Hi all. Pretty sure this issue isn't addressed elsewhere: the seatpost bolt (the screw that secures the seatpost in the frame) broke on a Casati Ellisse I acquired a year ago. A local bike shop replaced it with a too-large bolt with a washer tucked in to make up the difference. But however hard I tighten it, the seat gradually lowers over the course of a 20' ride. I suspect that what I need is the proper size bolt, but I have been unsuccessful finding what that size is, let alone finding such a bolt. And the shops I've taken this problem to also have not been of help in getting the right bolt. Any suggestions? I'll be grateful for any input.