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I have this bicycle given by me by a friend, I kept adjusting the height of the bicycle saddle for different members of my family and now the sides of the nut are rounded.

The nut is used to adjust the height of the bicycle saddle using a range.

I believe that the bolt is a carriage bolt? Now I am trying to find what size the nut is and the thread which I don’t understand.

Anyone know where I can buy the same nut online? Please tell me if the pictures aren’t clear, I’m still waiting to use the bike.

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  • Additionally, this was caused by frequent adjustment using an adjustable spanner that was not closed up enough. Adjustable spanners need to be adjusted almost every time they are put around a fastener. The proper tool is a ring spanner, unadjustable, which supports the fastener all the way around. Or one bike per rider is good too! – Criggie Sep 14 '20 at 23:35
  • You may be able to salvage that nut by hand filing the sides flat, and remove the burrs. But the right tool for the job will minimise the chance of this happening again. – Criggie Sep 14 '20 at 23:36
  • Take it to a bike shop or a hardware store and match what's in the bins. – Daniel R Hicks Sep 15 '20 at 0:05
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    Rather to a bikeshop and if you need frequent adjustments get a seat-post quick release bolt where you can adjust the height without the use of tools. If you want to stick with a wrench get a ring wrench that will not slide off and round the corners. – Carel Sep 15 '20 at 7:15
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This looks like 16 threads per inch, which would indicate a 3/8 inch bolt if you are using SAE sizing. It would take a 9/16 inch wrench. If you use a box wrench (or even an open end) you are much less likely to round off the nut than if you use an adjustable wrench. It could be 10mm, which would have a thread pitch of 1.5 mm and looks like it would take a 17mm wrench.

The threads on your bolt look fine, so take it to the store and test fit the new nut to see if it matches.

Note that your nut has a captive washer on it. If you buy a standard hex nut you should use a washer behind it. There may not be enough flat space for a standard washer. Nuts like this are fairly common, but not as common as the vanilla hex nut variety.

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