Is it possible to machine just one side of the rotors?

Dear Car Talk

Dear Car Talk | Oct 01, 1999

Dear Tom and Ray:

I have a 1987 Olds Cutlass Ciera in very good condition with 67,000 miles. The
brakes stop the car promptly, smoothly and without pulsation. The pads have been
replaced once, and the rotors have never been machined. When I took it to a brake
and muffler shop, they showed me that one side of my rotors was rusting. To me,
it looked like little sand holes. I suggested machining that side, but the
mechanic told me both sides would have to be machined, and that would make the
rotors too thin to use. He wouldn't replace the pads unless I replaced the
rotors. Can't I just have just one side machined? -- Abe

TOM: That's a good question, Abe, but the answer is no, you really can't machine
just one side.

RAY: Theoretically you could. But in reality, the lathes are set up so that the
rotors get pressed by blades from both sides simultaneously. If you apply
pressure to only one side, the rotor will "give" whenever there's a bump or
defect, and you won't really smooth out the surface, which is the whole point of
machining. In other words, you'll get a lousy job.

TOM: How does he know? He's done LOTS of lousy jobs on rotors!

RAY: At 67,000 miles, I suspect your mechanic is right. The rotors probably are
worn down to the point where machining them correctly would take them below
specification (make them too thin).

TOM: Moreover, if only one side of the rotor is rusted, I'd be concerned that a
caliper slide is stuck. Because when a caliper is working properly, the pads
should be applying pressure to, and scraping rust off of, both sides

RAY: So this is not the time or place to be a cheapskate, Abe. You really want
the calipers working properly and the pads and rotors in perfect, point-to-point
contact so you get optimal braking. After all, those doughnut shops can come up
on you awfully fast!

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