Have a seat and hear my tale: You probably know our trucks have 2 gaskets - a double profile that goes under the tstat and into the lift bracket mounted on the block, and a square cut o-ring that goes over the tstat. I have not ever successfully obtained that seating o-ring and have just cleaned up, silicone lubed and reused the old one. The square cut o-ring can be obtained from most parts supply houses (they refuse to admit the other one exists—might be in the service manual but not their inventory). BUT they also want to sell you a witch-hat (no offense to practitioners of the gentle wiccan faith) rather than a top-hat thermostat. The former are temp/pressure style rather than temp only so I have always been reluctant to use them. So…each time its been a Cummins t-stat and reuse the lower gasket.