I have a 95 F350powerstroke and just replaced the thermostat with one from NAPA #371-203. The old one had a rubber gasket tangled in it and was not allowing the TS to close. The new one came with a square o-ring that fit into a groove in the housing. It leaks like crazy. I cleaned both surfaces before installing. Does the old TS have a gasket that the new one doesn't? Can't figure this out .I bought my truck new and it has 400,000 miles. No TS problems till now. Waterpump replaced a few months ago but they didn't put a new TS in. Any ideas???