Nuts & Bolts & thread pitch 'n stuff

[Gandalf]

Here's the thing.  I have an SRAM X5 rear mech on which I have knackered the thread which accomodates the 'B' screw.  The mech is is good low mileage shape otherwise.

What I was thinking was that I could buy myself a cheap Tap & Die set and re-thread the hole.  I've seen a set for as little as £12.50,  I know it will be cheap and nasty but I'm sure it will be adequate for my needs.

In the bumpf it mentions 'thread pitch', about which I know nothing.  Can anyone put me straight about this and tell me if what I'm proposing might work and if so what type of bolt to stick in. The old one is 20mm long x a bout 4mm.

I'll probably bugger it up and end up buying a new mech, but I thought it would be more constructive to at least give it a go, more in the interests of learning something than saving money really.

[Zoidburg]

Just bear this in mind.

If you have stripped the thread entirely then you will have to drill out and retap at a larger size.

You can chase out a slightly damaged thread to save it - if it is completely stripped out though you can't put back thread that is no longer there, unless you put in a helicoil, but again that means drilling out to a larger size to restore the original size thread.

I would buy a new mech.

[Biggsy]

I would try glueing the screw in place with epoxy, or red threadlocker if some thread remains.

Pitch is the distance from one groove of the thread to the next.  It doesn't particularly matter what pitch is used for retapping as long as you have a matching screw.  Small screws tend to stay tighter with a finer pitch, though.

[rogerzilla]

Assuming the body is aluminium, you can drill it to the next size up (possibly M4) and then tap it.  A quick Google will reveal the necessary drill bit diameter to use; for instance, it's 4.2mm for a standard M5 or 3.3mm for standard M4.  Finer non-standard pitches will use a slightly bigger drill, because the threads are shallower.

When tapping: use lots of oil, screw in two turns then back half a turn to keep clearing the tap, and NEVER apply any bending force to taps, as they snap easily.  For aluminium, because it's soft, you can just use the normal DIY second-order tap.  Big threads in harder materials can require three taps of descending aggressiveness to produce a good thread without making a mess of things.