for them. New rotors typically measure about 1.310.
By the way, you're lucky your state doesn't have safety inspections or there would be no question here.Comment
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1.215"min spec'd w/ worn-out pads
As far as piston-in-bore position, practically (if not legally) you could go below this min disc thickness with sufficient meat on the pads.Comment
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But then you get into
the fun of having to eliminate runout from the new rotor/hub combo. The cost of having them machined can wipe out any savings you've previously accumulated.Comment
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Re: C-3 disc brake rotor minimums
Mike-----
For C2 and earlier C3, Chevrolet specified the minimum rotor finish after refinishing to be 1.215" and no discard thickness was specified. For later C3 the minimum rotor thickness after refinishing was specified as 1.230" and the discard thickness at 1.215".
Personally, I would not use a rotor that was below 1.215". In fact, I don't even like to use any rotor that has been re-finished, at all. Maintaining ALL of the rotor dimensional specs and tight tolerances in a field re-finishing operation is problematic, at best.In Appreciation of John HinckleyComment
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Addendum
By the way, I have several used ORIGINAL rotors here that are under 1.215". If anyone wants them, they're yours for the cost of shipping. Otherwise, they're scrap metal.In Appreciation of John HinckleyComment
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Re: C-3 disc brake rotor minimums
Thanks everyone for their input.
Joe, I take it that you would purchase new rotors or utilize NOS spares. Riveting or not riveting appears numerous times in the archives and it is well understood that since riveting was an aid to production that it is not required for functionality. Is there an advantage, i.e. reduced runout, in going to the trouble of having the hub re-riveted to the rotor? And, once re-riveted should a new rotor/hub assembly be checked for runout? Thanks again for your time.Comment
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Heh, Heh, god example of anal gone anal *NM*
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Re: C-3 disc brake rotor minimums
Mike------
Yes, usually I would use new rotors, either NOS or good quality aftermarket.
Whenever a rotor is changed, the assembly with the hub or spindle has to be checked for run out. If it exceeds specs, then the assembly has to be machined to get it into specs or other measures have to be performed to get it into spec.
Once the assembly is in-spec, the rivets can serve to keep it in spec. Even slight movement of the rotor with respect to the hub/spindle can affect runout. This can occur within the movement allowed by the lug bolts. The effect on runout of this movement is usually slight, but keep in mind that the allowable runout spec is very tight. I tap the spindle/hub rivet holes for 3/8-20 fastners. Then, I countersink the rivet recesses in the rotors to accept countersink-type, flat head, hex socket machine screws. These retain the rotor to the hub/spindle just like rivets and prevent any movement between the hub/spindle and the rotors. It might be regarded as "over-kill" but I'm an "over-kill" kind-of-guy.
The above usually applies to the rear only. The fronts were/are available as complete units with hubs and that's what I would always use for the fronts (I have 2 sets of NOS spares for the fronts [none for sale], so my front situation is taken care of pretty much forever). For the rears I've used the above procedure since years ago I foolishly pereformed rear spindle work which subsequently required me to seperate the rear rotors from the spindles. The above procedure can be used for the front hub/rotor, though, just like I used it for the rear spindle/rotor.
The whole key here is to AVOID the need to perform any service on the rotors. If one uses high quality, organic brake pads and replaces the pads before they get into the rivets or the backing plates, it should not be necessary to refinish or replace any rotors. The rotors on my 1969 have NEVER been refinished or replaced and they are currently near-perfect with respect to all specs, including being right at the original thickness of 1.250". All this with 200,000 miles on them.In Appreciation of John HinckleyComment
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