Do I have to get my disc brakes machined every time the pads are replaced?

Dear Car Talk

Dear Car Talk | Jul 01, 1997

Dear Tom and Ray:

My question concerns disc brakes. Every time my local brake shop replaces my
brake pads, they always insist upon machining (refinishing) the discs. After a
few pad replacements, the discs are then too thin to be machined and I have to
buy new ones at a 150 bucks a piece. Is this necessary? -- Phillip

TOM: There's no rule that says the discs have to be machined each time the pads
are replaced. It really depends on the condition of the discs.

RAY: If the discs are warped, scored or pitted, you machine them or replace
them. If they're not, you leave 'em alone.

TOM: So how can you tell? You can tell if the discs are warped with a run-out
gauge. The gauge is attached someplace on the steering knuckle (like on the
brake caliper bracket), and then the disc is spun around. If the gauge reads
more than a few thousanths of an inch, your discs are warped.

RAY: You can tell if the discs are pitted or scored by visual inspection. Discs
get pitted from rust, and you'll see the rust spots right on the disc's surface.
Discs get scored when the brake pads have completely worn out and the metal
backing of the pad has started to scrape the disc itself. This is also visible
-- just look for the deep gouges.

TOM: But if none of these conditions is present, and the discs have enough
remaining thickness, the mechanic is free to just slap a new set of pads in
there and send you on your merry way.

RAY: Now, if you don't machine the discs, it's possible that you may get some
brake noise for a few days until the new pads conform to the irregularities of
the old discs. And that's why a lot of shops machine the discs as a matter of
routine -- so customers don't come back and yell at them.

TOM: But if you're willing to rough it, you can decide whether to machine the
discs each time based on their individual conditions. Hey, it's a free country,
isn't it, Phillip? And last time I checked, the constitution gave you the
inalienable right to life, liberty and the pursuit of fewer brake disc

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