Exhaust Valve Seat

[Mike P]

I have a 18.5 HP Briggs and Stratton engine and the exhaust valve seat came loose. What would cause that to happen? Model 31P777-0348-E1. I also cannot find the seats listed in the B&S parts list. I guess you have to buy the head. Have you heard of this happening before?

I attached a picture.

 

[ILENGINE]

Normally due to overheading of the head. Grass buildup or from leaked oil and dirt sticking to it. See one every once and a while. When the head gets too hot the aluminum expands and the seat loosens and out it comes.