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I have a flip-flop hub that came stock on a Trek single-speed. I'd like to install a fixed-gear cog and lockring opposite the existing freewheel. How can I determine which cogs and lockrings are compatible?

I've read Sheldon's explanation. I've also read this SE question. I understand that ISO (1.375" diameter x 24 threads per inch) is the most common. But I'm not quite ready to assume that my hub is ISO-threaded. State Bicycle, for example, says that their hubs are British-threaded. And in my experience, Trek's part selections can be quirky. Unfortunately, Trek offers no reliable way to look up the part specs for a given bike.

Opinions seem to vary about which if any combinations of disparate thread standards are a "good enough" fit. Some say you should never mix and match. Others say British and ISO are compatible despite their differences. I don't know enough to form my own opinion.

I'm also confused about how the lockring threading relates to the cog threading. I understand that the cog and lockring threads are different sizes. But I haven't found any information about lockring thread standards.

So, I think my question boils down to three sub-questions:

  1. How can I determine my hub's thread standard? (I think calipers are the answer, but confirmation would be appreciated.)
  2. Assuming I have either an ISO- or British-threaded hub, are the two indeed compatible?
  3. Once I know what kind of cog I'm buying, how do I select a compatible lockring? Do I need to take another measurement?
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You are right that there is no magic way to tell which standard a hub is made to, if the manufacturer does not tell. If the threads are not damaged, calipers are indeed the best way to find out which standard the hub is. The thread pitch difference between English and ISO/Italian is too small to detect with pitch gauge.

The difference is small enough and aluminum soft enough that you can brute force a smaller thread cog on a larger hub. If someone already did this, measurement will get you the smaller size. It would be better to stick to the smaller standard afterwards.

With lock ring things are easier for two reasons: First, the standards are so different that measuring is easy. Second, you can just use the lock ring that came with the bike unless you managed to destroy it.

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