OK for the archives and the curious, before your plastic T-stat cover breaks, upgrade it to the aluminum one.
The upgraded one has an opening on-top for a plug and also for an air bleeder nut.

The crush washer it turn out, was for the plug bolt.

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD