I'm trying to help out a friend's child, age 13, riding an almost
grown-up sized mountain bike with a sloping top tube and not enough seat
post for his size. I measure the post, with a digital caliper, at about
31.5 mm. I didn't want to take it out since it had been slipping and I
just yesterday managed to get it to stay put, but I think it's roughly 9
or 10 inches in length.

Is 31.5 an 'official' seatpost diameter? This one slips a bit, in part
because it's a slightly sloppy fit, so I'm thinking maybe 32 mm is the
right size.

If anyone wants to provide a link to what we're looking for (right
diameter, longer than what we've now got), that'd be great.

Thanks in advance.

-S-