Here you go:
But first, just to be sure we are talking about the same thing, I'm NOT using flanged nuts when I'm using the summer wheels with spacers as they are ACORN style.
So the following is for
ACORN nuts style wheels only. Depending of your wheels, your results will be different.
Measurements with the 1/2in spacer and summer wheels (UM Scorpion):
Measured stud left: 1 1/32" (not all usable as acorn nuts sets before end)
Number of threads actually used by nut: 14 full turns or just shy of 7/8".
Rule of thumb for thread safety on steal bolt is
1 to 1.5X the diameter of bolt. In my case, threads used is exactly 1.5 (or 21mm on M14 bolt)
Considering these values, I would NOT recommend using anything thicker than 1/2" as spacers with OEM bolts. But at the end of the day, you are your own boss.