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I am in the progress of doing some work on the suspension. Can I reuse the nuts on the bolts holding critical parts (struts, tie rod, etc.) if I clean the threads and apply thread locker?

My aim with this question is to understand the reason for the manufacturer's recommendation to always replace the nuts:

  • Does the nut experience a change in the material (similar to torque to yield bolts), thereby lowering its strength?
  • Or is it just that the plastic locking ring gets consumed, thereby making the self-locking mechanism unreliable (correctable by applying thread locker)?

Note: Every bolt that gets torqued to a specific angle will get replaced, should I encounter a torque to yield bolt (unlikely) it will of course also get replaced. I am asking especially about the nuts. I do not want to be careless on safety relevant fasteners, I am just trying to tell the important things from the unimportant.

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    To be honest, I've never replaced one which still held. The main purpose of these is to ensure they don't vibrate loose. Almost every one I've ever seen has had no issue with this ... leaving this as a comment because there may be a real reason I'm unaware of and not just my speculation. – Pᴀᴜʟsᴛᴇʀ2 Oct 3 '16 at 21:32
  • @Pᴀᴜʟsᴛᴇʀ2 - Does the comment say what you mean? It seems to contradict itself "never replaced one which still held" vs. "almost everyone I've seen had no issue with this." Am I missing something? – dlu Oct 3 '16 at 22:28
  • @dlu - I'm sorry, it makes sense to me, and not sure how to make it clearer. If the nut still holds, it's still working and why should you replace it? I've probably replaced a few during my time, but these are far and few between. I have never really given thought about replacing them even after using them several times after being taken apart. Most all used lock nuts haven't had an issued holding. – Pᴀᴜʟsᴛᴇʀ2 Oct 3 '16 at 22:41
  • Makes sense to me now too. Read the first sentence to mean "I've never put one back on which held." – dlu Oct 3 '16 at 22:44
  • safety item. don't take chances when in doubt. – 1a Auto Mobile Jan 13 '18 at 7:45
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I'll make a couple of cogent points, but I doubt I'll be able to organize this coherently.

  • Go get and read Carrol Smith's Nuts Bolts Fasteners and Plumbing. This is religious tome-level knowledge that will transcend all time. Although mostly about racing and motorsports, the concepts still work and apply to all fastener connections.

  • Carrol will be quick to tell you that split washers, bellville washers, and so-called "Nylocks" are garbage. DON'T put your reliance on such things.

  • The reason fasteners stay fastened is ultimately due to TORQUE which creates CLAMPING FORCE. That split washer is NO deterrent to vibration-induced failure when it's crushed flat. Torque is king, and proper torquing methods of utmost importance.

    • (a) That said, I have not found many automotive applications (especially suspension) that use or require "critical" torque, "torque to yield", "stretch bolts", or other specific fastener requirements. (I'm assuming we are talking about road cars, not Formula 1.) Most applications are pivots in double shear, or have a weak female thread (e.g. weld nut in unibody). The torque requirements specified by the OEM for these applications will be far less than the maximum torque specified for the fastener by the fastener manufacturer, which will be perhaps 80% of the plastic deformation value -- for a so called "critical torque" application. Most cases these bolts will be Grade 8.8 in metric, not 9.8 or 12.9. The possible exceptions I have noticed are seat belt anchors and similar... but not suspension components.

    • (b) Keep in mind that as much as 75% of the final torque value is used up by friction, not by the bolt stretch which creates the clamping force.

    • (c) Clamping Force is what keeps the mechanical joint connected. Not friction. Friction only comes into play when the fastener is being torqued. This is why companies like ARP provide their own special lubricant to be used while torquing. The delusional fantasy that your wheels will fall off if you use anti-seize on the studs comes from a long history of misconception about how fasteners work. Lubricants may change [increase] the final torque value. They do not, however, create vulnerability to vibrating loose; in fact more torque makes more clamping force which makes the nut backing off less likely.

  • Threadlocker compounds, when used properly, work on anything. Even a Nylock, burned out or not. Even Carrol will say that if you can't safety wire, use thread locking compounds. It's still NO excuse not to torque properly.

  • Also realize that in any male-female threaded connection, there are only 3-4 threads in meaningful contact. Whaaat??? That's right, just like a barstool with more than 3 legs, only 3 bear most of the load. That nylony (probably not a word) bit isn't that meaningful. Super sticky alphacyanoacralate (no matter the color) might be significant.

  • "Torque to angle" on a suspension component? Never even dreamed of such a wild beast. I hope we are talking cars and not Superbikes or something. In any case, just because a fastener is "torque to angle" does not mean it cannot ever be reused. I submit ARP "American Racing Products" head studs/bolts as exhibit A. The very point of these expensive aftermarket fasteners is their ability to be reused, especially valuable in racing and motorsport applications where heads are removed and replaced frequently. On the other hand, if the OEM says "must replace", you won't finding me suggesting otherwise. (Although you might find me DOING otherwise, but that's my liability, not yours.)

Bottom line: if you have cheap, dirty, and/or rusty nuts, consider replacing them. But if you carefully clean and maintain your nuts, you can reuse them, at least until someone in authority tells you not to.

Proper torque, proper job.

  • Yes: "Torque to angle" on the suspension of a car. On a Opel Corsa the two bolts holding the strut to the steering knuckle get 80 Nm + 60° + 15°. – Martin Oct 4 '16 at 7:20
  • Thank you for your answer. Please help me with the technical terminology: What does "Critical torque" mean? Are "ARP Bolts" just a brand name or a very specific thing? – Martin Oct 4 '16 at 8:17
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    Great answer! I'm a big fan of Carroll Smith's writings, though I've not read the nuts & bolts one. I would think since most suspension bolts/connections are not statically loaded like a head bolt, you wouldn't want to use T2Y bolts. This seems ludicrous to me to use these types of fasteners for that application. I would also submit, ARP fasteners should not be considered T2Y. In most cases, ARP will give you a new torque value for their fastener. And yes, they should be reusable. I wouldn't buy them if they weren't due to the cost involved. – Pᴀᴜʟsᴛᴇʀ2 Oct 4 '16 at 10:23
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    @Myself I edited my content to [hopefully] further explain your questions. ARP is a brand of specialized reusable fasteners, and "critical torque" meant to refer to applications where the fastener is torqued to the fastener manufacturer's maximum specification. This automatically implies that both the male/female parts of the fastener are the same grade, and probably the same manufacturer - although if a fastener is certified 12.9, or AN, or some other aerospace specific rating, there's no reason why different provider of "nut" vs "socket head capscrew" cannot be combined. Very rare in cars. – SteveRacer Oct 7 '16 at 8:17
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There are two reasons why they may be "always replace" items:

  1. To (try to) ensure that there is good thread locker on the nuts, perhaps because thread locking compound doesn't work reliably on nyloc nuts - I could see it being difficult to reliably clean the threads, or perhaps the threads are slightly looser than they "should be" for use without the nylon insert. For whatever it is worth, VW uses locking nuts on the suspension that don't seem to be either nyloc or distorted threads, and the claim is made that the "locking mechanism" is one-time only.

  2. To ensure that the corrosion proofing on the nut is in good condition.

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