Jet valve issue

I was wondering what could cause the jet valve in my mitsubishi 2.6 liter motor to snap in half? Installed new piston rings, but didnt touch the valves. The clearances were up to spec so i just re installed the head. Went on test drive and began smoking out the tailpipe.got home and discovered the jet valve in piston 1 was cracked in half. The same piston that blew its rings. Did i do or didnt do something i was supposed to that could of caused this to happen. I would like to find the cause before installing a new one. Thanks in advance for your assistance.