Dear the Warden:

I actually replaced the thermostats on 2 different 300SD. On one of them, I simply used a toothbrush to "polish" the mating surfaces and put in the new thermostat with the O-ring. There has been no leak at all.

On the other 300SD, I just applied some Permatex sealant (for water pump/thermostat) on the mating surfaces, even though I believed that it would not leak without that Permatex sealant at all. The O-ring for the thermostat should be enough to prevent any leakage. You can buy that Permatex sealant at Kragens or Autozone for about $2.5, which is plenty enough for many uses.

BTW, if you want to save some money when ordering from Fastlane or any other online store, just get the "thermostat without seal" for $12 and a separate O-ring for $1 instead of the complete Behr thermostat set (thermostat + O-ring) for $21. The "thermostat without seal" is made in France (by Caloric or something like that), and it even looks more solid and better built than the Behr one. I ordered 2 such thermostats (French ones) and they've worked perfectly so far.

Hope this helps.

Best regards,

Eric

P.S. J'aime la France et sa langue .