How do I tell what bolt I need?

From memory, bike seat bolts usually have a 6mm Allen key socket in their head. I'd guess that the thread would be M6, M7 or M8. The head is usually recessed into the clamp, which can make finding replacement bolts more tricky.

You could try some local bike shops. They'll have a few shiny new bolts in blister packs on display, but the smaller shops will probably have a box of miscellaneous bolts under the counter to rumage through.

The message from "TC" contains these words:

It's almost bound to be metric if it's a recentish bike. Probably 8mm.

Apparently its M5 x 40mm - I assume the threads are always the same. Any recommendations for somewhere to get something strong on the web? Thanks

"Ask at a cycling shop".

"Probably ISO 8mm fine". HTH.

I thought it was "a bolt (from a seat post..)". Have you got a tandem? What bolt *exactly* is it? Why don't you spend 40p at a bicycle shop, or try your luck at an ironmongers?

In message , TC writes

A single bolt via the web doesn't sound like it makes much sense unless you are out in the sticks somewhat.

a local bike shop really is the obvious place - note that the heads on seat post clamp bolts are a sort of lower profile than your average Allen headed bolt, and some have shoulders on the body of the bolt etc.

St John Street cycles are good for odd bits/spares, but aren't cheap, and have (or did anyway) a fairly hefty minimum postage charge

No - different number of threads per mm for different apps. And I'd guess your app will use an uncommon one since it has to clamp hard but easily.

Count 10 threads and measure that.

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Perhaps that's why it broke. They're not normally something you break!

don't you have a Halfords or a bike shop near you?