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You need to know the diameter and pitch of the threaded part primarily.
If you measure the width of the threaded section I guess (judging by the size of your thumb…) it would be around 3 or 4mm. Pitches are generally standard unless it’s something specific and bike parts rarely are that niche when it comes to bolts. Buy some countersunk bolts of the right length and it will very likely be correct. Just compare the existing thread to the new thread, diameter is obvious enough just put the two ends together. For the pitch put the two bolts length ways against each other the teeth of each bolt should mesh perfectly together to show if it’s the correct thread pitch. If not then it won’t fit and don’t try to jam it in as it will damage the part.
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Dogtemple
Is there any way to ID this bolt without accurate measurement/any way to measure the thread? They hold my derailleur hanger on and one of them has disappeared at some point. Assuming I shouldn’t be using the bike with just one in.
Rough measurements using tape measure have it at 8mm long with an 8mm wide head. 2.5mm hex head.