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I had the most bizarre experience of trying to determine what kind of nut I have. I own a Schwinn Santis made by Pacific Bicycles.

The rear tire had a bolt and nut.

I took the nut to Home Depot and we determined that it was 3/8" diameter with a 24 thread count. We used a tool they had to do this and I called Pacific Bicycle to confirm.

It kind of looks like this nut here except for one thing:

The width is not 9/16".

I tried a 9/16 and it was too small. I tried a 10/16 and it was too big. I called Husky and they told me they don't make a 19/32 with would have been about the right size.

What did end up working was a 15mm.

What I find confusing is the threading is using english units while the tool part is using metric units.

This is not a problem, I could just use an adjustable wrench, but more of a curiosity question.

Has anyone else seen something like this? Why would they do this?

I do need to get a high quality caliper that can measure to this level of smallness.

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    A bicycle shop could provide you with the correct nut. And yes, the wrench used on a rear axle is 15mm. The wrenches are metric while axle nuts are imperial. – Carel Mar 15 at 19:11
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    Bicycle standards are a confusing mix of Imperial and metric units. Just wait until you need to replace a tire :) – ojs Mar 15 at 19:33
  • I found this bizarre. I guess I just have to accept it. – j.a. Mar 16 at 13:36
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You need a 3/8"x26tpi (threads per inch) axle nut. For decades they've all had 15mm wrench flats. (I believe that is something where you can find other examples in very old American, British, and probably other inch country bikes, but that is here nor there.) Bike shops have them.

3/8x24 is usually only for internally geared and coaster brake axles. Mismatching 24 and 26 tpi in either direction tends to go on with force but is likely to damage the axle threads and/or not hold well.

As for how it came to be that one of the most common types of axle nut has imperial threads and metric flats, there are a lot of other examples with bikes of the same thing. I don't know the story in this case and I wonder if anyone alive knows definitively, but factors can include costs for changing tooling at factories, consideration to what tools are in use where. For whatever reason, 15mm has been the nearly universal standard for axle nut wrench flats for a very long time, and that sort of thing has a way of being self-perpetuating because everyone's tooled for it. Meanwhile there's always clearly been some kind of economic draw to the big Asian producers of strictly OEM-level hubs staying with 3/8 axles and not switching to M10x1, a very functionally similar size that many higher quality hubs use. I don't know exactly what that is on the micro level, but on the macro you can bet it's just another example of the bike industry always running under-capitalized and barely in the black if at all. Regardless of what created the initial circumstances, if it's not causing huge problems and it's not going to be a selling point, there's not much impetus to change things just for rationality's sake.

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  • +1: Although do not entirely agree with the last bit. Seems to me the industry creates new standards each month - maybe its only once they have worked out a way to profit from it. :) – mattnz Mar 16 at 1:47
  • @Nathan - are you suggesting I have the wrong nut at 24 tpi and that it should be 26 tpi? – j.a. Mar 16 at 13:40
  • Almost certainly, yep – Nathan Knutson Mar 16 at 15:09
  • The tool at Home Depot probably only has choices for 3/8-16 and 3/8-24 because those are standard threads (coarse and fine pitch). It may be hard to tell that the 3/8-24 does not fit because there are not many threads for it to get out of synch, but if you look closely you can probably tell. They probably have a 12-28 on the same device. If you don't have to put the bolt through the hole, this would be the next one finer than your 26. – Ross Millikan Mar 17 at 0:37
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Sometimes this is your friend:

enter image description here

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    This is not a problem, I could just use an adjustable wrench, but more of a curiosity question. - from question. Also, they tend to not hold precisely enough and end up rounding the corners of the nut in some cases – j.a. Mar 16 at 13:39
  • Adjustable wrenches are never a good solution. On an emergency, yes but not for permanent workshop use. – Carel Mar 17 at 9:01

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