Seat tube 31.8mm dia. vs 31.6mm post dia.

ExFire

Member
I asked Suntour whether their 31.6mm post would work in the StepThru 31.8 tube.
Their reply - "You will most likely need a shim to make the 31.6 seat post work on that bicycle."

A shim would have to be .01mm thick, to fill the .2mm gap(both sides) .

I can find no source for a shim which is that thin. I've checked Cane Creek, for example - no shim is even
close to being only .1mm thick.

Other than some "makeshift" shim (tape, tinfoil, etc.), does someone know from experience that the 31.6 post will be clamped sufficiently in the 31.8 tube, or know of some thin-shim source?

I am aware that some have hacksawed a vertical slot in the tube, allowing the two sides to be squeezed together, but I am not willing to do that.

Thanks for any replies ;-)
 

Darceman

Well-Known Member
I asked Suntour whether their 31.6mm post would work in the StepThru 31.8 tube.
Their reply - "You will most likely need a shim to make the 31.6 seat post work on that bicycle."

A shim would have to be .01mm thick, to fill the .2mm gap(both sides) .

I can find no source for a shim which is that thin. I've checked Cane Creek, for example - no shim is even
close to being only .1mm thick.

Other than some "makeshift" shim (tape, tinfoil, etc.), does someone know from experience that the 31.6 post will be clamped sufficiently in the 31.8 tube, or know of some thin-shim source?

I am aware that some have hacksawed a vertical slot in the tube, allowing the two sides to be squeezed together, but I am not willing to do that.

Thanks for any replies ;-)
You do not need this.. just put it in and tighten up... this is been discussed numerous times ..it works fine with no shim
 

Nova Haibike

Well-Known Member
does someone know from experience that the 31.6 post will be clamped sufficiently in the 31.8 tube, or know of some thin-shim source?

So you already have or bought a 31.6 seatpost? If not, buy the smaller 27.2 diameter size and use a thicker shim.
 

Nova Haibike

Well-Known Member
You do not need this.. just put it in and tighten up... this is been discussed numerous times ..it works fine with no shim

As a working professional, using the wrong size and just tightening up is a bad idea. In order to do what you suggest requires over-tightening the bolt or quick release with more torque than it is designed for.
 

Taylor57

Well-Known Member
I asked Suntour whether their 31.6mm post would work in the StepThru 31.8 tube.
Their reply - "You will most likely need a shim to make the 31.6 seat post work on that bicycle."

A shim would have to be .01mm thick, to fill the .2mm gap(both sides) .

I can find no source for a shim which is that thin. I've checked Cane Creek, for example - no shim is even
close to being only .1mm thick.

Other than some "makeshift" shim (tape, tinfoil, etc.), does someone know from experience that the 31.6 post will be clamped sufficiently in the 31.8 tube, or know of some thin-shim source?

I am aware that some have hacksawed a vertical slot in the tube, allowing the two sides to be squeezed together, but I am not willing to do that.

Thanks for any replies ;-)
The Suntour 31.6 will fit perfectly in your XP. Just stick it in and tighten.
 

myers830

Member
If you want a perfect fit...buy the 27.2 SunTour seat post and also order a Cane Creek 27.2-31.8 spacer. Thats my setup. For the record...I accidentally bought a 31.6 seat post a few moths ago...did it fit..? well I could tighten it down but it eventually worked loose to the point of no return. So yes, a 31.6 will "work"... for a while.
 

rich c

Well-Known Member
.2 mm is .008" That's assuming both are at nominal dimensions. Neither tubes are ground to dimension. Have you measured the actual dimensions with a micrometer or veneer caliper? You will not have an issue tightening as is.
 

Nova Haibike

Well-Known Member
You will not have an issue tightening as is.

Not to beat a dead horse, but this is asking for trouble. I see it all too often, people not using the correct part. There is no penalty for doing it right, but there potentially is you do it wrong. Just get the correct smaller diameter post and the appropriate shim.
 

Taylor57

Well-Known Member
Not to beat a dead horse, but this is asking for trouble. I see it all too often, people not using the correct part. There is no penalty for doing it right, but there potentially is you do it wrong. Just get the correct smaller diameter post and the appropriate shim.

I popped the 31.6 into the 31.8 a couple of months ago and my seat hasn't budged.
 

Gionnirocket

Well-Known Member
I don't think you'll have a problem.
I just went through this purchasing a new suspension seatpost
My bike has a 27.2mm tube and I've been using a 26.8mm post for the past 12 years.
 

MMC

Member
fine grit sandpaper works well as a make-shift shim.

Or use Finish Line Fiber Grip anti slip gel. I have a tube of this and it's like gel sandpaper as it's got lots of grit in it and will fill that micro mm
 
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harryS

Well-Known Member
I'm pretty sure you could snip off a piece of aluminum soda pop can and it would fit nice. I'm checking out a piece at slightly under .1 mm.
 

FlatSix911

Well-Known Member
I'm pretty sure you could snip off a piece of aluminum soda pop can and it would fit nice.

I'm checking out a piece at slightly under .1 mm.

This is the correct answer... and has been used for decades by bike and moto mechanics.

Make sure you use Coke... Pepsi won't work! ;)

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Darceman

Well-Known Member
As a working professional, using the wrong size and just tightening up is a bad idea. In order to do what you suggest requires over-tightening the bolt or quick release with more torque than it is designed for.
That is a bunch of baloney I weight 210 pounds I have 3000 km on my XP and the seatpost has never sliped. And it is 31.6 SunTour suspension post
 

Darceman

Well-Known Member
Not to beat a dead horse, but this is asking for trouble. I see it all too often, people not using the correct part. There is no penalty for doing it right, but there potentially is you do it wrong. Just get the correct smaller diameter post and the appropriate shim.
It doesn't slip what will it hurt
 

ExFire

Member
.2 mm is .008" That's assuming both are at nominal dimensions. Neither tubes are ground to dimension. Have you measured the actual dimensions with a micrometer or veneer caliper? You will not have an issue tightening as is.

Beg pardon, but .2mm is .078(.08)"

.008 is one too many zeros, no?
 

6zfshdb

Well-Known Member
The soda can trick is tried & true. I've used it for years on my old Diamondback. Make sure you use aluminum. Brass or steel will cause corrosion.