I have had two seats drop, one on each head. One aft and one fore. The first one 10yrs ago. It happened one night in LA. Temperature in the high 50s. I went out about 9pm and then got back to the Jag about 11pm and ‘clik’, the valve dropped at start up. No over heating occurred at all. Left bank.
This last one occurred one morning with a temperature of -15 deg F. Did a short fun to the dentist and an hour later starting the car, ‘clik’, dropped valve seat again. I thought it might have been a lifter and drove it for a while thinking the lifter would loosen up. It never did and did some damage so a new head altogether this time.
Both beginning runs were under a half hour and of course no over-heating occurred. Both instances were not in summer nor daytime, runs were short and no prior overheating.
Any ideas why the valves dropped? The machine shop guy said he has seen both drops due to cold or no heat and due to over-heating and in many different makes and manufacturers. Key to all is aluminum heads with steel valve seat inserts.