Back in the dark ages when engines were expected to last forever, or at least well past the warranty period, the designers added some pretty redundant parts. Nowadays, the same manufacturers figure how many engine failures they can get by with during warranty and still make a buck. Palnuts were just one of those efforts to add reliability, but it was discovered they really didn't add any degree of safety to the assembly. As far back as the 1950's, I can remember my father tossing them out on every engine he rebuilt.
Jerry
