Giant TCR seatpost shim - why and how?

[FONT=Calibri]Hi - just realised my new old frame had one of these inserted in the seatpost, [/FONT]
[FONT=Calibri]http://www2.giant-bicycles.com/en-pa/partsaccessories/indoorcycling/332/26213/[/FONT]
[FONT=Calibri]Anyone know why they are necessary and should it be inserted fully into the frame or should there be a centimeter (approx) showing for the seat clamp to attach to? Finally can anyone confirm what size seatpost clamp I should use with a 27.2 seatpost? It's a a 98 Giant TCR Compact Aluminium frame. So far I've tried 28.6 (too small) 30.6 (fits but stops me inserting seatpost into shim), and 31.8 (fits nicely but won't torq down tight enough to stop seatpost slipping even when seatpost is inserted in shim). All the research indicates it should be 31.8 but mine isn't working - uinless it's just taht its an NOS seatclamp and maybe I need a new one which has a lip to rest on seatpost frame? Any advice appreciated[/FONT]

I can only suggest that the previous owner didn't want to splash out a new seat post of the correct size and used a shim to reuse a post they already had.

Good point, lesson learned, next time I get a frame thats been completely stripped I need to check headset fork and seatpost dimensions. Oh well, it's been a learning curve, I've subsequent;y found this on the www
http://problemsolversbike.com/blog/comments/new_seatpost_shim_size