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As stated in my previous question, I am still trying to find the size of my flange nut on my seatpost. I need a new one because the old one was worned out. I bought a 3/8-16inch g/8 flange nut off of ebay and it was way too big for the bolt. I have reposted the pictures over here and hoped that someone can help me determine the flange nut size and where to buy it. Btw, due to covid19, I am trying to stay home so I hope to be provided with online help. Thanks to those who previously answered my question and I hope for more answers here. Any answers or comments with be appreciated. Thanks!
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  • Have you measured the bolt? And, since you have the new nut and the old bolt there, insert the bolt inside the nut and see if the threads match.
    – Daniel R Hicks
    Sep 24, 2020 at 22:10
  • I would suggest to invest a little money into calipers (en.wikipedia.org/wiki/Calipers#Vernier_caliper). It's one of those bread-and-butter tools that will come in handy whenever you need a precise measurement. They can measure outer and inner widths/diameters, as well as depths of holes, and all with submillimeter precision. In the case of your nut, you would simply measure the outside diameter of the bolt, and there you've got the size of the nut you need to buy. Trying to measure the bolt with a lineal is rather tricky.
    – cmaster - reinstate monica
    Sep 25, 2020 at 9:58
  • These days (@cmaster-reinstatemonica) digital calipers (aka digital Verniers, though that doesn't make sense) are cheap and much easier to use for the uninitiated. I have traditional Verniers here and like them, but use digital ones in work
    – Chris H
    Sep 25, 2020 at 12:54
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    @ChrisH Depends on how much money you are prepared to spend. Any type of calipers will be better than a meterstick, though. And that's all I said.
    – cmaster - reinstate monica
    Sep 25, 2020 at 13:25
  • might be easiest to just buy a new seatpost binder bolt and nut together, tbh.
    – Nathaniel Hoyt
    Sep 26, 2020 at 1:27

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The thread pitch looks too coarse to be metric. Since a 3/8-16 (3/8 inch diameter with 16 threads per inch) was too large I would speculate its' a 5/16-18 (5/16 inch diameter with 18 threads per inch). The standard wrench used with this size nut is 1/2 inch. This is a common size on older American made bikes.

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Do you have access to a thread gauge and a more-accurate measurement tool like a vernier caliper or a micrometer? The "Caliper and pitch gauge" section of https://www.wikihow.com/Measure-Thread seems approachable.

Basically you measure the outside of the bolt's threads and aim for the nearest "standard" size.

Then use the thread gauge to line up the points with the valleys and read off how many Threads Per Inch (or TPI)

The nut's thread has to match the bolt's thread.

Note that the outside of the nut is not directly related to the thread specs. As long as it fits in any recess on your bike frame, then its okay.

Another option is to carefully hand-file the daggy bits off your original nut, and create six flats.

Finally, stop using an adjustable spanner on this. You should use a ring spanner, or change out the whole bolt to a QR instead.

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  • I can't even find a seatpost binder bolt at home to check sorry. Can try some other people's bikes at work next week though.
    – Criggie ♦
    Sep 24, 2020 at 22:39
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FWIW, I'm guessing your clamp looks like this, more or less:

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Or is it like this:

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    I though OP was talking about a bolt for the pinch-clamp in the frame, to hold the seat-post in the seat tube. The pictured one has a square head on one side, whereas these ones have a square in the middle and a nut on either end.
    – Criggie ♦
    Sep 26, 2020 at 21:55
  • The second image is it!
    – ClariisseDeem
    Sep 27, 2020 at 12:41

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