seatpost binder bolt
 

My Falcon has a seatpost bolt that tightens with a 10mm nut - I want to change it for a more modern recessed allen wrench bolt - are these standard sizes or is this something I have to measure? I'd like to just bop by the LBS on the way home to pick one up.

The frame is Reynolds 531.

you are much better off takeing the bike along. there are a few sizes, lenght mostly, and two type of anchors. one fits snug with a knurled section the other a 'key'