I'm restoring an old Atala for my son. The frame takes a 26.8 post. The binder clamp does have notches on both sides. I measured the outer distance and come up w/18.8mm. Nearly all the binder bolts I see are 19mm. Am I going to need to shim it with a washer or do I need a shorter bolt? And I measured with the post snugly in place.

Thanks for any help.