According to Southerland's, BSC (English) and ISO freewheel / cog threads are a Class A fit, which means that they are interchangeable. This is because, although there are very slight differences in dimensions, they are less than the clearance differences between the dimensions of the external threads on the hub and the internal threads on the freewheel / cog. Threaded parts such as nuts and bolts are not meant to be interference fits, which means that there must be a small amount of play in the connection until it is completely tightened. A BSC cog on an Italian threaded hub is a Class B fit, which means that it will work and be serviceable, but will damage the threads slightly. Bottom line, don't lose any sleep over using BSC or ISO cogs on the same hub. Oh, and Soma makes excellent cogs from 16T thru 22T in 1/8" thickness for a lot less than EAI.
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