I'm with idoru2005, check the fully compressed width of the bolt to make sure it's not bottoming out on itself:

1. remove bolt, tighten bolt-end all the way in
2. use calipers and measure inside distance between the underside nut & bolt heads
3. measure the ouside distance between the seat-binder ears

Is #3 larger than #2?


I've had good luck with using a star lockwasher between the nut and seat-binder ears. A lot of frames have a little notch on the seat-binder where the tab on the binder-bolt nut fits.