binder bolt size??

[oldguy00]

I'm looking to get some replacement binder bolts for my steel tsx tubed merckx.
How do I measure to see which of these sizes I need???
thx!

http://www.nashbar.com/profile.cfm?category=&subcategory=&brand=&sku=11371&storetype=&estoreid=&pagename=

[sw3759]

i also have a TSX Merckx(circa 93').the 18mm should work fine,might have to sand out some paint if the diameter is a bit big,the 25mm definitely will not.i think the Campy bolt is an 18mm as well.