i have an '03 specialized stumpjumper hard tail, and the stock seat post is 30.9 mm. i need a new one, but it seems the only mtb posts available are 27.2 and 31.6. specifically, i want a thomson masterpiece. can i use a 27.2? i'm assuming the 31.6 will be too fat. am i F'd or what?! somebody please give me good news...