I honestly don't know what the set up is in your model; however, when I changed out my t-stat and gasket last month I ran into the following problem. Went to the parts place and they said they had the parts for my model - I bought them, got home took everthing apart, dug out the old stat and gasket only to find that not only did I not get the right stat, I also got the wrong gasket. What I was given was an o-ring, but that is not what I needed. What was needed was gasket that fit around the outside rim of the stat. I had to reorder elsewhere and wait four days to get it. Anyway, like I said defer to someone else who knows your model, but on mine, it doesn't take an "o-ring" in the strictest sense, but rather a rubber seal.

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'95 E300D ("Tank") - 231,000 miles
'79 240D ("Biscuit") - 197,250 miles (Sold)
'83 240D ("Ding-Ding") - 217,000 miles (Death by deer)
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"Back off, man. I’m a scientist” ~ Peter Venkman