Hello,

One of the two 5mm Allen nuts bolts that hold the eccentric bottom bracket on my city bike no longer tightens: It just spins without holding its place.

Should I try a 6mm nut bolt, or maybe some kind of locktight material?

Thank you.

Sounds to me like it's stripped. If that's the case and it threads into the frame I'd look at a TimeSert.

https://www.timesert.com/

Thanks !

Remove the bolt and check to see if its stripped. Inspect the hold in the frame and see if the thread in it are stripped. You might run a new bolt of the same size into the hole and see if its loose. If only the bolt is stripped, you could try a new bolt. Perhaps use a longer bolt and add a backing nut if the back of the bracket is square with the hole.

It's likely that both the bolt and the frame threads are stripped. If so, and you understand how to do it, you can use a time-sert as stated above. To install a time-sert, you need adequate access to the hole (you have to drill the bad threads out, and use an oversize tap to cut new threads, after which you screw the time-sert insert in. It might not be feasible to fit the tools in.

If so, and the back of the threaded bracket in the frame is flat and square with the hole, you could just use the longer bolt with a backing nut. A self-locking or broaching nut might be useful (see below). You could even use a new bolt with a special nut that has leveling, or a spherical washer plus a nut.

Or a HeliCoil: https://www.stanleyengineeredfasteni...ands/heli-coil Timeserts look similar, but the HeliCoil is made of square-section wire; being thinner and having no top flange it may take less space and remove less base metal during installation.

dsbrantjr, I agree. My concern is not just getting the insert in place, but the drilling out that must be done. Might end up having to enlarge the bolt hole on the unthreaded side of the clamp.

But your suggest of helicoil inserts over time-serts (or ez-lok, too) seems to give the best chance of success.

If there's no clearance, or if the back of the threaded part is not square with the axis of the bolt, one could use a reverse counter-sink tool and an allen nut. The reverse counter-sink tool pic is from Huele+, the nut is from mcmaster-carr. The reverse counter-sink tool is inserted, spun up so that the cutting wing extends, and then pulled back into the hole to give a counter-sunk hole that is aligned with the bolt. This would avoid increasing the size of the unthreaded hole.

...it's difficult,if not impossible, to offer you good advice on the simplest practical solution that will work without taking the machine screw that no longer holds ouit, and inspecting the threads on both it and the hole as well (as stated above).

It honestly might be as simple as replacing the machine screw with one that still has good threads. Or as you suggest, rethreading the hole a size bigger and using a larger screw (unlikely, but still a possibility).

If you can post decent photos of the thing, removed, and of the hole, empty, you'll get better advice.

Or take it to someone who works on bikes regularly, or anyone you know who might be a tinkerer with mechanical stuff.

Or maybe drill and tap to the next larger bolt size. If that's truly an M5 bolt (not just a 5mm wrench size) an M6 thread could be cut* with almost full form and better strength than M5. If it is actually the more likely M6 bolt, then a full M8 thread could be cut*.

* In any case, including helicoils and inserts, check first for sufficient material in the boss before cutting.

Or, more elegantly, see if a longer through bolt with an external nut (top or bottom) can be fitted -- drill through the boss if necessary.

In my view, if it's a blind hole, then options are fewer for repairing the threads because you have to get in with the tools from the same side as the portion that supports the screw head. If it's not a blind hole, a longer bolt and external nut would seem to be the next thing to consider.

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I would be careful enlarging the hole to put in some kind of insert nut, helicoil, etc. Enlarging the hole might not leave you enough meat on the frame for a secure hold.

Simplest is best. You might be able to run a tap through there so a slightly longer bolt will catch enough for to hold. Or, as someone suggested, use a longer bolt with a nut. This is not a very elegant solution, it will look awful. But you can totally back out of it with nothing lost.

Another possibility might be to go up to a 6mm thread size. This might require drilling out the hole a bit more, but not nearly as much as an insert would require.

Be curious to know how you decide to handle this.

You might also be able to go with a M5 tap and bolt.

Thanks everyone for the feedback.

I found another M5 bolt of roughly the same length, and… it tightened just right. Problem solved :-)

If it didn't, I would have tried finding an LBS that could drill and tap both threads as M6.

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