A while back, somebody posted a tech article about turned rotors vs new. If I remember correctly, the tech data show that turned rotors increased stopping distance by as much as 60%. The spiral grooves that result from turning on a brake lathe cause the brake pads to "skip". In the past, I used to get rotors turned once before replacing--now I just replace with new.

J. M. van Swaay

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1994 E320 Wagon, 230,000 miles
1995 E320 Sedan, 106,000 miles
1994 E500 Sedan, 79,000 miles