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Thread: BE WARNED: Caliper Bracket Bolt Failure

  1. #1
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    BE WARNED: Caliper Bracket Bolt Failure

    Yesterday I had a second caliper bracket bolt failure within the last 60 days. The first was on the rear driver's side, and was the lower bracket bolt, so the caliper just jumped a little until I discovered it and fixed it. This time it was the upper on the front driver's side which caused the caliper to push into the rotor and grind.

    Both times the bolt just backed itself out. The first time this happened when I went to BMW parts in Dallas the guy in parts was a hobbyist and said that BMW had replaced the original part number with a different bolt some time ago and that he had seen this failure before.

    Everything was tightened to spec last year when I did the brakes. I figure this could be a disaster waiting to happen in the case that a bolt backed out and the caliper somehow locked up a rotor. I'm going to go ahead and replace all the remaining bolts with the new versions and would recommend to our members that you replace these bolts next time you do your brakes.

    Cheers.

  2. #2
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    I just had this happen to me as well. Noticed the only difference between bolts was that the new ones has what looked like dried up locktite on the threads. Just went and put locktite on the caliper mounting bolts and called it good

  3. #3
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    bolts backing out is typically due to not enough tension through the bolt. This can be fixed by installing a longer bolt with some thicker washers or being able to go to a slightly higher torque.

    Looks like realoem has one that is Zinc plated, and one black oxide. Same length and geometry though. (front)


    Looking at the rear, the bolt looks to have some thread lock. Are we sure they're not all supposed to get thread lock? anyone check the Bentley?
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  4. #4
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    This would be a good question to ask the track forum. Nothing tests like the track. I don't track but have not had an issue with caliper bolts in 9 years. However I have had the calipers off a few times but always torqued them properly.

  5. #5
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    Quote Originally Posted by pbonsalb View Post
    This would be a good question to ask the track forum. Nothing tests like the track. I don't track but have not had an issue with caliper bolts in 9 years. However I have had the calipers off a few times but always torqued them properly.
    Agreed I would be interested to hear what they say. I've been working on cars for over 20 years and never had a caliper bracket bolt back out. So having two in 60 days is either really bad luck or part failure considering they were torqued to spec.

  6. #6
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    Never had an issue in any of our track cars ... we do use locktite though on all the important suspension bolts. Maybe you some odd vibration that has caused the multiple failure? Seems unusual.
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  7. #7
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    Quote Originally Posted by ScotcH View Post
    Never had an issue in any of our track cars ... we do use locktite though on all the important suspension bolts. Maybe you some odd vibration that has caused the multiple failure? Seems unusual.
    My engine and tranny bushings, and most of my suspension bushings have been replaced with poly so I probably have more vibration than a lot of folks, but realistically, most of that has calmed down over the years and I'm sure it's not as severe as you guys with track cars. My biggest issue with vibration/noise at this point is the horrible drive lash you get out of light weight flywheels. >.>

  8. #8
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    For the LTW flywheel, have the tune programmed for an 850 rpm idle and use Redline MTL fluid. Takes care of most of it. Also, skip the bolt though poly trans mounts. Use isolated bolt poky trans mounts.

  9. #9
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    What program did you used to change the idle? I do have Redline MTL. It worked for a whole and eventually came back.

  10. #10
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    The bolts backing out are only an issue if not installed properly.


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  11. #11
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    This caliper carrier bolt problem has come up before. The general consensus among some of the the experts here has been 'don't worry about it, just reuse them', this in the face of BMW along with every other manufacturer's instructions to replace these bolts every time they're removed. Every time you torque a nut or bolt it loses a large percentage of it's maximum torque value. You can look up your own tables for this. Also, these aren't low grade fender bolts. IIRC, grade 10.9, or 12.9 is better if you track. Red loctite is stock from the factory for these. If you're gonna cut corners maybe the brakes aren't where you want to do it.

    By the way, the caliper pins that BMW sells for about $20 apiece can be gotten at any Dodge dealer for about $6 each. They use the same setup on lots of Dodge vehicles. Just show one to the parts guy.
    Last edited by tjm3; 06-29-2014 at 07:34 PM.
    See ya later,

    tony
    '98 M3, '92 Dinan3, '05 R1100S BCR, '07 R1200S, Aprilia T

  12. #12
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    I've had a caliper carrier bolt back out, but that was on an old 4 wheel truck driving across the country on old dirt trails with many many miles on a washboard road that caused your fillings to fall out

  13. #13
    NeilM is offline Member BMW E36 M3 Expert
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    Every time you torque a nut or bolt it loses a large percentage of it's maximum torque value.
    What? No — here's what happens to a fastener when you torque it:



    Normal fastening torques are specified to fall within the bolt's elastic range, below the yield point (see diagram). In this range clamping force rises linearly with bolt torque (excepting friction effects, see more below), and the bolt returns to its original length, without loss of strength, when released. Bolts so fastened may be reused a large number of times.

    A bolt tightened beyond its yield point goes into the plastic range, and will not recover either its original length or strength when released, and should never be reused. This can happen when you substantially over-tighten a bolt, such as by using the wrong torque value for it, or hammering it inappropriately with a big air wrench.

    However the flatter stress vs. strain curve as you enter the plastic range can be exploited to get a more accurate clamping force than in the elastic range, where the steep curve means large changes in force for small amounts of additional tightening angle. These are termed torque to yield bolts and must never be reused. They are recognizable by installation instructions consisting of an initial torque, followed by one or more final turning angles. Examples are BMW head bolts (30 Nm, then +90˚, finally another +90˚) and crankshaft main bearing caps (20 Nm, then a final +50˚). Torque to yield bolts are used mainly in critical applications, and often call for lubrication of the threads to minimize variations due to friction.

    Sometimes a manufacturer might specify using new bolts out of an abundance of caution — or being averse to liability. The specified mounting torques of both the M3 front (110 Nm/81 ft-kb) and rear (67 Nm/50 ft-kb) fall well within the normal tightening range for these bolt sizes/types.

    I can think of a number of potential reasons for fastener failure:
    1) Bolt not adequately designed/specified for the job
    2) Substitution of a different bolt that doesn't meet design needs
    3) Manufacturing defect
    4) Bolt substantially over-tightened
    5) Damage to the bolt/cross threading
    6) Bolt under-tightened

    In this case we can certainly eliminate the first reason, and likely the first three (assuming the proper use of original BMW parts). Over-tightening is certainly possible, but in practice it's less likely than you might think, since there's usually a pretty good margin between specified torque and the amount that would permanently weaken a fastener (unless the fastener size is quite small, not the case here).

    The last two points are somewhat related. If a bolt (or the receiving thread) is damaged, cross-threaded or very corroded, then some portion of the tightening torque is dissipated in overcoming friction (or in mangling the thread), and insufficient clamping force results. The same happens if the part is inadequately torqued, either in error or by mechanical failure (bad torque wrench). Either way the bolt is insufficiently stretched to be properly loaded in tension to clamp the parts being joined, and instead gets flexed by shear forces in use. Eventually it fatigues and breaks. (This is what most often happens when wheel bolts/studs fail.) A qualified person can tell the difference between fastener breakage due to over- or under-tightening by examining the fracture.

    Neil
    Last edited by NeilM; 07-01-2014 at 11:12 AM.

  14. #14
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    Thanks for posting that, I didn't have the time for the hassle, and you wrote it a lot better than I could.
    Last edited by Moron95M3; 07-01-2014 at 11:17 AM.
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  15. #15
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    Quote Originally Posted by NeilM View Post
    Sometimes a manufacturer might specify using new bolts out of an abundance of caution — or being averse to liability.
    Neil
    I like the tech stuff and chart. I have some equipment to test and retest nuts and bolts under load. The theory is excellent but practice varies.
    Bottom line is BMW thinks you have a better chance of keeping these parts in place by replacing the bolts. Any shop who turns a car loose that was repaired differently than what the manufacturer recommends, especially the brake system, has a potential failure on their hands so why would you do that if you're in business? Only needs to happen once to be ruined.
    Restated, if you're gonna cut corners maybe the brakes aren't where you want to do it.

    I'll add this. How would you feel when you picked up your car at the shop and they told you that BMW recommended these brake bolt replacements but they saved you $4 by using the old ones?
    Last edited by tjm3; 07-01-2014 at 01:40 PM.
    See ya later,

    tony
    '98 M3, '92 Dinan3, '05 R1100S BCR, '07 R1200S, Aprilia T

  16. #16
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    Quote Originally Posted by tjm3 View Post
    I like the tech stuff and chart. I have some equipment to test and retest nuts and bolts under load. The theory is excellent but practice varies.
    Bottom line is BMW thinks you have a better chance of keeping these parts in place by replacing the bolts. Any shop who turns a car loose that was repaired differently than what the manufacturer recommends, especially the brake system, has a potential failure on their hands so why would you do that if you're in business? Only needs to happen once to be ruined.
    Restated, if you're gonna cut corners maybe the brakes aren't where you want to do it.
    They HAVE to say to replace fasteners that are safety related to cover their ass when an indy overtorques something and BMW gets the blame.

    If you are your own mechanic and know that you A. Have a good bolt. and B. Know that you are the only one to tighten it, then you can reuse assuming you've torqued it properly.
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  17. #17
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    Like I said, practice varies. You're right, though.
    Personally, if I start with a new bolt on my car I'll reuse it a second time only. Then replace.
    On a 'not my car' it gets new stuff. It's just not worth spending the weekend worrying about the guy who's going to try to drive a hundred miles of coast highway at 80+ mph.
    See ya later,

    tony
    '98 M3, '92 Dinan3, '05 R1100S BCR, '07 R1200S, Aprilia T

  18. #18
    NeilM is offline Member BMW E36 M3 Expert
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    Quote Originally Posted by tjm3 View Post
    How would you feel when you picked up your car at the shop and they told you that BMW recommended these brake bolt replacements but they saved you $4 by using the old ones?
    However as I recall BMW does not recommend that the caliper bracket mounting bolts be replaced. But my memory about that could certainly be faulty, so I just looked it up in the TIS (ref. 34 11 519, 34 11 2AZ), and indeed they do not need to be replaced. Loctite isn't called for either.

    I agree about replacing fasteners if they're getting ratty from multiple assemblies/disassemblies, if they're corroded, where the manufacturer calls for it, or sometimes even if it just seems like a good idea. I keep a substantial stock of new fasteners on hand for all of those reasons.

    But since this caliper bolt breakage happened twice to the OP, and on different corners of the car, I'd tend to suspect either a torque wrench problem (mechanical or human), or perhaps previous abuse to the bolts, maybe by a previous owner or a hack mechanic. And then there's always the strictly human factor. Which of us hasn't been interrupted by someone and lost track of what we've been doing on the car? I know I have — ask me how I know what a loose wheel sounds like when you're driving...

    Neil
    Last edited by NeilM; 07-02-2014 at 07:46 AM. Reason: duplicate word

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    Happened to my 325is, years ago.

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    If we replaced every bolt after 2 uses on the race cars, we'd have to have a warehouse full of nuts and bolts ... After each weekend (and during), many nuts/bolts gets loosened tightened. As long as the hardware is in good shape, and it's not a torque-to-yield, and you use proper tools, they are perfectly fine to reuse.

    But as you said ... not everyone uses the proper tools or techniques, and that's very likely the issue in this OP case.
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  21. #21
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    this is old yes....but Im under car and doing brakes....

    97 m3...

    what is torque spec for these frt brake caliper bracket bolts?

    Ive always just gone "really really" tight on my e30 track car.

    - - - Updated - - -

    Im seeing/searching 81 ?


    ok thats really tight kinda like a lug nut

  22. #22
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    RRSperry is offline Senior Moment Member BMW E36 M3 Expert
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    IIRC it's something like 37ft-#. Oh and while the thread locker on the factory bolt is pink, it's not locktite RED. If it were, you'd need a lot of heat to remove it.
    37 or even 47 #-ft is not doing to deform that bolt. The important thing is getting all the old thread locker off the bolt and out of the hole.
    Last edited by RRSperry; 06-03-2024 at 07:24 AM.
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  23. #23
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    Hey. I have been reading 81 ft lbs in multiple places. Your response is surprising

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    BE WARNED: Caliper Bracket Bolt Failure

    Quote Originally Posted by jrkoupe View Post
    I have been reading 81 ft lbs in multiple places.
    Confirming from Bentley, TIS would be better…
    Last edited by bluptgm3; 06-03-2024 at 01:59 PM.

  25. #25
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    Was already written here, by NeilM in post #13

    "The specified mounting torques of both the M3 front (110 Nm/81 ft-kb) and rear (67 Nm/50 ft-kb)"
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