This is an example of a failed Briggs exhaust valve guide. It has extruded almost to the rocker. This is caused by overheat due to low or dirty oil and / or dirty cooling fins. Also if it 95 degrees or over just wait till evening to cut your grass.
– A temporary fix:
1) Disassemble the head
2) Create a grove with a dremel type tool less than a quarter of an inch down from the top of the guide.
— Note: I use a small piece of metal tubing slipped over the guide to cut a level groove.
3) Drive the guide down till the middle of the grove is just level with the surface of the head.
4) Peen the aluminum into and over the grove.
5) Using a punch with a small sharp point, peen dots into the head forming a spiral circle radiating out from the guide about a half inch. This will further expand the aluminum into the valve guide.
6) Now assemble and install the head making sure to perform a valve lash adjustment on heads that require it.