90s GT - seat post sizing issues


Dirt Disciple
I have a 1990 GT Team Avalanche that I've just built up. Everything is working great, except I'm struggling with the seat post, and I was wondering if I could get some tips.

(measured with callipers)

The seat post tube is: 26.9mm internal, 30mm external

I have 2x seat posts:
1. 26.6mm seat post diameter, using 30mm clamp - after 5-10 miles has completely fallen down
2. 27mm seat post diameter, no clamp needed - this will go in about 80mm but no further

Is the answer to just get a 26.8mm seat post? Is that the difference here? Or is there something I can use with the 26.6 or 27? Both seat posts are old GT posts, so I assume that maybe the seat tube of the GT has expanded over the last 20-30 years and therefore doesnt fit properly anymore?
I had a similar problem with a Marin. You’ll likely need a 26.8 post. If the post is not textured enough to grip the frame, you’ll have to clean the grease from the seat tube and add a gob of assembly compound (friction paste).
I've noticed that on some bikes after 30-odd years of clamping the end of the seat tube can become, to use a technical term, 'squished'...So what was 27.2 can become 27... what was 27 can become 26.8mmm
If a 27 goes in 80mm, then I would assume the seat tube is 27 (a gt size @Northernlight ) but might be a little bent, corroded or dirty.
They rarely get larger below the clamp slot.

First try running a wire "Bottle brush" up and down inside the seat tube with an electric drill to clean and smooth it out, then try the 27 again.

If your 27 still doesn't fit, try a 26.8, which as @ishaw said is also a common gt post size, and double check the clamp slot is parallel when the clamp bolt is tightened.

We see loads of frames killed by the wrong post - always check this slots are parallel👍
Team Avalanche in Prestige Concept was a 27.0.
If measuring 26.9, as above, it's got crimped slightly.
Original post was a Tioga IIRC
Ironically, I’m looking at seatposts too, for my GT RTS Team ‘94……. but for different reasons (I’ve broken my Syncros again, cracked repair) and needing a new post. Mine measure 26.8mm and fits beautifully.
I have a 1990 GT Team Avalanche with original Tioga seatpost. I can check the DIA sometime tomorrow as it’s currently hammering down with rain and lighting out the garage is pretty poor!
... so if it is a 27.0 and you've cleaned the inside of the frame, and it still doesn't go in beyond 80-100mm then either the post or the frame tube is bent.

Look along the post, or put a straight edge against it.
If it's bent, then it goes in the recycling👌

If it's not bent, examine the frame for dents in that area.
Dents could be removed and the tube can be straightened, but it's probably not worth it if it's not a yo eddy😉

You could cut the post down to what fits, or taper the bottom end to get more insertion. Basically in this case you'd just do what you can. 100mm is probably adequate for leisure use.
A lot of 60s 531 racers had less than that. Quite a few of them broke after 30 years though🙄