Seat Tube Woes - 1989 GT


Dirt Disciple
I'm in a bit of a pickle. I've got a 1989 GT Team Avalanche frame. It came with a 26.8mm seat post, which was definitely too small. I messaged @SeeingisBelievingGTMuseum who kindly checked out his and confirmed it's 27.0mm from the factory. I then ordered a nice NOS Tioga 27.0mm from a bloke in The Netherlands. Happy days I thought.

Unfortunately, whilst it's very snug front to back, there is a good amount of side to side play and in order to defeat the play and make it so I can't turn the post side to side, I have to clamp it really tight. So tight that the seat pin is bent slightly and the braze ons are nearly touching each other.

Alas, I think some sort of shim is needed but only on 2 sides as it appears to be an ovalised problem. If I shim all the way around it will never go in as it's very snug front to back.

The other issue is how to keep the shims in position, especially given the detailing at the top of the seat tube. Aaaargh, what to do!?!

Any advice really appreciated as I've plowed a lot of money into parts and I don't want it to be a non-starter. TIA

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This is a tricky beast as that is a pretty rare frame with original paint that looks to be in amazing condition!

Something certainly looks amiss from the above shot as it looks quite "gappy" if that ever is a word?

A shimmed seatpost like a USE might work, but this won't be period correct and they come with their own issues.

The best person I can think of is @danson67 who may have a few tricks up his sleeve. He's saved many a retrobike with his talent, skills and know-how so perhaps drop him a PM. My concern is that the required action will require the paint to be harmed...

Good luck! 💪🤞
Thanks mate. It's not original paint FYI, it's a re-paint but well done. This build was going to be about as perfect as can be, so I absolutely need to invest in a proper solution that allows for the period correct post if possible.

It does look gappy, but I think that's an optical illusion caused by some touching up of paint I had to do.

Shimming in the traditional sense is unlikely to work as the seat tube is extremely snug at front and back. It needs shimming either side only, somehow. The only other alternative would be to ream it out for a 27.2 but I don't like what that might do to the integrity of the frame.

I fear I may have caused this, there were some serious burs in the seat tube that were scratching posts in a major way, so I ran a 27mm ball hone through there a few times. I really didn't think it was possible to remove that much material doing that though - it was 120 grit.

Understood. If it's perfection you're after then best get the professionals in.

You might be able to ream out to 27.2mm DIA as the tubing on these era GT's won't be crazy thin, plus with the triple triangle detail there's more chance of heat distortion or some break-through of the weld from the seat stay bridge on the back of the seat tube that can cause a tight spot.

You running a 27mm DIA hone may have caused an issue, hard to say as I have no idea how you did it - by hand, or in a pillar drill? I owned a few GT's in the 90s and have to be honest none of the seat tubes were particularly well finished on the inside and used my bike shops expandable reamer (carefully) and then needle files around the slot to deburr.

I'd contact Dan, he builds racing wheelchairs by day and resurrects old retro MTBs by night!
If you put a 27.0 post in there and it's wobbly and won't clamp, that's a lot like it's now a 27.2.
The way the lugs are closed up also makes it look like that.

If it was (still) 27.0 and oval, the post would be tight once it's in. You haven't changed the diameter if it's distorted, just squashed it.

It seems unlikely your honing took off 0.2mm (so evenly uneven!) but it must be possible.

It must be possible it was 27.2 in the first place, but unlikely.

I assume you checked the post was accurate? I've seen plenty of posts 0.2mm out in sizing.

You need to try a decent 27.2 post to see how that is.
The outside tube diameter will be 28.6 to take a front mech - so reaming it out to 27.2 should be fine - it's very unlikely you only have 0.1mm Metal spare anywhere before failure, assuming the tube isn't bent.

usually the seat tube is fine and the distortion is in the welded and clamped area.
Thanks guys. Anyone know anybody in the Sheffield area that knows their way around a reamer? Or that can be trusted to diagnose this and take careful action to save the frame.
Thanks guys. Anyone know anybody in the Sheffield area that knows their way around a reamer? Or that can be trusted to diagnose this and take careful action to save the frame.
Before you go gun-ho with another tool, as @bikeworkshop said get hold of a 27.2 seatpost to test the potential fit of that. Do you have access to a digital caliper at all? You can pick them up quite cheaply these days and they are so useful to check dims/sizing on loads of bike parts. Plus you can check the true size of the Tioga post you have to. Measure twice, hone/ream once!