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Eccentric Seat Tube

Old 12-14-06, 08:53 AM
  #1  
Rattlebag
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Help! My Seatpost Doesn't Fit!

I have just come in to posession of a used steel frame, and when I tried to fit my existing 27.2 seatpost I found it was too large to fit in the seat tube (which I was told was 27.2 when I bought it). I measured the tube and post, the post is pretty much 27.2 all round, but the tube's inner diameter ranges from 25.8 to 26.8. For the time being I have jammed a cheap seatpost in there, supposedly 27.2 as well but it also turned out to be pretty eccentric when I measured it. I suppose I could take it to a shop and have the tube reamed out to fit the post I had in mind.

What I really want to know is how hard this would be to do (would a small-ish shop have the right reamer? Would doing this affect the strength of the frame? how much would it cost?), and if there are any alternatives to this that I could do at home (at the moment I'm a bit hesitant to take a file to my new favourite object).

All suggestions appreciated

Last edited by Rattlebag; 12-14-06 at 11:27 AM.
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Old 12-14-06, 07:00 PM
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Butted tubes have varying I.D.s (Inner Dimensions); usually it's just the area right around the seat tube collar that determines the seat post size.

You do not say what brand of frame this is, but It sounds to me like the frame takes a smaller sized seat post than 27.2mm.

Does the person you got the frame from have the seat post that was originally installed?

There are reamers available to open this area up and if used properly will not weaken the frame. Hopefully any shop that has these tools knows how to use them.

If you cannot get the original post I suggest letting the shop measure the frame first.

FWIW, unusual sized seat posts go pretty cheap on eBay because there is not much call for them.
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Old 12-15-06, 02:34 PM
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have your lbs measure the seat tube and go from there it may be that you need a 27.0 seat post however nominal the difference do not ream your seat tube. if you were to stand on top of a soda can without imperfections it would support your weight, with any imperfection it would crumble. your seat tube is a straight guage tube by reaming it out you remove the designed integrity of the frame.
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Old 12-15-06, 08:26 PM
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Todd Richards
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27.2 or smaller setpost needed

I'd bring it to the local LBS on a monday or tuesday a.m. (God NOT saturday!) with 1/2 dozen donuts and coffee. they'll have 27.2, 27.0. 26.8, and all other seatposts available. insert each and crank on the bolt 'til you discover the one that fits. It'd be so much easier to do it this way than to measure and try to discover the true originally intended diameer. the tube diameter itself is probably distorted due to bad welding techniques (sea tstay's etc torque this area some), but all will come back into true round when clamped around the right seatpost diameter. The key here is to be nice, not demanding, and bring a freindly offering for the guy(s) who will rumage thru a box or 2 to discover the right thing you need.
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Old 12-15-06, 09:02 PM
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Originally Posted by jp_rcr
have your lbs measure the seat tube and go from there it may be that you need a 27.0 seat post however nominal the difference do not ream your seat tube. if you were to stand on top of a soda can without imperfections it would support your weight, with any imperfection it would crumble. your seat tube is a straight guage tube by reaming it out you remove the designed integrity of the frame.
Is this your experience or your opinion?
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