Originally Posted by whiney

Don't know if it helps (surly website) :

NOTE: Two reasons why cogs don’t always fit perfectly on hubs the first time:

1. Our cogs use ISO standard threads: 1.375" x 24TPI (34.92mm x 1.058mm). This threading was chosen due to its compatibility with other threads used on bicycle hubs. ISO threads fit English and Italian threads almost perfectly. It is important to not switch back and forth between cogs and/or hubs with different threads. FYI, the only hub that an ISO thread is not compatible with is one with French threading (34.7mm x 1mm).

2. We've found that the diameter of hub threads vary wildly from brand to brand. As such, we have tweaked our cogs to fit all hubs safely at the expense of being a little bit tight on brands that tend to be on the large side of an acceptable tolerance. American ClassicTM comes to mind; our cogs require a little extra force to install on these hubs than usual. In certain cases, only a professional mechanic will be able to tell the difference between a cog that is simply tight-fitting and one that is crossthreading on. If in doubt, call a pro!! We've also found that thread lengths vary from manufacturer to manufacturer. Unfortunately, there is no standard at this time. Our cogs are 7.4mm thick. They work perfectly on most hubs, but you may occasionally find a hub that has threads longer than 7.4mm. Or, you may find a hub that has cog threads substantially shorter than 7.4mm, leaving only 1 or 2 lockring threads exposed. In these cases, contact your local bike shop for recommendations and assistance.