Originally Posted by
Andrew R Stewart
As a bit of background, the world wide bike industry uses a (sometimes) confusing mix of SAE, Fractional and Metric specs. A classic example is a campy NT rear hub axle that is a 9.5mm diameter (not 3/8") but has 26 TPI. Due care when getting replacement parts is needed to have a properly working bike. Using almost but not quite parts is a good way to have problems and cause damage to the parts or fittings. Andy
The metric diameter/inch thread pitch combination arose because the non-English makers were using lathes made in England which are only geared to cut inch thread pitches but which can cut arbitrary diameters.