Sounds like you have the 8mm variety if you have another one that is too big. The thing you want to check is to see if the bike frame has a notch in the bolt hole on one or both sides into which a protusion from the bolt head fits into. If the bike has this, you should get the notched variety of seat post bolt, although I believe the friction fit bolts will also work in this case. If the bike frame does not have notches in the bolt hole, then you MUST use the friction fit type bolt, or be prepared to do minor frame modification work (creating a notch).