Tidying up damaged threads (yes, caused by ham-fisted fettling)

[mattc]

I've never owned them (perhaps it's time??) but obviously taps-n-dies are the preferred way to do this.

But if it's just a couple of turns that are damaged, any advice on fixing them "manually"?

[LittleWheelsandBig]

It depends on exactly what is damaged. Is it a bolt or a frame component, a large diameter, coarse thread or a small, fine thread, steel or aluminium, through hole or bottoming thread?

[mattc]

Threads plural!

Shimano square-taper crank-bolt - male AND female are damaged.

I'm resigned to binning the lot - it's probably only a UN55 at best - but it might be worth a little dicking about to save me the time required to buy/swap-out the flippin thing!


[I think if I didn't have the Internet to ask, I'd just grind off the damaged sections ... #bodgetastic ]

[LittleWheelsandBig]

How did you manage to damage the axle thread? Clean out the axle thread with a pointy metal thing first.

I'd just replace the bolt but filing off a couple of threads wouldn't be a major problem.

[Brucey]

the female threads in the spindle are in hardened steel and are quite difficult to damage. However it is possible, e.g. if the end of a crank extractor tool is sufficiently knackered.

It is usually possible to pick crud out of those threads as LWaB suggests, and/or to grind a relief in the last couple of threads using a dremel tool. Even if you had a nice tap with the right thread on it, I wouldn't advocate running it into the spindle because it will soon be ruined by the hard steel.

You will know when you have succeeded because a (good) bolt will go into the spindle properly.  The damaged bolt is best replaced, but it is often possible to salvage one by careful dressing even if it is damaged.  By all means try the bad bolt in the other end of the spindle, having dressed the threads with a swiss file or something.

cheers
 

[Torslanda]

I think if I didn't have the Internet to ask, I'd just grind off the damaged sections ... #bodgetastic

Then watch the whole thing break in the middle of Shap!*

The BB bolt uses an M8 x 1.0mm thread, this is NOT a common size and almost certainly not part of any generic tap & die set. A tap will cost a few quid but you need to exercise caution. Not all steels are the same. Shimano axles are 'bloody hard'**. Carbon steel will just not do!

As regards the crank bolts, a replacement pair will cost considerably less than the matching die. Given those odds I would exercise extreme prejudice and replace them.

Matt, don't bugger about. If it was mudguard bolts then bodge away but this really is safety critical...

*Insert remote, inconvenient location of choice.

**So hard that I'm surprised you've managed to damage the thread in the axle. If the bolt is stripped it's most likely debris from the bolt that's lodged in there.

ETA: X-posted