I try to keep seatpost slot in the seat tube, as parallel as possible, as well as the inside of the seat tube. a) Squeezing slot past parallel has caused fatigue crack over time (this happened with an OEM plastic bushing there, compressed too much, later models had aluminum bushing. b) If only tight clamp up top and flares open going down, seatpost may rock, plus concentrate load and fatigue there.

Once slot is opened to parallel, if you need a tighter fit with the seatpost, I make a shim from the side of a beverage can, easily cut with scissors because thin and soft, I cut careful for it to reach almost 360 degree wrap around the seatpost but no overlap as that will be a high-spot. Be careful, the edges are sharp. You can flare over the top edge as tabs or all the way around to keep the shim from sliding down.

OTOH you can get something like this:
https://www.amazon.com/PRAZTECH-KNUR...dp/B09SZJYM2R/
and build it into your own jig using bits of scrap (if you don't have a lathe to use it in). It won't be as neat and pretty, but it will knurl aluminium just fine.

Loosen or remove the clamp bolt. Insert the 26.8 post 2-3". Bash it with a rubber hammer in the direction of the slot. If it's wobble loose then use a 27.0. If it's not then you're gold with the 26.8

I have a 5" piece of 27.2 post that I use for this to relieve the "Z" factor of abused post clamps. It may sound stone age but it's way better than the screwdriver/chisel ploy

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"The post isn't slipping, despite the fact that I am 100% sure it is the wrong size by .2mm." Kontact
To borrow one of Francis's lines: "never fix a working part" (and I might have paraphrased it). It seems to me that the 26.8 post is right enough.

"The quandary is whether it will be more damaging to correct it than the potential damage of just leaving it alone and riding." Kontact
Before I would try to do any "corrective" work on the bike I'd want to have a thorough look at it in person, so I really can't add much to the value judgement you ask about. Andy

Yeah, I pulled the post again and took a good look - everything looks pretty normal. Maybe the 26.8 post I got is on the large size (seen a lot of variation in post diameters), or enough of the clamp area is distorted that the post is very evenly clamped.

Conclusion - leave it alone. The $5 post I got at the coop is nice looking.


Thanks, everyone!

Yes I saw that one when I searched, No lathe in our shop. Like I said nice tool, just not sure we can justify it.