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#1
11-19-2018, 10:20 AM
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W124 Rotor Retaining Screw Broken
So it looks like when a local indy did my front rotors and brakes ~4yrs ago, he broke the rotor retaining screw on my front right rotor. Instead of fixing it, he left it figuring I would never notice.
Well, I noticed while doing the bearings but did not have the time to address immediately. Is this a huge issue? My gut says YES cause its part of the brakes....but clearly I've been driving for 4 yrs with no catastrophic problems. Wondering if this can wait till the spring to drill it out and tap a new one in or if I should deal w/ it now.
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1995 W124 M104 E320 Wagon. 175k Miles and Counting 
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#2
11-19-2018, 10:31 AM
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The retaining screw is more of a convenience item to keep the rotor from flopping around when you have the wheel off. When the lugs are tightened in place, the wheel is what actually holds it in place against the hub.
If the screw is broken off down in the hub, I don't think I'd waste my time drilling it out. It won't affect brake performance.
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Current stable: 1995 E320 149K (Nancy) 1983 500SL 120K (SLoL) Black Sheep: 1985 524TD 167K (TotalDumpster™) Gone but not forgotten: 1986 300SDL (RIP) 1991 350SD 1991 560SEL 1990 560SEL 1986 500SEL Euro (Rusted to nothing at 47K!)
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#3
11-19-2018, 02:54 PM
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It also indexes the rotor to the hub and prevents the rotor from coming off with the wheel rather than staying with the hub. Any stub of broken retaining screw will do the indexing. I don’t know how likely the rotor will galvanize more to the wheel than the hub. I’ve had to pound on wheels to get them off the hub but I think that’s the hub centering ridge rather that the rotor face. Shouldn’t be an issue with a regularly maintained or carefully stored car.
Sixto 98 E320s sedan and wagon 02 C320 wagon
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#4
11-19-2018, 03:37 PM
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^^^^indeed^^^^
In wetter parts of the world wheels rust onto discs and then you're in a pickle if the retaining screw isn't used. Hammering a wheel off a wriggling disc can knacker a caliper. I'd drill the snapped off screw out and re-tap if necessary. A thin thin thin smidge of copper ease on the mating surfaces between the wheel and the disc ain't a bad thing either
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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!
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#5
11-19-2018, 04:03 PM
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Quote:
Quote:
There are a couple of other reasons why bolt attached wheels have a retaining screw. 1 It keeps index of the rotor holes and hub holes when the wheel is off. 2 A flake of rust "could" fall between the hub and rotor causing a braking vibration but , again , stud mount wheels survive just fine. ( Brake vibration is caused by a thickness variation more than it is caused by a wobbly rotor. ) Wheels corrode to the hub index and not to the face of the hub so the rotor coming off with the wheel won't be an issue. The caliper will prevent losing the rotor during a wheel change.
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#6
11-21-2018, 08:37 AM
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 I have swapped that wheel out many times since that last brake change and never had any issues with the rotor. I am thinking of redoing the suspension in the summer so while I have it jacked up, I will re-tap it
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1995 W124 M104 E320 Wagon. 175k Miles and Counting
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#7
11-21-2018, 09:17 AM
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What specifically did the tech do wrong to break the bolt and what could they have done to not break the ( non critical ) bolt? In rust belt areas, bolts constantly seize and or break during removal. If a shop was liable for every stuck bolt, they would be out of business. Think of this, labor time guides are based on new cars without rust. An older rusty car can easily add 30% more time to the job. Should the shop be liable for the extra labor? Some MB dealers won't work on a car that is over 10 years old, rust bolts are one reason.
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#8
11-21-2018, 09:33 AM
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I suppose it would be asking a lot for the tech to check before he started and use a bit of PB blaster or something of the sort...That would really add up for him since the guy usually does old euro cars. I will say though I think it was kinda unprofessional to not atleast tell me it happened and sweep it under the rug...or under the rotor as it may be in this case 
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1995 W124 M104 E320 Wagon. 175k Miles and Counting
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#9
11-21-2018, 05:22 PM
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#10
11-21-2018, 09:04 PM
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Had this been a critical bolt, I'd agree that it should have been repaired but at customer cost. If a tech damages non repair related parts, yes they should fix it. ( the brake fluid on wheel example ) If an exhaust system needs to be removed for a repair and the exhaust breaks apart during removal, should the shop be liable for replacing it? How about this. The next time you have a car repaired, state in detail your expectations up front. Use the broken brake rotor bolt as an example and that you expect any bolts / parts that break during removal to be repaired at no extra charge. If they agree, great for you. For more context, I've been around the auto repair / industrial machinery / construction equipment business 40+ years and have seen lots of broken rusted stuff. PB blaster and other penetrating oil won't help every badly rusted bolt or one that was stripped in during a prior repair / or from the factory.
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#11
11-22-2018, 11:23 AM
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The rotor screw is for assembly line production, it doesn't make any difference whether the screw is there, whether the rotor indexes to the hub pattern the same way or anything else. The screw is there to keep the rotor from falling off the hub while the chassis is going down the assembly line before the calipers are put on. You can run without it just fine.
If you just have to repair it, get a left hand drill set. You start with the smallest drill in the set and step drill up until you either get the screw drilled out to where you can pick the remaining threads out, or if you are lucky, the drill hangs up and the screw backs out. Left hand drill set can save you hours of work in many cases.
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#12
11-22-2018, 01:34 PM
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I worked on a '02 CLK with a busted off retaining screw. As has been said this is not a critical part, but you might want to have it based on your personal preference. This was not the worst problem however, all 4 dust caps had been left off the caliper pin slide tubes. And the owner assured me that the brakes had previously been done by the local Mercedes dealer....... sad.
Anyway, I went to drill out the remainder of the screw in the hole. I had the hub off since I was doing the bearings also. I know I had a right hand drill bit, but I don't remember if I was drilling from the front or back of the hub. Anyway, the screw spun right out easily. It was actually loose. So the question is then, how do you break off a screw that is not seized? My guess is the mechanic had his impact set the wrong direction and snapped it off. Maybe he does them all that way and leaves all the caps off to help the pads wear faster... The caliper brackets hadn't been cleaned at all either. I remember being to this dealer and marveling at how clean their shop floor is (they have a big window you can see it through). Now I know why it is so clean!
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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine) 1988 F150 144,000 miles (leaks all the colors of the rainbow) Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra
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#13
11-22-2018, 02:21 PM
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"I know I had a right hand drill bit, but I don't remember if I was drilling from the front or back of the hub. Anyway, the screw spun right out easily. It was actually loose. "
Yes, if you use a right hand bit, it will push it on through. Left hand removes it from teh front side. If there is no issue with the screw coming out the back and getting hung up, right hand will also work. " Anyway, the screw spun right out easily. It was actually loose. So the question is then, how do you break off a screw that is not seized? My guess is the mechanic had his impact set the wrong direction and snapped it off." I often end up removing screws and bolts that have been broken off both on assembly by overzealous impact users and also on disassembly, when the shank of the screw is rusted or corroded in the clear drilled part being held by the screw. In both cases, the screw can usually be removed with your fingers if it were not broken off below the surface. I have also removed some by using a sharp pick and simply unscrewing them until it can be held with your fingers. Only gotcha is when the screw wrings off and buggers up the first couple threads or the threads themselves are corroded stuck. In that case, sometimes you can TIG weld a nut to the screw through the center and remove with a wrench or it may just be hopeless to remove short of drilling out and re-tapping the holes. Worst case, you helicoil it, weld up and re-drill and tap the hole, or replace the part, in that order.
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