Bicycle Mechanics - Bianchi San Jose FW question

I ride a 2006 Bianchi San Jose with the stock Fixed/Free flip flop hub. I picked up an English threaded FW today to increase my gear ratio (going from 17 to 16 tooth), and went to install it except I could not get it to go on the hub without crossing threads. I know for a fact that the FW is not metric and I don't know of any commonly used threading schemes beside English/Metric for modern single speed FWs so some input would be appreciated. The current FW (which threads on and off like butter so the threads are not damaged) is an ACS Claws the only other marking on it is (C2), the Freewheel I was trying to replace it with is also an ACS Claws but was marked (C7) if that makes a difference. Also, I threaded the new FW on a BMX wheel no problem, so I know its threads were also intact, the old FW would not thread on the BMX hub however; and a metric FW was too small to even begin to thread on either hub.

Thanks.

So did you already screw up the threads on the hub?

I attempted to install the FW by hand and as soon as I felt resistance I stopped inspected the threads which are fine and reinstalled the old FW which went on without resistance and I rode on today with no issues. This isn't a thread problem unless some how the threads on the hub and existing FW got deformed together over two years of riding, but that seems far fetched to me. The old FW threads on easily as I sad before with no signs of being stripped... I climbed a pretty steep hill that had me out of the saddle today.

Does anyone know if it could maybe be ISO instead of English, same TPI and 7/1000 an inch difference in diameter, could explain it. But I didn't think they really used ISO that much or at all for single speed FWs.