OH160 valve seat

five years ago i replaced the head with a loose exhaust valve seat on a OH160 engine with a new tecumseh head. this is my dad's '77 16/6 mower last week it would not start checked it out for him and it also has dropped an exhaust valve seat. it has run good all this time no problems and not over heating at all. puzzling to say the least .has anyone else had this happen to them ? is this that common of a problem with this model engine ? i dont mind fixing stuff with new parts but at over $200 for five years of service seems less than ideal. all thoughts , ideas , insight , welcomed thanks TKW / tim

I also have a '77 16/6, bought it not running, I knew what had happened. This seems to be a common issue on this motor, but should not have happened already to you. I bought a good used head of ebay, actually a good deal for one for $75 right now, and installed on mine last fall, been running good ever since. There is just a lot of heat differences between the aluminum head and steel valve that causes them to come out.
Have you pulled head? If so, is there any damage to the head where seat sits? I ask as cause if it just "fell" out, you can buy some seat adhesive from NAPA, glue the seat back in, then you can use a small punch and put dimples around the seat and actually pinch the seat in the head. Surprisingly, the Tecumseh 8-18 Hp manual actually tells you to do that when replaceing seats. I wasn't lucky enough, my seat had damaged my head beyond repair.
I make sure mine stays clean under the tins and also try to run it a little on rich side which will help keep head cooler.

thanks for the information about the seat adhesive from napa. no damage was done to the head or valve , it appears it was just the right place at the right time when it was shut off that the seat fell out only a quarter of an inch which held the valve open so it wouldn't start. thanks TKW / tim