What can I do to keep a threaded bolt from falling out of its slot?

Question

I bought a new Chinese bike, 250cc. The bike was delivered in a crate, and the handlebars had to be screwed in by the end user.

One of the bolts keeps loosening, and if I forget to tighten it routinely, it will fall out. I’m looking for effective ways to keep this from happening. The first method that comes to mind is welding it in there, but what if I want to change the handlebars later in my product's lifecycle? Also, welding might not be the most economical solution.

What are some solutions to keeping my threaded-bolt from loosening and eventually falling out?

Answer 1

The easiest, most economical method I can think of is to use Loctite Blue 242. You apply it to the thread, torque it to spec, then within 24 hours it will keep the bolt held in place without issue. If the Blue ever comes loose, you can step up to the Red, which requires heat to bust it loose. It will not come loose without a LOT of effort.

NOTE: I have no affiliation with this product.

Answer 2

I agree about the Loctite products but a simple lock washer or lock nut might be another possible solution as well (depending on the length of the bolt available). Some handlebars are "secured" within a U shaped clamp with a bolt going through the ends of the "U" to squeeze the clamp down on the handlebar assembly and the harder you ride that bike the more those handlebars will come loose as the metal rubbing against metal slowly, or not so slowly, wears away the metal or the finish (street cruising, check it occasionally and tighten it as it gets loose ... back country motocross then you could have a catastrophic failure during your ride). A 250cc seems like you'd be having some of the off-road fun.

Answer 3

There is something that bothers me: Handlebar bolts, as safety critical items, aren't supposed to become loose. Either you torqued them wrong (too loose or too tight) or there is something fundamentally wrong with it.

I would inspect the bolts / entire assembly and check if

• The thread is intact and fits the matching hole. Does the issue persist if you swap bolts?
• All the bolts look identical (Size, threading, metallic shine).
• Does the problematic bolt look distorted/stretched?
• Does the clamp look distorted/overpinched?
• Is the receiving thread intact? You can check that by "feeling" the torque on tightening the bolt: If it gets harder and harder the thread is good, if you can "spin" it forever with moderate torque the thread is gone.

Depending on the root cause (Ordered from most likely to least) I now list possible solutions

• There is not enough tension on the bolt. Solution: Try to find the correct torque (there ought to be a specification) and use a torque wrench. "Feeling" the right torque depends on experience and often lead to mixed results (bad in things that could put you in danger).
• The receiving thread is bad (either from the factory or during assembly). Solution: You can use something called "heli-coil". Search this site for further information.
• The bolt is bad. Solution: Get a matching replacement, but pay attention to the right grade.
• The supplier took some shortcuts and gave you handlebar clamps made of some cheap pot metal with the strength of a wet sponge. The creeping clamps will make the bolt loose tension. Solution: Ask you local motorbike shop for an adequate replacement.

Answer 4

If you can replace the bolt with one equipped for use with a lock wire/safety wire (e.g. one with a hole drilled through the bolt head from the side, or possibly a castellated bolt), and you can find something to tie the end of the safety wire to, this is one of the most reliable ways of keeping a bolt in place which is subjected to vibration stress. (This is the technique used by aircraft and other heavy industrial uses where a bolt failure can be catastrophic.)

Answer 5

I have to agree with some of the other posters: Something is not right with the fastening, and/or the proper clamping pressure is not being achieved, perhaps due to under-torque.

That said, I would rank the simplest solutions from best to worst as follows:

1. castellated nut with cotter pin, hitch pin, safety wire on nut or bolt head
2. adhesive threadlocker "Locktite"
3. double nuts
4. locking nuts, either distorted threads or "Nylocks"
5. lockwashers, star washers, etc.

Note that #5 really shouldn't be on the list, as once they are compressed, they are nearly worthless. Note also that #1 may be very difficult or nearly impossible to achieve for your application. Any or all of these solutions can be combined, but past #2 you are looking at very diminshing returns.

All of these assume a properly designed and torqued fastener for the application. Perhaps that should be solution #0...

Once again, I will seriously recommend Carrol Smith's Nuts, Bolts, Fasteners, and Plumbing as a must read for anyone desiring a real understanding about how fasteners work, and how to keep them working reliably. It should be mandatory reading for any "mechanic" or mechanical engineering student. Simple and readable, this is not a math book, but an encyclopedia of fastening with knowledge gained from decades of experience with race vehicles and aircraft - a place where a loose fastener can represent loss-of-life, vehicle, and mission, as opposed to a mere maintenance headache.

It bothers me to this day that otherwise very mechanically inclined folks still believe that lubricants or grease can cause a properly torqued fastener to loosen, that lubricant causes less than measured torque, that left-hand threads resist loss of torque from parts that have right-hand rotation, that some dirt or rust upon installation is a good thing as it keeps fasteners from loosening, multiple clicks from a torque wrench is a good thing, the torque wrench I inherited from my grandfather is still accurate, that all bolts can be reused indefinitely, and that you can trust your life to a lock washer.

[sigh]

Answer 6

The thread locking compound and/or lock-washer are excellent suggestions assuming you're not loosing clamping force due to bolt stretch, bolt length (too short or bottoming), threads pulling or martial softness of the clamping surfaces.

I'd check all of these things out because bolt "torque" does NOT equal clamping force which is what holds the handle bar in place.

Answer 7

You could also try re-tapping the connection

https://www.instructables.com/id/How-to-Tap-Screw-and-Bolt-Threads/