My mountain bike came with a quick release clamp that reads '34' on its side (embossed or debossed, am forgetting. Reading online there are about 3 sizes for seat post clamps I went with i believe '32mm' something (lower than 34, since the next higher size 'sounded' too big to me), but it turned out noticeably larger, so i had to return it. So what does the '34' stand for? And how can I determine size without going to a bike shop?


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    A big help would came from having a caliper to measure it. Otherwise, even putting a strip of paper around the seat tube top, then measure the length, then divide the lenght by 3.14 then we have a guess (which can be easily off of 5mm, but it can help). Which model/year is your bike? that may help, in absence of other informations.
    – EarlGrey
    Mar 11, 2021 at 8:50
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    There are cheap basic calipers available in department stores. Those would already be quite up to the task.
    – Carel
    Mar 11, 2021 at 12:30
  • yea, thought of caliper, but see absolutely no other need for one other than for this one task, hence it'd be about 7 bucks or so, probably close to lbs fee.. Mar 12, 2021 at 1:04
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    Once I bought a caliper gauge I found myself using it a lot and I would guess you would too, they are very useful. You will do el_oso a favour if you accept his answer as a solution.
    – pateksan
    Apr 28, 2021 at 8:31
  • @slackexchange The whole purpose of life is to acquire tools. Nothing else comes close to the rush you get when you have the right tool for the job. Buy some calipers.
    – Criggie
    Apr 28, 2021 at 8:46

1 Answer 1


Easiest way without access to calipers is to lookup the bike specifications online. What make/model/year is the bicycle in question?

  • I looked at the specs and it only saysmaterial (alloy q.r), and have no idea for how long they've run this model (i believe the model is gravity basecamp). Mar 12, 2021 at 1:03
  • Looks like your seatpost diameter is 27.2 mm. Given that and maybe roughly 1mm seat tube wall thickness for steel, I would expect a seat post clamp size of around 30mm. My old Trek aluminum frame with 27.2 mm seatpost has roughly 3mm thick walls and takes a 34mm post clamp.
    – Armand
    Mar 13, 2021 at 0:15
  • cool, will try 31.8 mm. Mar 15, 2021 at 6:20
  • The size 31.8 mm worked, thanks. Mar 29, 2021 at 6:31

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