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Re: To cut or not to cut

To: Douglas Shook <>
Subject: Re: To cut or not to cut
From: "Michael D. Porter" <>
Date: Mon, 21 Feb 2000 13:20:00 -0700

Douglas Shook wrote:
> re: cutting rotors

> I described the situation regarding the rotors that were turned
> on a lathe, and he explained that the problem with cutting rotors
> on a normal lathe is that if you do not cut both sides at the
> same time, and if there are high spots, the cutter can deflect
> the rotor, leaving a high spot.
> Watching the knowledgeable local auto parts machinist cut the
> rotors, he only skimmed enough to true them, and he did it in
> multiple passes, taking about 15 minutes per rotor (he has an
> older machine with a tired motor).

This is quite true, but has more to do with hard spots, rather than high
spots. The cutting tool is more likely to deflect at points where the
rotor has work-hardened from heat (these areas are sometimes
0.010-0.015" deep and must be undercut, which requires more tool
pressure). And, quite so, tool pressure on both sides of the rotor
ensures that the cut is true on both sides in the direction of rotation.

> I guess what I am trying to say here, is that there is little
> reason not to turn the rotors if the person doing the job knows
> what he is doing.  You only need to skim off whatever amount is
> required to true them.  If the rotors are in good shape, then
> almost nothing will be removed. If they are warped, then, yes,
> more will be taken off. I have had the two pairs of rotors cut
> now a total of eight times, and they all still have enough "meat"
> left to be turned more times.

Keep in mind that every rotor and brake drum has a recommended minimum
wear limit, and by law, a machine shop cannot turn the rotor or drum
beyond that point. Another consideration is the intended use. Racing, of
course, requires different equipment. But, hard occasional use would
probably not be advised with rotors or drums at or near the legal limit
of thickness. I have worked on cars with exceptionally worn brake rotors
(an MG Midget with rotors 1/8" thick comes to mind) and the brakes were
still functional, but marginally. Hard use with rotors such as those
would have certainly resulted in disaster.

Note, too, that even with new rotors or drums, the surface may not be
absolutely true, and the surfaces must be skimmed a few thousandths.
That happens with perhaps 3% of all new rotors. 

In short, Doug seems to be saying, have them done on the proper
equipment by a trained operator, and don't be afraid of having the
rotors turned if there's available material. All good common sense. I
would only add that one should _thoroughly_ clean the hubs out and
regrease after having the rotors turned--it's very easy for small chips
to stick in the grease and eventually find their way into the bearings.
For those who want best results and longest bearing life, be sure to use
a waterproof grease with a temperature rating suitable for use with disc
brakes (this is usually a grease with a liquefaction point of about 500
deg. F).



Michael D. Porter
Roswell, NM

`70 GT6+ (being refurbished, slowly)
`71 GT6 Mk. III (organ donor)
`72 GT6 Mk. III (daily driver)
`64 TR4 (awaiting intensive care)

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