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  1. #1
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    Seat tube question: Boris and Thudbuster

    I recently picked up a Thudbuster LT for my Boris X9. It is a 31.6 seat post, but I am having no success getting it in.

    The specs say the Boris has a 31.6mm seat tube, but the post that came out is marked 30.4??

    Any suggestions?

    eta:
    Frame ALLOY 6061 FRAME, W/DISC MOUNT ONLY, TIG WELD, replaceable rear derailleur hanger, H2O bosses, Rear Rack Seatstay Brazeons, 170mm Symmetrical Rear triangle spacing
    Fork 26" 25.4x28.6x30x250L Straight blade CRMO 1 1/8 W/DISC MOUNT ONLY
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    Front Derailleur SRAM FD-X7 2x10 HIGH DIRECT MOUNT
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    Saddle WTB SPEED V SPORT SE VINYL TOP
    Seat Post Alloy 31.6mm x 350mm aluminum
    Seat Post Clamp 34.9 ALLOY, Q/R
    Sizes See pull down
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  2. #2
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    Reputation: tigris99's Avatar
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    U can't put a seatpost in that's bigger than the one that came out, just not doable.



    You have to always check stock sizes of what's on the bike for things before just ordering and hoping it fits.

  3. #3
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    @ Tigris
    I agree with your point, but the specs say 31.6mm seat tube. Others have used a 31.6 seat post on the same bike.

    My thought is they put in a smaller seat post and "pinched" the seat tube to a smaller size. Not sure that is the case, but if it is, is there a way to "unpinch" the seat tube safely?

  4. #4
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    The specs on the x9 say 34.6 seat tube. They say 31.6 seat post.
    By those specs your new post should fit. Did they put in a smaller post but stick in a shim that you're not seeing?

  5. #5
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    @ tfinator
    No shim that I can see. I ran my finger in there looking for burrs, paint, etc. Tube looks clean.

    Apolgies for the mixup on seat tube vs seat post. Gets confusing for a noob like me

  6. #6
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    I used a digital caliper and the seat tube measures 31.6mm longitudinally and 30.9 laterally. Is there a safe way to expand the tube?

  7. #7
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    The X9 FS With bluto states 30.4, did they update the frame on the non FS to the same. That's my guess.
    MacGyver is my spirit animal

    -Until proven wrong, assume you are the weak link in any system-

  8. #8
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    @WSU
    That would suck
    If so, they should update the website. That particular model is the only Boris with that size listed.

  9. #9
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    @WSU
    Looking at the Boris report thread, it seems the newer X9s have a 30.4mm seat tube. BD needs to update their site.

    I re-calipered the seat tube and it measures F-R: 30.6 / L-R: 30.3 so some slight oval deformation but stock seat tube fits perfectly and is pretty tight as it is pushed all the way in.

  10. #10
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    I went to Harbor Freight, purchased the adjustable reamer set, and went to town. After about an hour and a half, the seat tube is now at 31.6mm.

    No need to sell the Thudbuster!

  11. #11
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    Could you have just exchanged/returned the Thudbuster?

    And if I read that right, you bought tools to expand the seat tube? Is that a potential safety issue?
    it's a challenge some of us are ultimately worthy of.

  12. #12
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    Quote Originally Posted by 5 String View Post
    I went to Harbor Freight, purchased the adjustable reamer set, and went to town. After about an hour and a half, the seat tube is now at 31.6mm.

    No need to sell the Thudbuster!
    I've considered doing just that... Please report back in a bit if it affected frame in anyway, shaving that half mm off the walls.
    MacGyver is my spirit animal

    -Until proven wrong, assume you are the weak link in any system-

  13. #13
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    Quote Originally Posted by tyriverag View Post
    Could you have just exchanged/returned the Thudbuster?

    And if I read that right, you bought tools to expand the seat tube? Is that a potential safety issue?
    I bought it used from PB, so no return/exchange. To convert it into a smaller seatpost was going to be a lengthy multi-deal solution that I ultimately chose not to pursue.

    I used a reamer to slowly remove enough material to allow the seatpost to fit. Tubing thickness is nominimally 1.8mm. I imagine there is some compromise in strength and I will report back with any long-term issues.

  14. #14
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    I had a similar issue. The specs say 31.6 actual was 30.4. I bought a set of reamers and corrected their cost cutting measure myself. It was not fun. I'd recommend returning the post and exchange it for the correct size

  15. #15
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    Quote Originally Posted by rollertoaster View Post
    I had a similar issue. The specs say 31.6 actual was 30.4. I bought a set of reamers and corrected their cost cutting measure myself. It was not fun. I'd recommend returning the post and exchange it for the correct size
    Too late! I did what you did. Have you had any issues?

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