Seat tube gradually expanding?

[Tommy27]

Jack Taylor Tandem (1975)

We've been lent this by a friend, with the view to buying. Mostly original according to him and he's a fair bit of experience with tandems. Front hub seems a later addition as with handlebars, but 650b super champion rims, Lefol mudguards, Mafac Tandem Canti's, Campag Rally and T.A. cranksets...

www.retrobike.co.uk

So, we've been lent this and will most likely be buying it, however the rear seatpost is worn out and also only fits when shimmed. Obviously the shim has only been used for brief test rides.

The original seat posts are 26.0, would a 26.2 be enough or should I look for a 26.4?

 

[Jonny69]

It sounds a bit like the seatpost is too small anyway. If you can physically fit a 0.1mm (or bigger) shim all the way round then the seatpost is too small for the hole. It's not because it's worn.

You need to get a vernier caliper in there and measure the inside diameter of the seat tube in a few places. You want to be sure the clamp isn't squeezed closed (giving you a false reading) so take the bolt out completely and work the clamp back open with a broad screwdriver if you need to.

 

[Tootyred]

Its a little hit and miss measuring as verniers (unless huge) don't have enough jaw length to get to the true section of tube below the clamped split tube....which obviously changes size as you open and close the slit. If it feels ok with a 0.1mm wall shim on a 26.0mm post then your at 26.2 (the thickess of the shim is on both sides).

This is where a donor post to try or hanging onto a whole series of knackered posts come in for the future.

If your still not 100%, drop into a bike shop as they will have a stepped measure....they may even sell you the post too!