I need a double check on minimum rotor thickness for my rear brakes on my 126, specifically my 86 300 SDL. Thanks

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83 300D 227,xxx miles, sold
86 300 SDL 130,000 miles, sold

If the Rotor is off of the Car 90% of the time it is stamped on the rotor somewhere. Sometimes on the outer edge.

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84 300D, 82 Volvo 244Gl Diesel

Yes, I thought that also but can't find any numbers. It does have a recess cut in the outer lip but its pretty thin. I was wondering if,as long as the recess is complete then the rotor is good. Just thinking out loud. Thanks

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83 300D 227,xxx miles, sold
86 300 SDL 130,000 miles, sold

I've got data for W123 disc brakes that may be close=>

Front new = 12.6mm thick
Front service limit = 10.6mm

Rear new = 10mm thick
Rear service limit = 8.3mm

Maximum run out is specified as 0.12mm for both front and back

If your discs are of a different starting thickness then I guess a guide would be that the front and back can wear by about 15% (judging from the data in my Haynes manual I've given above)

'Hope this helps

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

Fronts; 28mm new, 25.4mm wear limit.

Rears; 10mm new, 8.3mm wear limit.

Army, the 126's have a vented front disc, hence the big difference from the 123's.

I also think the recesses are there for a reason, but am not sure.

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83 SD

84 CD

Thanks everyone, my local auto parts chain had it wrong and wouldn't turn the rotors since they said they were under. I tried to reason with them on the problem but it just irritated them. When they set the caliper to what they said was spec it rattled around on the rotor with enough gap to throw a medium size cat through. I knew it wasn't right but didn't have any real numbers. So I returned to my garage and chucked them up on my lathe and cleaned them up. All is well now, I can turm rear rotors so if anyone gets in a real bad jamb or needs some cleaned up that are too close to min spec for mass production shops I might be able to help. It's a real pain but beats buying new when they are still with in range. Thanks

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83 300D 227,xxx miles, sold
86 300 SDL 130,000 miles, sold

You're welcome, MB recommends against turning, but if you're within specs, I don't see an issue.

I would think on the front simply replacing with new is the way to go, due to the time consuming bearing hassle.

If it's close to minimum, then wears below that and the pads wear down below their limit, your caliper piston may push completely out of the caliper resulting in no brakes on that axle.

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83 SD

84 CD

Could you quote a MB publication that makes said recommendation? None of the service manuals that I possess recommend for or against rotor machining.

How is the "bearing hassle" avoided when replacing with new?

A loud, metal-on-metal squeal should provide plenty of advance warning prior to "piston departure."

Interesting to see that the front vented discs on the W126 have a wear limit of about 10% where the solid W123s have 15%...

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

it's not a percent of wear overall thickness, it's wear of thickness of the surface steel.
the 126 has a large chunk of air in the middle...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
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If you decide to turn the rotors and put 'em back on, just to swap 'em out again relatively soon, by all means. I cannot however, locate anything from MB right now. If they need turning, they may be warped, meaning to turn them down to flatten them out. Most folks don't access to a lathe, (the OP mentioned mounting them on a lathe was time consuming, I mean "a real pain") so simply replacing them would be prudent.

You don't avoid the bearing hassle when installing a new rotor, but why do this twice at short intervals? You have to go through the hassle putting a nearly too thin rotor too.

My first MB blew a front bearing at interstate speeds, seemingly because the fella that put 'em on didn't go by the book. Cost me $1100 to have it corrected. The rears do not have such an issue, it's simply unbolt the caliper and remove the rotors.

It's your call to take chances with the braking system. I don't know how much extra is built into the thicknesses, but I would rather replace a close to too thin rotor than risk a potential caliper piston disengagement. I wouldn't trust a caliper once that happened, just more money to be spent than otherwise budgeted.

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83 SD

84 CD

That theory doesn't necessarily hold water. I machined the front rotors on my W123 twice in 150k miles. And they still comfortably exceeded minimum thickness.


The minimum thickness specification for brake rotors is driven by the rotor's ability to absorb and dissipate heat.

See para on Rotor Thickness:
http://www.aa1car.com/library/2003/bf110322.htm
At no time have I recommended "taking chances" with the brake system. But, from all indications, anyone who relies on an internet discussion forum for brake service information is doing just that.

Correct. I always thought it was for heat as well, until I saw this happen.

In the Chicago area market, new rear rotors for an 86 300 SDL run $18 to $35 each depending on brand & vendor. At this price, it just doesn't seem worth turning old rotors.

$9.95 to turn a rotor in Atlanta. Just saw an ad.