Paul B. commented:
"No. Stainless steel is FAR weaker than good quality mild steel. For most, non stress application, stainless steel RULES! I am a BIG stainless fan. But for stressed applications, like say, holding the wheels on, or, suspension components, original spec. mild steel (#5) nuts only. I mean, there really is no reason (other than for a show bike) to deliberately REDUCE the strength of the fasteners holding your (or wifes in this case) wheels on. Especially when the correct fasteners are readily available from Piaggio. Also, your original wheel changing tools will no longer fit the wheel nuts since, as you say, the stainless nuts require a 13mm wrench, and the original nuts are 14mm."
Thank you again Paul B. for commenting. I owe you big. You opened up a great conversation and I began to research it a bit by emailing my mentors for how they handle fasteners on their restorations as well. The bottom line is I will probably use galvanized steel now as opposed to stainless steel, based on your's and Tom G.'s suggestion.
Here is what I learned from Tom G.:
"It is true that stainless is weaker in tensile (and probably compressive) strength than mild steel, and of course this applies to fasteners.
That being said, I can't really think of an instance on a vespa where this will make any difference from a safety perspective. I can't begin to imagine that stainless wheel nuts would be unsafe to use.
That being said, I don't really prefer the look of stainless to galvanized for wheel nuts, and most of the time I use galvanized for them. I do frequently
use stainless for attaching body parts (like attaching front fender to fork on vespa, and attaching most lammy body panels to the frame). Is this really a
good idea? It is an open question. Where you use dissimilar metals next to one another, the less "noble" metal corrodes preferentially. In other words, if you use stainless fasteners to hold steel panels in place, the steel panels will corrode
(marginally) instead of the stainless fasteners that are in electrical contact with them. If you were to use bare steel fasteners on painted steel panels, the
fasteners would corrode more quickly (and the panels would not corrode in the vicinity of the fastener as long as electrical contact is maintained). This is
why roofers don't use iron nails to hold copper roofs in place -- the iron nails will completely corrode away, while the surrounding copper will not corrode.
So strictly speaking, from a corrosion and protection of metal standpoint, the best solution is to use galvanized steel fasteners on painted steel panels.
It is an extra "bonus" that this will also give you the correct look. But only anoraks will care.
Good luck getting wheel rim nuts in 14" size anyhow. They ARE available from some specialty shops, but are quite expensive. I never bother. Frankly, I'm not
even sure that the 14" nuts were used on the SS anyhow. Do you know for sure? They were used in some places on the GS 150, and I think also for the GS160.
But I'm pretty sure that by the Rally, the 8m hardware was all 13mm. I'm unclear on the SS.
If you can itemize the locations you want to use stainless, I can give you my best guess as to whether that might present any problem." -- Tom G.