Is it bad to just replace one brake rotor...
 

...even if the other one isn't to worn?

Yes. Do brake work in pairs, fronts or backs, never one side and not the other.

Yes I agree...always in pairs....

Safety
 

What he said.:)
Uneven braking could result.:eek:

Always in pairs per axle... like they said above..

IGNORE PEH when he comes on and says ' don't fix it until it is broken' ... because you may break more than that with the results of uneven breaking and driving....

I'll toss in my $0.02 worth. It depends. If I replaced the set and 5000 miles later, one broke, I would replace that one, provided I see that the problem is that one only. Now, if I had a bunch of miles, I would replace both.

I'm wondering
 

Why only one is worn.

That is why I said, replace both unless one of a set failed soon after install. Could be bad machining. I replaced both fuel level senders on my 99 C280. Guess what, 2 weeks after that, one of the senders failed. Good thing I bought them from the dealership and they had to go to parts warranty to get it fixed. I was thinking of getting it here but decided the risk wasn't worth it since I would have to drain the tank and it would be a ***** to do in the driveway.

Even on my $500 beater I would never do a single rotor.

How does having one new rotor and one old rotor create uneven braking? Assuming that the old rotor is still within spec and in good shape.

wear pattern......new is smooth the other is not....casues on wheel to grab before the other...not a good thing in curve in the rain.

and then the saving of the cost of the part doesn't seem like such a savings

Rotors are so cheap! I replaced rotors, pads, and wear sensors, and rebuilt my calipers for less than $120. Replacing a single rotor is FALSE economy.

Old. You may laugh but someone came into the dealership with a set of rotors so chewed up that the service manager made him sign a waiver since he didn't want work done. It was so thin that they didn't feel it was even roadworthy and they guy thinks he can squeak a couple miles more out of it. They even took pictures of it. I think the next time he came in he got it fixed. This time, the rotor was missing half of it. Imagine the two side of the rotor with ribs in the middle. Imagine when he came in, you saw some sanded off ribs and one thin side of the rotor. :eek: You drive that. I won't!! I am staying the hell away from it.

I'm doing my W123 in the next two weeks.....I have one sticky caliper but found good enough prices I am doing all four corners Calipers, rotors, and pads....I don't want to have to go back in with snow on the ground and fix something else...