Thank you all so much. My seatpost is 27.2 x 350mm. Should I look for a seat clamp that says exactly 27.2mm (=1.07"), or slightly more, or slightly less? Will
this one, or
this one, work? I see some nicer-looking black ones, but either 28mm or 31mm, which are too big?
(LBS is quite far from from where I live, so I'm not going soon.)
No you probably only need a seat binder bolt, like
this. You probably shouldn't need to replace the entire clamp just pull out the quick release and put in the binder bolt. As TugaDude stated bring the bike to the LBS and get the right size bolt.
PS by the way you 27.2 is the seat post diameter not the seat tube diameter. The clamp that you linked to, needs to fit around the seat tube not the seat post. If you were to place one of those clamps on the seat post, it may not go down any more but the seat and seat post will pivot in the seat tube and be very irritating