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 replacements.

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© 1997 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.

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