if it is a modern brooks seat you will have issues. yea you can do the ol flip the post upside down trick but i have found if you clamp a brooks as hard as you can doing this it still wont tighten all the way. this leaves you with two options. tighten it as hard as you can then spot weld the seat into position, or the better either find a wald addapter that goes from the schwinn size to a modern seat or even better a wald post that goes from 5/8 to 7/8 these are still avalible through most shops. the shims are not
You need the clamp center portion from a late 60's/70's Schwinn or Columbia, or certain AMF coaster models. Sorry, mine are all in use. I have yet to find anyone who supplies them new, even though they still sell the posts!? :? :x :roll:
You can buy a 7/8 clamp for around $5. if the top of your seatpost is 5/8 you can go to a hardware or home depot type store and get a 5/8-7/8 bushing/spacer for $2. Ive used them many times to use an existing seatpost. Buy the spacer and cut a slot in it to allow it to clamp onto the post.