from the phil catalogue

"The use of “stamped” or plated cogs with minimum thread engagement and beyond normal spec thread tolerances are not recommended with quality track hubs. Our thread is 1.370 x 24 tpi. This is compatible with most high quality cogs. Caution should be taken with some cogs marked as ISO thread – the pitch may actually be closer to 1mm than to 24 tpi. A simple test is: Thread the cog onto a quality freewheel type hub. If it easily threads to the freewheel stop, it’s 24 tpi – if it jams before, it may have a metric or other pitch – NOT COMPATIBLE with ours and other quality track hubs."