Correct-o, Matt! Cast-magnesium, also known as "compressed corrosion" in the aircraft industry, has the peculiar property of stress-aging. That is, as a wheel is run through turns, they actually bend slightly, and this bending stress tends to build up over the years until they crack, unless they are stress-relieved. The stress-relief can be done in Mom's oven at 375 degrees for a 3-4 hours, then very slowly cooled down, but this will brown the protective paint, too, forcing a retouch. The only other use for magnesium wheels is on high-performance military fighters, and such wheels are not cast but forged. They are routinely scrapped after a certain number of hours of use.