Check your P to V clearance... but if thats plenty then
I would pull the spring and see if the valve is bent
or the spring ins broke... if thats fine then I would
see what the rockers can take for max open pressure..
if it is all fine then I would just replace the
rocker and move on.... I've only seen a break of
that nature when something bottomed out... even a
over tightened lock nut shouldnt do that... that
would tend to rip the threads out... JMO