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.

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.

^ Agreed.

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...

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.

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.

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.

I machined the original rotors on my '82 300D twice. When I replaced them after 150k miles, they were still slightly above minimum thickness.

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.

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.

Did a shop do it for you or did you do it yourself?

Automotive machine shop. (Two different ones.)

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.

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

If you follow MB's recommendations to the letter, you wouldn't be asking the question to begin with. You would take your car to a MB dealer and say "Fix it."

MB's recommendations have their bottom line in mind. Not yours.

Do you ever drive with your windows down? Is that a practice positively endorsed by MB?

No doubt the profit margin on new rotors is a lot higher than with machined rotors.

So you get all of your brake work done at the dealer?

In practice, I replace the W123 rotors with every second pair of brake pads (both front and rear). The W123 rotors are just too inexpensive to bother machining, and my experience indicates that a new set will outlast two sets of pads without any performance issues. In this case, I believe the OE recommendation is excessive.

That is a totally subjective evaluation and says nothing about the technical feasibility of the practice. Clearly, anyone who finds the practice a "bother" is free to avoid it.

From a technical standpoint, it is entirely possible that a rotor machined on the hub (in the case of front brakes) might have less runout than a new one just installed.

I agree, it can be done. For me, it would be more trouble than it's worth. The new ones are very cheap and work well for me; but it's not the only way.

I typically change the rears every other time.

I can only remember changing fronts once... they are much thicker than the rears

I am OK with your Choice.

I was just saying what I do in General.
As an example if it is going to cost $10 to get a Rotor turned and new ones are $13-$15 I might opt for the new ones.

Another issue not mentioned is that when you Measure the Thickness of the Rotors you are measureing the high spots not the bottom of the Groves.

And, if you add a little bit of posible warpage that would have to be removed; your measuremt ends up being mostly and Educated Guess concerning if haveing the Roto turned will work or not.

Another issue is that Guy that turns the Rotor does not want to do it twice. He is going to make as deep a cut as he feels will get the job done at one time.

So I am thinking of all of the above stuff when I am deciding if I am going to have th Rotors Turned or not.

No recommendation here but just a story.
When I took the Brake and Suspension class in Trade School it was taught by an Instructor who owned his own Brake Shop.
He said that in his shop he would reused Rotors that after they were Turned were slightly below the minimun allowed thickness.
He said he felt that it was still safe because the minimum thickness on the Rotors had some several thousands of safety margin to them.

I know at least 2 people who have said they would never again have their Rotors turned while they were still on the car because they had problems with them later.
I suspect the reason is may Cars have a lot of Wheel Blearing end play/clearance. If you do not remove the end play/clearance before you turn the Rotor I do not see how the turning job can be accurate.
I also do not see how they can line up the Machine accurately either.

"On the hub" is not the same concept as "on the car."

This is not a comment on Brakes but just a comment in general. There is a lot of times I have read on the forum there ar things you should not do on Mercedes.

Because to Me a Mercedes is in fact just another Car/machine to me.

So when someone tells me something cannot be done I compare it to what is being done with other Cars or vehicles and try to look for a reason why it should not be done on Mercedes.

If I cannot find a reason why something I want to do on my Mercedes should not be done I use my best judgment and decide what to do.

I continue to enjoy the discussion. For myself, it seems that the slightly increased price to replace rather than machine is worth it. I can pick up a pair of Balos or Brembos for $110. What would I get charged to get two rotors properly machined? Whatever the resultant price difference is doesn't seem worth it. But to each his own....

Machining a rotor should cost about $15.

Don't forget to include the cost of new caliper and rotor bolts, where applicable. Speaking of positive endorsements, MB positively does not endorse the reuse of those bolts.

Answer
 

You have e-mail. :D


Have a great day.

What a timely thread. I have started to feel the pulsing from the brake pedal that would be caused by a warped rotor. If my memory serves me the issue of not turning the rotors was the same on my 89 range Rover (I bought it with 58,000 miles on it) I never saw it writing however everyone I spoke with on that forum said to change them not machine them. So I did. I may go ahead and turn these ( I have access to a quality brake lathe) and report back. The only issue I can see is that I need to tun them and wait while that process is going on. If I buy a new set the whole thing goes smoother and faster. Of course they is only because mine are warped without a doubt. If they were not and were found to be within spec I would just resurface/dress them and save the money.

I'm pretty easy on brakes. I only change them if they are warped or too thin. I use standard pads or ceramic. The ceramics will last longer but might reduce rotor life. The clean wheels are worth it though. I haven't tried turning any in decades.

I'm on the original front rotors at 400kmi on my 1977 300D. I use Pagid soft pads, commute gently on highways, brake gently, and have driven this car for ~305kmi. There's essentially no lip on the rotors, but when it comes time to renew the rotors, the new ones are so cheap and easy to install that it's foolish money to machine them. Actually, I have an almost new pair of rotors in the garage attic from a wreck I bought for the engine. Pads are cheap and easy to replace. I've got the wheel covers so brake dust is not a big issue.

'77 300D 404kmi
'95 C220 190kmi
'83 Dodge B150 336kmi
'72 Norton Interstate 169kmi

Sorry, I did not catch that detail.

There are undoubtedly those who would argue that any money spent on a 33 year-old, 400k miles car is "foolish money." You won't get your money's worth out of those new rotors unless the car lasts another 400k miles.

Antique rotors!! That is just unheard of. Unbelievable!