Another Learned Lesson - Thermostat O-ring
 

Don't even THINK of trying to reuse one. Even if it's fairly new. I know it's cheap, but I didn't have one, MB dealer is a distance, It was less than a year old, bwaaah, bwaaah, three hours later, a big mess on the garage floor, and I need a new O-ring anyway. Tightened and torqued the neck evenly to the water pump and everything.

A great thing about this forum is we can learn from each other's experience, as long as we do like you did and post it :)