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  #1  
Old 09-29-2010, 07:23 AM
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Always Replace Rotors When Replacing Pads?

As I understand it, MB designed rotors to wear with pads so that it's a good idea to replace both at the same time....which is what I've done. Is that what everyone around here does? Has anyone had success just replacing the pads and then replacing rotors every other time?

I'm glad to save money where I can....but I hate brake annoyances even more.
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91 300D "Otis", Smoke Silver, 113k mi, DD
98 E300D "Murray", Silver, 84k mi, Stage 2 Rocketchip chip tuned, other DD
99 E300D "Winston", Black, 153k mi, son's DD

Ten other MB's owned and sold
2002 Honda Odyssey 181k mi (want to name it "Homer," wife won't let me)
1961 Very Tolerant Wife (OK, I ripped this off from another board member...but my wife was born in 1961 and she is very tolerant.)
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  #2  
Old 09-29-2010, 07:36 AM
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I only replace the rotors when they get down to or below spec thickness, which is printed on the edge of the rotor. I bought a digital calipers so I can measure them. The other indication I use to change the rotors is if they get warped at all. You can tell this by the pulsating of the brake petal.

I think the responses you get to this question will vary, but there is a definite bias on this forum toward frugality so I would venture to say that a majority of the members do not change them with each change of the pads.

When I did replace my rotors, I purchased a good quality brand. I stay away from cheap metal that will likely warp sooner. They will be a few dollars more but of way better quality.
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  #3  
Old 09-29-2010, 08:10 AM
toomany MBZ's Avatar
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^ Agreed.
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  #4  
Old 09-29-2010, 09:03 AM
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Ditto!

Although, I will advise anyone with the 123 series to get a micrometer, as the outer edge makes it difficult to measure the caliper thickness when there is a lip, and the non vented rotors are more likely to warp from heat than the SD 126 rotors are...
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  #5  
Old 09-29-2010, 09:19 AM
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I agree with the above. But, when I have needed them I have always bought new rotors for all of my Cars and never had any issues with them.
Also, I did not have to buy Rotors for my Mercedes as I found 2 new ones in the trunk after buying the Car.

If you reuse the Rotors and do not have them machined you pads will not last as long and the can be noisy.

I agree with the better quaility Rotors to the extent that if you buy high quality Brake Pads you should have High Quality Rotors.
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  #6  
Old 09-29-2010, 09:58 AM
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Quote:
Originally Posted by Diesel911 View Post
If you reuse the Rotors and do not have them machined you pads will not last as long and the can be noisy.
Why do I have the impression that MB rotors shouldn't be machined? Somewhere along the way I've heard reuse or replace, but don't machine.
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91 300D "Otis", Smoke Silver, 113k mi, DD
98 E300D "Murray", Silver, 84k mi, Stage 2 Rocketchip chip tuned, other DD
99 E300D "Winston", Black, 153k mi, son's DD

Ten other MB's owned and sold
2002 Honda Odyssey 181k mi (want to name it "Homer," wife won't let me)
1961 Very Tolerant Wife (OK, I ripped this off from another board member...but my wife was born in 1961 and she is very tolerant.)

Last edited by shertex; 09-29-2010 at 03:23 PM.
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  #7  
Old 09-29-2010, 10:03 AM
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Quote:
Originally Posted by shertex View Post
Why do I have the impression that MB rotors shouldn't be machined?
Because you listen to rumors and old wives tales. Ask one of the rumor mongers to show you in writing where MB prohibits the machining of rotors.
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  #8  
Old 09-29-2010, 10:44 AM
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Originally Posted by tangofox007 View Post
Because you listen to rumors and old wives tales. Ask one of the rumor mongers to show you in writing where MB prohibits the machining of rotors.
My understanding is that they are made of a softer metal and therefore more effective at stopping. But this makes them last not as long.

I took mine to two different shops to see if they would machine them for me when I had a slight warp. The one guy where I take my cars for inspection told me that he cannot get a good smooth cut on them because the metal is too soft. I went to another guy I know who did some body work for me. He said something about how his machine could not hold them and keep them from chattering, I think he said. So after those two negative opinions, I decided to just get new ones. In reality, they are not all that expensive and probably close to what it would cost to have a set machined.

If I had the equipment to machine them, I would give it a try, but I do not. I would be curious to hear if anyone has had their rotors successfully machined. Certainly they were machined in the manufacturing process.
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  #9  
Old 09-29-2010, 10:47 AM
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Originally Posted by RML View Post
I would be curious to hear if anyone has had their rotors successfully machined. Certainly they were machined in the manufacturing process.
I machined the original rotors on my '82 300D twice. When I replaced them after 150k miles, they were still slightly above minimum thickness.
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  #10  
Old 09-29-2010, 10:54 AM
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Tip: To get the calipers over the raised "lip" at the edge of the disc problem.

You can use regular calipers instead of micrometer, by grabbing a couple of common washers and hold one to each side of the disc and caliper that thickness. If you zero your caliper on the 2 washers alone, you don't even have to do any math to subtract the thickness of the washers to arrive at the disc thickness.
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  #11  
Old 09-29-2010, 10:55 AM
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You do not have to change out both unless they are worn below minimums. The FSM states a value above the minimum that when your changing out the pads the rotors should also be changed.

I haven't recently priced rotors, I've just changed the pads and all was well, rotors were above minimums. Sometimes it's more cost effective to change the pads and the rotors vice having the rotors turned.

Personal choice i guess. The FSM doesn't say you can't turn the rotors.
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File Type: pdf Checking brake rotor pad thickness.pdf (203.5 KB, 67 views)
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  #12  
Old 09-29-2010, 10:56 AM
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Quote:
Originally Posted by tangofox007 View Post
I machined the original rotors on my '82 300D twice. When I replaced them after 150k miles, they were still slightly above minimum thickness.
Did a shop do it for you or did you do it yourself?
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  #13  
Old 09-29-2010, 11:01 AM
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Quote:
Originally Posted by RML View Post
Did a shop do it for you or did you do it yourself?
Automotive machine shop. (Two different ones.)
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  #14  
Old 09-29-2010, 11:12 AM
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I am as big a cheapskate as the next guy but I don't cut corners when it comes to brakes. My own personal policy on Mercedes cars is I always replace front rotors with every set of pads. I do this because I have found that the rotors are not all that expensive, and, often times even if the rotors have more meat on them than the FSM calls out for they will develop warping at some point in their lives. Warped rotors have a much more pronounced effect on the front than on the rear wheels so I just have accepted the fact that they should be done every time front brakes are done.

Now rears are the place where you could use some judgment and save some money as long as they are in good shape. Personally I replace them every time too (because historically the rears last about 2X as long as the fronts) but each time I have done it I felt it was probably overkill.

The bottom line is this...if your rotors aren't worn out and aren't warped you don't have to replace them, but front brakes work hard and if your time is worth anything and you are as annoyed by warped rotors as I am you will just learn that it is good practice to simply replace them every time - at least up front.
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  #15  
Old 09-29-2010, 11:12 AM
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Quote:
Originally Posted by tangofox007 View Post
Because you listen to rumors and old wives tales. Ask one of the rumor mongers to show you in writing where MB prohibits the machining of rotors.
No....various indie mechanics over the years I think.

But anyway....I checked the FSM and wasn't able to find ANYTHING about machining brake discs (replacing and cleaning, but no turning/machining). It seems that the burden of proof should be assumed by those who think it's a good idea. In other words, what we should look for is not a prohibition, but a positive endorsement of the practice by MB.

Maybe it's there and I missed it. Anyone know where MB recommends machining rotors?

http://www.w124performance.com/service/Index/Groups/42BrakeSystem.htm
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91 300D "Otis", Smoke Silver, 113k mi, DD
98 E300D "Murray", Silver, 84k mi, Stage 2 Rocketchip chip tuned, other DD
99 E300D "Winston", Black, 153k mi, son's DD

Ten other MB's owned and sold
2002 Honda Odyssey 181k mi (want to name it "Homer," wife won't let me)
1961 Very Tolerant Wife (OK, I ripped this off from another board member...but my wife was born in 1961 and she is very tolerant.)
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