[Dominik]

David, you said:
"...so with a 1/4 hex it would be 3/4" of engagement (of thread) which is fine."

The thread must go all the way through, else you "bottom out" when fitting the nuts.
The wheel stud is too long!

And that the taper have different angles doesn't help either...

Yes, it was 1:30am, now it's 8am. I do the float test later - with red wine!

Dom