Rocker arm guide slip

[EquippedNut]

I have a L20B motor that I rebuilt parts that were changed were new pistons with new rings, connecting rod bearings, valve seals, timing chain, upgraded oil pump for a 240sx. I was able to fire it up for the first time after properly priming for a multiple crank overs ensuring it was properly lubed. Connected spark and was able to fire it with no issues minus a vacuum leak that made it run rough as all hell. Found the vacuum leak and once the vacuum leak was sealed it ran amazing for about 10-15 minutes. After that I started to hear ticking so I shut down immediately. Next day I opened valve cover and noticed on cylinder 4 a rocker are had slid off the exhaust valve guide sitting on top of valve. Inspected cam no damage and there is no damage to rocker arm. My question is what might have caused this? Will it happen again? And when I remove the cam cradle and place rocker arm back on how can I do a check to make sure it will not happen again? Thank you!!

[datzenmike]

Extremely loose valve lash can do this plus high revving. But you would have heard this after starting.

 

The others are less welcome...

 

1/ Exhaust valve seat fell out and valve cannot close. With the valve prevented from closing there is extreme rocker to valve tip clearance and the arm is free to fall off the lash pad. Turn the engine over by hand and see if the valve spring rises to it's closed position like the intake beside it.

 

2/ Exhaust valve was accidentally bent while the head was off causing it to jam in the guide in the head and not close. May have been alright till the engine heated up (about 10 min) then jammed, valve doesn't close, too much clearance and rocker falls off the lash pad on the valve tip. A valve can easily be bent by laying the head face down with a valve open and exposed.