Seat Bolt size?

[ruddlad]

Can anyone let me know the size of the above? Mine felt slightly stiff yesterday replacing the seat so want to tap it, don't have bike with me to check. I am guessing it's 8mm

[Sid Squid]

From memory M6 - you're not the first to need the threads tidied up .

[ruddlad]

Excuse the idiotic question, so I'll need a 6mm tap & die then??

[Sid Squid]

If the thread in the frame has gone completely and the bolt will not grip then material is missing from the hole and the original size will not work, if the thread is just a bit graunchy you may be able to tidy it up with a suitable tap, but carefully check the bolt too, if the thread on that is damaged then putting it back in will undo any tidying up you've done. If your bike is a Curvy then a common trick is just to use a nut behind the lug on the frame, it's a little fiddly to fit, but obviously it's an easy fix. Similarly, I've welded a nut behind the original one on Curvies a couple of times.

If the threads are only slightly damaged you can make your own tidying up tools with another nut and bolt of the same size and a junior hacksaw. For the nut cut through a flat, across to the thread on the opposite side and into that about a millimetre or so, so the nut resembles a 'C' rather than an 'O', tidy it up so there's no burrs, if the nut is a sloppy fit on the bolt crush the gap slightly so it's snug, then run it up and down the bolt a few times and if the damage is only slight this will clean the thread grooves up. For the bolt cut two or three grooves equidistant around the bolt into the thread as close to the axis of the bolt as you can, use a wire brush to remove any burrs and run that through the hole in the frame a few times, removing it each time and cleaning any debris from it.
Obviously these tools are not as hard as taps and dies, so won't cut a new thread, but if the damage is only slight they'll tidy up the existing thread.

If the hole is too badly damaged and too much material is missing you'll need to have the thread repaired with a Helicoil or similar, or drill it to the next size up, tap it and use a bigger bolt.

[ruddlad]

Job done, quick tap and sorted, that said, the bolt was missing!! I recall gettiing hit on the neck riding back yesterday, thought nothing of it, but must have been the bolt.