I am with Beltfed on the flat washer deal, but only if you plan on flying an airplane with your Stovebolt engine!

On land, a tuned up Stovebolt doesn't vibrate enough to tell that it is running at idle, let alone vibrate a side cover bolt loose.
BTW, I can only hope that someone who goes by the name of "Beltfed" has a good sense of humor, and realizes I'm just pulling his leg a little.
Carl