When is it time to replace brake rotors ?

[gerryvz]

I noticed recently that my front brake rotors are quite worn down -- both sides have definite lips on the edges. I don't recall ever replacing the rotors in the past, in the 17 years that I've owned the car, so it is past time. I am also extremely happy to replace them, as I've been waiting for years to replace the pads with the Akebono Euro pads that I've had in my parts stash.

I've ordered a set of coated rotors for the stock E500 320mm front brakes. You guys might know that these are "directional" rotors, meaning that the front left and right rotors are separate parts with separate part numbers.

So, I guess this will be another part of my job, but I'll save it until the end.

 

[Jlaa]

I noticed recently that my front brake rotors are quite worn down -- both sides have definite lips on the edges. I don't recall ever replacing the rotors in the past, in the 17 years that I've owned the car, so it is past time. I am also extremely happy to replace them, as I've been waiting for years to replace the pads with the Akebono Euro pads that I've had in my parts stash.

I've ordered a set of coated rotors for the stock E500 320mm front brakes. You guys might know that these are "directional" rotors, meaning that the front left and right rotors are separate parts with separate part numbers.

So, I guess this will be another part of my job, but I'll save it until the end.

Wow that has to be a record! 17 years with no rotor changes.

 

[gerryvz]

Wow that has to be a record! 17 years with no rotor changes.

I'll have to double-check that in my maintenance records.



Yep, brake rotors (front and rear) have never been replaced according to my records.

Distributor caps and rotors were replaced two times before now -- once in early 2004 at 74,900 miles and once in 2007 at 94,000 miles. Which makes sense, because the caps I just replaced had a Fall 2005 date code on them. They lasted nearly 50,000 miles and were working fine !!

 

[gsxr]

Measure brake rotor thickness first, Gerry... no need to replace if they are above FSM specs! But order a new set of OE to have on the shelf, just in case of NLA.

 

[gerryvz]

I will measure them, but when I did my 560SEC front rotors some years back, they had even slightly higher lips ... and they were pretty worn out of spec. But that's a good point. Since I have one of the car's front wheels up in the air, I can pull it and check the thickness in a few minutes.

 

[LWB250]

Measure brake rotor thickness first, Gerry... no need to replace if they are above FSM specs! But order a new set of OE to have on the shelf, just in case of NLA.

Isn't there a wear gauge cast into the edges of these? The little crescent shaped notch on the inside? I'm not sure if they do this across the board on all their rotors.

Dan

 

[gsxr]

Isn't there a wear gauge cast into the edges of these? The little crescent shaped notch on the inside? I'm not sure if they do this across the board on all their rotors.

Not that I'm aware of, but if so, we neeeeeed pics!

 

[gerryvz]

I have always just used a digital caliper to measure wear. Simple, and accurate enough.

 

[Jlaa]

Isn't there a wear gauge cast into the edges of these? The little crescent shaped notch on the inside? I'm not sure if they do this across the board on all their rotors.

Dan

Not that I'm aware of, but if so, we neeeeeed pics!

Is this a wear gauge or is this something else?



Also, Mercedes picture --- Zimmerman rotors:

 

[JC220]

Is this a wear gauge or is this something else?

View attachment 110573

Also, Mercedes picture --- Zimmerman rotors:

View attachment 110574 View attachment 110575

Most likely these are factory grind marks of material removed in order to balance the new brake disc. I do not believe these are a minimum disc thickness indicator!

The min disc thickness is stamped on the edge of the disc usually and should be checked with a caliper.

 

[LWB250]

Is this a wear gauge or is this something else?

View attachment 110575

That's what I"m talking about. I've seen these little crescent shaped notches on factory rotors, I'm sure. I assumed they were wear indicators. One way to tell would be to determine if they're on all of the rotors and in the same location. That would eliminate the likelihood that they are there as a balancing function.

Dan

 

[gerryvz]

A good rule of thumb for brake discs that I've always known with MBs, is if there is a lip on the rotor that you can catch with your fingernail -- a substantial lip -- then it's time to change the rotors.

Of course, there is a spec for the rotors that you want to measure against with calipers, and this will DEFINITIVELY tell you if the rotors are in or out of spec. But when you're casually under the car, as I was, checking various things -- a quick fingernail feel on the edge of the rotor will tell you whether they are worth investigating further.

 

[gsxr]

That's what I"m talking about. I've seen these little crescent shaped notches on factory rotors, I'm sure. I assumed they were wear indicators.

Those are for balancing, not wear indicators. There's no wear indicator on factory or aftermarket rotors that I'm aware of.

Also, the typical wear limit is >2mm thinner than new, which would translate into a full 1mm lip on each side before they need replacement (or, "need replaced" (sic) as the kids say today). Your fingernail will catch on half that, which is why the fingernail test is only good for telling you if you need to dig out the micrometer to measure properly.

Note there is a difference in spec between maintenance service limit (when installing new pads), and wear limit (absolute min thickness).

 

[a777fan]

Those are for balancing, not wear indicators. There's no wear indicator on factory or aftermarket rotors that I'm aware of.

Also, the typical wear limit is >2mm thinner than new, which would translate into a full 1mm lip on each side before they need replacement (or, "need replaced" (sic) as the kids say today). Your fingernail will catch on half that, which is why the fingernail test is only good for telling you if you need to dig out the micrometer to measure properly.

Note there is a difference in spec between maintenance service limit (when installing new pads), and wear limit (absolute min thickness).

View attachment 110597

Dave, is the 'Maintenance Service Limit' what you would use as the replacement limit when replacing pads? IE- if you were slapping on new pads, you'd use the 'Maintenance Service Limit' not the 'Min Thickness' to determine if you needed to replace the rotor?

 

[gerryvz]

Correct. MSL is where you'd want to replace them. The wear limit is the max wear spec where they MUST be replaced.

 

[JC220]

Generally when brake discs (rotors ) get worn much at all I just get new ones on my cars.

With an edge visible on them they have fairly significant wear anyway and resurfacing etc is a false economy In my opinion at least where I live. (It would cost 75% of a new disc to have them resurfaced by a machine shop!)

As a rough rule of thumb on a daily driver I get through 2 sets of pads on a new disc before it's ready to come off. (Ie one pad set replacement) On older cars which dont go far each year the discs can get damaged by rust before they are worn out.

 

[gerryvz]

Resurfacing brake rotors is not really permitted in MB maintenance literature. I've NEVER seen it specified nor recommended. They always say to just replace the rotors outright. Which IMHO is the way to go.

Turning rotors, as is common on American cars, is a joke. Particularly given the price of brake rotors.

 

[gsxr]

Resurfacing brake rotors is not really permitted in MB maintenance literature. I've NEVER seen it specified nor recommended. They always say to just replace the rotors outright. Which IMHO is the way to go.

Turning rotors ... is a joke. Particularly given the price of brake rotors.

Turning rotors is generally a Very Bad Idea, if done on a lathe which leaves a spiral groove like a vinyl record. There was a great video explaining this but sadly the website is DOA, and because it was a Flash Player embedded video, Archive.org did not capture anything beyond the introductory language selection. Anyway - click here for a written explanation of the phenomenon. If rotors are machined/turned, they need to be Blanchard ground, or otherwise finished with a criss-cross pattern to eliminate the grooves.

Unless a rotor is NLA from absolutely every source, don't machine them. Replace them.

 

[sheward]

I remember that video. It was well done and solved for me a bit of a mystery why for years I could get no further life out of turned rotors. I agree it's just better to replace them.

drew

 

[aliking]

I noticed recently that my front brake rotors are quite worn down -- both sides have definite lips on the edges. I don't recall ever replacing the rotors in the past, in the 17 years that I've owned the car, so it is past time. I am also extremely happy to replace them, as I've been waiting for years to replace the pads with the Akebono Euro pads that I've had in my parts stash.

I've ordered a set of coated rotors for the stock E500 320mm front brakes. You guys might know that these are "directional" rotors, meaning that the front left and right rotors are separate parts with separate part numbers.

So, I guess this will be another part of my job, but I'll save it until the end.

Hello gerry, please i want to ask about the "directional" rotors for the E500, its only for the late E500 models? I mean my car model year is 1992 500E, it also comes with directional rotors with different part numbers?
Thank you

 

[dionphaneuf]

Data point here: my brake light was on for a not v long period of time and my pads were mega toast up front, so glad I changed them. Rears were kinda fine, but I was a bit surprised there wasn’t more margin behind the wear sensor.

 

[gerryvz]

Hello gerry, please i want to ask about the "directional" rotors for the E500, its only for the late E500 models? I mean my car model year is 1992 500E, it also comes with directional rotors with different part numbers?
Thank you

There are three different front rotor setups for the 500E/E500, depending on the VIN number of your car, and depending on if it is a North American + Japan or "rest of world" car.

For early North American + Japan cars -- if your VIN/FIN (vehicle ID number) ends in 930494 or less, then you DO NOT have directional rotors, and only need to order TWO of the following front brake rotor part number: 124 421 26 12 (I believe this number has been superseded to 124 421 26 12 64). Current list price is $75 each (you can easily get discounted to around $55-60 each).

For later North American + Japan cars -- if your VIN/FIN ends in 930495 or more, then you have directional rotors with different part numbers. The part numbers for these rotors are 129 421 17 12 (left side), and 129 421 18 12 (right side). Both of these part numbers may have been superseded, to add a "64" onto the end of each part number. Current list price is $102 for each parrt number (you can easily get discounted to around $75.80 each).

If you have a non-US car, you need to do more research, but I believe the following is correct information:

For early non-North American/Japan cars -- if your FIN (vehicle ID number) ends in 927760 or less, then you DO NOT have directional rotors, and only need to order TWO of the following front brake rotor part number: 129 421 20 12 (the number has likely been superseded to 129 421 20 12 64). The US list price for this rotor is $105, and you can get discounted to around $80 each).

For later non-North American/Japan cars -- if your VIN ends in 927761 or more, then you have directional rotors with different part numbers. The part numbers are the same as for the later North American cars: 129 421 17 12 (left side), and 129 421 18 12 (right side). Both of these part numbers may have been superseded, to add a "64" onto the end of each part number.

If you have an AMG model, then your brake rotors are a separate part number, and that is outside the scope of this post.

 

[aliking]

There are three different front rotor setups for the 500E/E500, depending on the VIN number of your car, and depending on if it is a North American + Japan or "rest of world" car.

For early North American + Japan cars -- if your VIN/FIN (vehicle ID number) ends in 930494 or less, then you DO NOT have directional rotors, and only need to order TWO of the following front brake rotor part number: 124 421 26 12 (I believe this number has been superseded to 124 421 26 12 64).

For later North American + Japan cars -- if your VIN/FIN ends in 930495 or more, then you have directional rotors with different part numbers. The part numbers for these rotors are 129 421 17 12 (left side), and 129 421 18 12 (right side). Both of these part numbers may have been superseded, to add a "64" onto the end of each part number.

If you have a non-US car, you need to do more research, but I believe the following is correct information:

For early non-North American/Japan cars -- if your FIN (vehicle ID number) ends in 927760 or less, then you DO NOT have directional rotors, and only need to order TWO of the following front brake rotor part number: 129 421 20 12.

For later non-North American/Japan cars -- if your VIN ends in 927761 or more, then you have directional rotors with different part numbers. The part numbers are the same as for the later North American cars: 129 421 17 12 (left side), and 129 421 18 12 (right side). Both of these part numbers may have been superseded, to add a "64" onto the end of each part number.

If you have an AMG model, then your brake rotors are a separate part number, and that is outside the scope of this post.

Very useful and great info
Thank you so much

 

[aliking]

There are three different front rotor setups for the 500E/E500, depending on the VIN number of your car, and depending on if it is a North American + Japan or "rest of world" car.

For early North American + Japan cars -- if your VIN/FIN (vehicle ID number) ends in 930494 or less, then you DO NOT have directional rotors, and only need to order TWO of the following front brake rotor part number: 124 421 26 12 (I believe this number has been superseded to 124 421 26 12 64). Current list price is $75 each (you can easily get discounted to around $55-60 each).

For later North American + Japan cars -- if your VIN/FIN ends in 930495 or more, then you have directional rotors with different part numbers. The part numbers for these rotors are 129 421 17 12 (left side), and 129 421 18 12 (right side). Both of these part numbers may have been superseded, to add a "64" onto the end of each part number. Current list price is $102 for each parrt number (you can easily get discounted to around $75.80 each).

If you have a non-US car, you need to do more research, but I believe the following is correct information:

For early non-North American/Japan cars -- if your FIN (vehicle ID number) ends in 927760 or less, then you DO NOT have directional rotors, and only need to order TWO of the following front brake rotor part number: 129 421 20 12 (the number has likely been superseded to 129 421 20 12 64). The US list price for this rotor is $105, and you can get discounted to around $80 each).

For later non-North American/Japan cars -- if your VIN ends in 927761 or more, then you have directional rotors with different part numbers. The part numbers are the same as for the later North American cars: 129 421 17 12 (left side), and 129 421 18 12 (right side). Both of these part numbers may have been superseded, to add a "64" onto the end of each part number.

If you have an AMG model, then your brake rotors are a separate part number, and that is outside the scope of this post.

I think that im using the wrong rotors in my car, its a 1992 euro 500E, im using the sebro rotors pn: 129-421-10-12(06-12) and brake pad pn: 005-420-01-20 41
After i ride my car for half an hour i hear squeezing from the front brakes while driving even if not pressing the brake pedal, i think i should change those rotors asap.
Thank you

 

[gsxr]

129-421-10-12 rotors are 300x28 and are correct for early 500E. Don't change the rotors, it won't help.

The issue you describe may be due to the Brembo calipers. See TSB linked below:

http://www.w124performance.com/docs/mb/W124/Brembo_brake_noise.pdf

 

[aliking]

129-421-10-12 rotors are 300x28 and are correct for early 500E. Don't change the rotors, it won't help.

The issue you describe may be due to the Brembo calipers. See TSB linked below:

http://www.w124performance.com/docs/mb/W124/Brembo_brake_noise.pdf

Thank you gsxr, i will adjust the wheels bearing first, if that doesnt help, i will look for fixed calipers! Thank you so much

 

[gsxr]

The "repair" listed in that TSB is to remove the lightweight Brembo calipers, with heavy iron Ate calipers (available cheap, as they were used on most R129's). The Brembos are pretty rare and valuable, either keep them if you change calipers, or you could sell them to recoup some $$$.

 

[aliking]

The "repair" listed in that TSB is to remove the lightweight Brembo calipers, with heavy iron Ate calipers (available cheap, as they were used on most R129's). The Brembos are pretty rare and valuable, either keep them if you change calipers, or you could sell them to recoup some $$$.

I like the mercedes white star on the brembo caliper, i think i will keep them in my store and use the r129 calipers.