What the hell is going on with my seatpost sizing?

I’m having real trouble establishing the size of the seatpost needed for my bike. It’s a vintage track bike that I bought as a frame. I’ve now gone through 3 different seatposts and still haven’t got the right size!

26.6 is too small, 27.0 is too big and 26.8 is so tight I basically have to screw the thing in. What’s going on here? Has the clamp Ben over tightened crushing the collar? That’s the only thing I can think of. Is there any way to solve this or is it fucked?

The build is so nearly done and I just can’t seem to get past this hurdle

Has it been resprayed? Over spray inside the seat tube?

No it hasn’t. I think it’s been over tightened tbh. Is there any way to correct this?

How far have you tried to get the 26.8 in? Does it get easier after the first couple cms?

Does the seatube seem straight or have any knocks in it?

Any idea who made the frame and what it's made of?

So on closer inspection it appeared one side of the seat collar(?) where the bolt goes through was at a different angle to the other. I used a screwdriver to bend it out to the same angle and the 26.8 fit well! Clamps down solidly too.

Simoncini, it’s Columbus cromor. It appears someone had over tightened it. I’ve gently bent it out slightly and it now seems to be okay

that's good, I can rest easy in my bed tonight

Oh won’t we all!

I used a screwdriver to bend it out to the same angle and the 26.8 fit well!

Yep, this isn't all that uncommon; the giveaway is if the edges of the slot aren't parallel. It's a good idea to tickle the inside of the slot with a file after levering it open again, to ensure you haven't created a slight dag that will scratch the post.