Hi

I have a old OS 19, I can't work out what the prop nut size is

Thread is similar to a os 20 or 40 ( 1/4 x 24 ) but smaller.

Thanks

Johnkpap

Could it be a 5 or 6 mm metric thread? The smaller O.S. engines had metric threads when the crank was smaller than the "standard" 1/4" shaft diameter.

We don't have that information. The .19-size engine was long out of production when we became the O.S. service center.