That's what I thought. It's a 1/2-20 thread, which is very close to something metric 12-1.25 or 13-1 (I forget which exactly) I took the prop valve down to my local hardware store, which has an impressive collection of fasteners... The metric bolts didn't engage cleanly, the 1/2-20 bolt was the best fit. Inline Tube confirmed its 1/2-20 thread.
They're just made bad, like so many other parts these days.
