>  > I have them in my hand.  The heads take a 17mm wrench, I guess
>that makes them
>  > 17mm.

Last time I hung a propane tank on a non-Westy , I used cap nuts from
an engine head of a dead watterboxer engine.  Since the cap nuts
cover the end of the bolt, they keep the bolt threads from corroding
and/or filling with dirt under the chassis.   Removal will be much
easier in 10 years.....I had to get the appropriate length metric
bolts at a hardware store.

I guess you could also use regular nuts and coat them with some kind of gooo.