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This is more "AQA" than "tester approves"
FWIW, if you run a 27.0 reamer into a thin walled tube, the walls will push away from the cutting edges and then spring back to an undersize when you extract the reamer.
Seat posts are not a high precision fit, and I imagine you'll be fine under the circumstances described
gbj_tester
I have a dilemma and hoping for some sage advice.
I have an IF Planet X that’s at least 15 years old. Seatpost size from that era seems to be variable but I think I’ve narrowed it down to 26.8. I used a new 26.8 post and it was a very close fit for the first couple of inches and then far too close a fit to go further. I backed off and took to the LBS to clean out properly.
They had a good look, did some cleaning and said they’d reamed out to 27.0. All good. But a 27.0 doesn’t fit at all although the 26.8 will go in nicely although with a tiny bit of play. I’ve used the 26.8 and nipped up the seat collar, it seems ok but haven’t put any weight on it.
I suspect the answer is to get the tube reamed properly to exactly 27.0 but I’m a bit wary about taking any more material off the tube (it’s 853) as well as taking it back when the LBS already says it’s 27.0 so wondering if it will all go wrong using the 26.8 (it’s 400mm long and a good 280mm or so I s in the frame)?