LogosA

Im fixing up a old bike I was given and when I ordered a new seat post clamp I ordered too big.After talking to the seller, searching google, and talking to my local bike shop (deals with road bikes, not BMX) Ive been unable to find an answer.

So do I loosen the clamp and measure from the inside with calipers? or the outside?

The silver is the old one, the red one was listed as 34.9mm on ebay and is way to big

Ive got a 2005 Specialized Fuse IV with a 33.71mm outside diameter

proileri

Obviously the measurement is the smallest diameter the clamp can be tightened to, and you want a clamp that's a bit smaller than your external post diameter - in your case a 31.9 mm clamp seems to be the closest.

CMcMahon

This

LogosA

Well that's questionable.It would seem weird to sell a clamp based on what its measured once its tightened since everyone tightens slightly differently.Apparently from anotehr thread I started on vitalbmx.com the someone who replied told me the following:

"specialized is weird and uses a larger diameter seat post"

So it seems I need to get a 33mm one since my post is about 33mm

Street rider

Measure the outside of the seat tube without a post in it or clamp around it. Order accordingly. And the shop wouldn't measure and tell you what you needed? Sounds like you need to find a new place to go.

proileri

I think "weird" is a reference to Specialized using same sizes as on non-BMX bikes - 31.9 mm is a standard bike clamp size used in road bikes and MTB's, which make most of Specialized's product range. Usually BMX clamps are a bit smaller, 28.6 mm or so. In any case, the adjustment bolt in a 31.9 mm gives you more than a couple of mm of range.

There's an easy answer here: tighten the old one so it closes completely, then check the interior diameter. Should be 31.9 mm. You're just overthinking it

CMcMahon

Or you could, you know, but a 31.9mm clamp, try it, and then return/exchange it if it doesn't fit. There's only so many standard sizes to work with...

edit: For the record, Specialized was stupid for using a different-sized clamp/post than every other company making freestyle BMX frames.

IthaDan 

What size is your actual seatpost?

... and what's your frame made of?