It was measured with a Vernier caliper - sorry if that wasn't clear. The pointt was that the seat tube measured 27.0mm at the cluster, but only to a depth of 2cm owing to the length of the caliper jaws. I could insert the 27.0mm post to about the bottom of the cluster lug, but no further.
That led me to speculate that the seat tube might actually be a 28.6mm one that had been splayed out a bit by a previous owner. I bought a 26.8mm post to see, and it inserted just fine, pretty snug, and tightened up just so. So it looks as though it is indeed the smaller size.
I also managed to get the fixed cup out last week, so now I have to make time to get going on the paint. I'm rapidly running out of excuses ...
I'm finally (but still slowly!) getting round to building a TI Team replica with the 753 fastback frame I bought last year. I'm having trouble with the seatpost though! I was told that the seat tube was 27.0mm and when it arrived I measured it with a caliper and it read 27.0. All good, so I bought a nice DuraAce post - it's stamped 27.0 on the shaft, and measures 27.0 with the same caliper - but I can't inset it more than ~3cm into the frame! It doesn't clear the minimum insertion level, and just about gets past the bottom of the slot in the seat tube.
What's going on here? Is it possible that the frame is 26.8mm but has been slightly flared out at the top. What size post do I need?? The frame is an SB stamped one from the early 80s, so what would the norm have been back then?
I would be inclined to get a vernier caliper and measure the internal diameter, simples
Here's one for under a fiver.http://www.mandp.co.uk/productinfo/5296 ... ue-Vernier
Once you have done that you will know what size seat post you require.