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got into a slight accident at home and my saddle has slid off my seat post. It looks like the rear bolt is now bent out of shape and the front bolt won't tighten at all into the nut. I wasn't sure where I could find spares for these, as I don't know precise lengths or whatever – all I know is that a 5mm Allen key is what I need for fastening these.

Does anyone here know why my bolts won't fasten, and where I could get some spares? Thanks!

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    If your seatpost got whacked in a way to bend the bolts, most likely 6mm, make sure to check the integrity of the post, its head, the saddle rails and even the frame.
    – Carel
    Jan 10 at 19:44
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    If they won't tighten then most likely the nuts got stripped in the crash. Jan 10 at 20:07
  • Thread in the silver 'nut' looks stripped to me. How did this happen - sudden impact accident or came loose got worse over time? If the former, then have the frame checked by you local bike shop, and probably replace the seat post and seat. If the latter, replace the nuts and bolts, or maybe the whole seat post, your local bike shop will be able to supply.
    – mattnz
    Jan 10 at 22:07
  • I think the latter. I've been adjusting my seat little by little over the past few months because my replacement saddle wasn't that comfy. Thought I swap to my old saddle, then as I sat down, it came right off. Maybe too much Christmas weight! Jan 11 at 9:00
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Most likely they are standard bolts having a metric thread. To be sure, use caliper to measure the thread width (you measure the major diameter from a "hill" to a "hill", not the minor diameter from a "valley" to a "valley") and thread pitch (pick 10 threads, measure the distance between these, divide by 10).

If the thread matches the coarse threads at https://en.wikipedia.org/wiki/ISO_metric_screw_thread (for example if the width is 5mm and pitch is 0.8mm), you will very easily find a replacement. If the thread is a fine thread, finding a replacement will be slightly harder but not impossible.

It's also possible it's an imperial thread measured in inches: https://en.wikipedia.org/wiki/Unified_Thread_Standard

Also you need to be sure to replace the bolt with one having equal strength. If there's a marking such as 8.8 or 12.9 in the old bolt, be sure to select at least this strength. If the seller of a bolt doesn't specifically state the strength grade, most likely the bolt is junk -- don't buy such a bolt. If unsure, pick preferably 12.9 or at least 8.8.

You also need to select stainless vs ordinary steel. Stainless bolts usually can't be made to the highest grades, so ordinary steel bolts have a higher tensile strength and thus you can find bolt grades such as 12.9. However, in environments such as riding in salted roads without fenders where the bolts are subject to corrosion, stainless may be preferable.

Lastly, don't forget to select the correct length (measure the length of the bolt and pick a similar length), and don't forget to grease the bolt threads when installing the new bolt.

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  • wow this is super comprehensive! thanks so much. It doesn't help that I don't really have measuring tools at home but this is a great start. :D Jan 10 at 19:44
  • If you don't have measuring calipers take the cylindrical nut and the bent bolt along to the store.
    – Carel
    Jan 10 at 19:47
  • But first take you seatpost & saddle to the bike shop to find out if they need to be thrown away as damaged and unsafe. Maybe this is a good excuse to buy a new seatpost you've had your eye on?
    – Armand
    Jan 10 at 21:16
  • If the bolts have been bent and stripped, the nuts also need replacing.
    – mattnz
    Jan 10 at 22:11

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