o-ringing t-stat housing

When I bought aftermarket chromed themostat housings, I could never get one to not leak. I purchased a cast iron housing and sent it out and got it chromed. I never had a problem after. Later after a few years as a precaution I purchased a Proform rubber gasket that has a built in "O" ring. It's been in use for about 20 years removed about 5 times and reinstalled, and I've still never had a problem.