1990 Volvo 740 4 cyl. Hole burned through top of piston 4.
What is the cause? on 1990 Volvo 740

I pulled the head and there is a hole burned through the top edge of the piston.

Asked by for the 1990 Volvo 740
either something poked a hole or it ran so lean it burned a hole. lean could have been from a number of things