27.2 seatpost in a 26.8 seat tube

[handbaked]

Hi guys

So i have finally got the frame i want for my build, an old skool steel dmr trailstar. Lovely frame and freshly powder coated however i thought it was a 27.2mm seat tube so bought myself a new swanky sr suntour ncx suspension seat post. To my annoyance it arrived and i quickly realised it is in fact the more unusual 26.8mm size that i need.

The ncx is not available in thia size as seems the case with other good suspension posts. I know it is not wise to force it in as that .4mm will cause issues for sure. I was thinking about cutting down this new post and fitting a 26.8mm post inside it (using a shim) so that it fits the seat tube.

Has anyone done anything similar or have any other ideas? Really want this post on this frame!

Cheers.

 

[anotherkiwi]

A dowel that is just under 26.8mm and some emery paper and sand out your .4mm. That is not going to weaken your down tube by too much.

 

[Gringo]

New A dowel that is just under 26.8mm and some emery paper and sand out your .4mm. That is not going to weaken your down tube by too much.

if I rember correctly the NCX post has a very thick wall So I'd be more inclined to find someone with a lathe and turn down the seat post to 26.8mm.
You'll be lucky if you can hone out your frame to size evenly and if it goes wrong you've lost your frame.

 

[anotherkiwi]

If you turn the post down you will need to have it powder coated as well in order to prevent it being in contact with the steel frame. Having just this weekend had to seperate some steel screws from an aluminium frame trust me you do not want direct contact between the two.

Maybe if I say 0.00787401575 inches on each side your fears will go away? .2mm on all sides is a couple (OK three...) up and down movements with the emery paper and it doesn't have to be perfect. You would have to be very heavy handed to sand your way through a steel down tube. Or even put it out of round.

 

[handbaked]

If you turn the post down you will need to have it powder coated as well in order to prevent it being in contact with the steel frame. Having just this weekend had to seperate some steel screws from an aluminium frame trust me you do not want direct contact between the two.

Maybe if I say 0.00787401575 inches on each side your fears will go away? .2mm on all sides is a couple (OK three...) up and down movements with the emery paper and it doesn't have to be perfect. You would have to be very heavy handed to sand your way through a steel down tube. Or even put it out of round.

I really didn't think this was even a possibility. What grit rating of paper would you recommend? And is this a tried and tested method?

 

[anotherkiwi]

The reason I even mentionned it is because the other day I tried fitting another seat post to my frame and had the same issue and that was the sollution I though might work for me.

Did some googling and it won't work for you, your tube is too thin.

Sorry

More Googling and the lathe idea can also work the other way - you could have the .2 machined out at the machine shop. He will tell you if there is still enough material for strength.

 

[eHomer]

If it was me I'd much rather emery the seat post than the inside of the frame tube.

As well as leaving the frame tube unaltered, I would find it easier to consistently remove material from the outside surface of the alloy seat post than the inside surface of the steel seat tube.

Although your plan of wrapping the emery paper around a close fitting dowel would make radial accuracy fairly easy, applying effort from the open end of the seat tube would inevitably taper the inside bore, making it thinner at the clamp end than further down to the depths of the seat post socket.

Whereas, if you emery the seat post with a concave curved block, you can keep trying it in the frame tube as a very accurate template/former, and the emery paper can be much more accurately applied to not taper the seat post.

I would make the concave curve block by drilling a block of wood slightly larger than the seat tube, then cutting it in half.

 

[KTM Bike Industries UK]

Its steel frame, so its not a massive job for a competent frame builder to swap the seat tube for one with a different internal diameter.

I've had it done ones to an old frame, I remember the cost not being a huge problem, however it was a good few years ago now. But it might be worth contacting your local road bike shop, they will know a good frame builder locally who can do it.

 

[handbaked]

Oh wow. This all sounds so long winded.

 

[jonathan75]

Oh wow. This all sounds so long winded.

Lol just sand the ncx down until it fits ;-)

(using appropriate respiratory protection and sandpaper)

Maybe spraypaint it using poundland paint in case it sticks.

 

[jonathan75]

Oh wow. This all sounds so long winded.

P.s. I recommend this for respiratory protection http://r.ebay.com/iBoZ7I

 

[handbaked]

Well to confirm i went the sandpaper route. It took at least an hour with grade 40 paper to do just the bottom 1/4 of the tube but it has worked!