Wow, that's the first I've heard of that. I've heard of the breather tube blowing off due to condensation building up inside of where it connects to the airbox, then freezing, which causes pressure build-up. Maybe you're having that same problem but instead, it's blowing the side cap off. Is this happening at really cold temperatures?
i think i am gradually getting to the bottom of this, i believe when engine ran hot i blew a head gasket and that put pressure in base, and blew out plug, also antifreeze in the clyinder, before i realize this i had screwed new plug back in cover with lock tight on thread, when engine turned but won't start, i took out spark plug and found antifreeze on plug, that when i realize i probably had head gasket gone, took head off and sent to machine shop to be checked out also. now i have to take new crankcase plug out, to time engine, but the lock tight had done it's job, and plug it only made of plastic, so i striped out the allen key hole. i still have to find out why engine ran hot, fan working fine, so i will change thermstot and hope alls well