Okay, this is a quick one, I promise. 
I know bike seat clamps go by OD of the frame or something like that, so I want to know if a Salsa bike clamp that’s spec’d at 30 mm will fit my 25.4 mm seatposts. It seems like 28.6 mm is what people usually go with, but the clamp I’m looking at only goes down to 30 mm and I wonder if that 1 1/2 mm would make that big of a difference.
Many thanks!
I have a Coker with a 25.4mm seat post. The tube it fits into measures 31.8mm OD (I just checked it). The Salsa quick-release clamp is marked 32.0.
It fits fine.
I have changed a lot of seat post clamps on my bikes and I would not use that clamp on a unicycle with that much difference. 1 mm is .0393 inches and by the time you tighten it enough to work it would no longer be round and it may not tighten evenly. I also have some nice Thompon seat post clamps left over from my bikes and I would not use them but I would buy the double bolt clamp that fits it properly.
This explanation may shed some light (from Compulsion Cycles website):
http://www.compulsioncycles.com/index.php?route=product/category&path=102
By the way, I’ve bought gear from them and had good experiences.
To clarify, you probably need a 28.6mm clamp. A 30mm WILL NOT work.
Got it, thanks guys.