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I'm currently rebuilding an old steel frame. After measuring it I was sure it'd take a 27.2mm seat post (see picture) so that's what i bought. Sadly it doesn't fit (despite some "encouragement" with a mallet). So I'm now in the position of needing to buy another.

Anyone got any suggestions on what size I should go for. You can see from the picture how it's measuring and I don't want to go wrong again.....

enter image description here

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  • Odd. That is a standard size. The post might be mis-sized? Maybe borrow a post from another bike to double check.
    – paparazzo
    Mar 21, 2017 at 18:03
  • @Paparazzi the post was from BLB so I'm pretty confident it's not mis-sized. I'll try the post from my other bike though.
    – Darren L
    Mar 21, 2017 at 18:06
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    Seat posts pretty much increment in 0.1mm sizes. Although 27.2 is one of the most common, especially steel, when you say 'old steel', how old? You may be better researching the common sizes that brand of frame used. Measuring at the collar is not reliable (as you have found) as someone before you may have hammered a 27.2 in making a earlier 27.0 or 26.8 bigger. Option B is take it to an older LBS.
    – mattnz
    Mar 21, 2017 at 19:16
  • @mattnz I don't actually know the year or brand of the frame which has made the whole process a bit more difficult. I think I'm gonna have to go with option B and just try out whatever different sizes they have in stock.
    – Darren L
    Mar 21, 2017 at 19:29
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    Mattnz is right, you'll just have to try several sizes and find one that fits. Also, if you need to measure something to within 0.1 mm, then a ruler won't do you much good.
    – Mike Baranczak
    Apr 17, 2017 at 20:47

3 Answers 3

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Usually at the "Cut out" in the top of the seat tube, they measure a little bigger than the actual size of the post. I'd say your best bet is to try several sizes at your Local Bike Shop, there even used to be a tool to measure it, but it's rather hard to find these days. If I had to guess, it's probably a 27.0 or 26.8 just judging by your measurement.

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    JA Stein SZ-1 Seatpost Sizing Rods. Currently available, no problem. There's another one out there that I always forget who made or makes it.
    – Nathan Knutson
    Apr 18, 2017 at 7:50
  • I wasn't familiar with those! I'll start recommending them to shops looking for that function. Thanks
    – CardMechanic
    Apr 18, 2017 at 18:47
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Trial and error was the eventual solution. It took a 26mm post in the end so the measurement at the clamp was WAAAYYY off!

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The seat clamp is stretched a little so your measurement is off.

You can use a vernier caliper to measure a bit further down inside the seat post. Definitely a tool to buy if you can. Even cheap plastic ones are handy.

Another rough estimate is to use a flexible tape measure around the seat tube, to get the outside circumference. Then divide that measure by Pi to get the diameter. Then subtract 2 x the wall thickness (which looks like about 1.2mm)

I have a bunch of old short seat posts to help figure out how big a seat tube is inside.

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