What's the thing with 26.8 mm seat posts?
simple they spec builds with a straight wall seat post ... 28.6 minus 26.8 = 1.8 , diameter.. Tube wall is 1/2 that [ D = 2x r ] so its an 0.9mm tube wall .
for a 27.2 post the seat tube wall is thinner at the top. .. 0.7 vs 0.9.. not even 1.5mm difference.. still 0.9 at the bottom (butt) ...
So, ream out the seat tube on your own, maybe?
I shined up my 27.0 RB 1 with a reamer , till a 27.2 post just fit .. and now very little effort to clamp the post in the frame...
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