Duwem if you have the old thermostat just take it with you to get the diameter at any parts store. You might even get a matching gasket if you take the thermostat housing. If not, buy some gasket paper and make one. According to my service manual the switch closes at 221-232 degrees. Again, take the old one with you to match the threads. I think it is NPT. I put a gauge on my 1450 but kept the switch too. There is a plug on the water pump that I removed and put the gauge sender in there. If you ever have to replace the oil sender, it is not NPT or metric, it is British Standard Pipe thread. There an adapter available for that. I'll come back with the number.