Well, looks like my cooling project did not end with the new waterpump.
I noticed a slow drip that looks like it's coming from the thermostat housing, I assume this means my thermostat will need to be replaced.
The only gasked I could find is the thermostat seal
http://www.***************/search/product.aspx?sid=fuz0rzajmra4xf2daveeys55&makeid=800016@Mercedes&modelid=1193802@300DT&year=1987&cid=16@Cooling%20System&gid=2136@Thermostat%20Gasket/Seal

What about the gasket fro the housing (cover for thermostat)? Could not find it anywhere...Am I missing something?
this is for 1987 300D

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1987 300D
2005 E320 CDI

The gasket that is on the thermostat is the seal.

The thermostat cover on a 617 does not use a paper gasket. It just uses a thick rubber seal (like the oil filter housing) supplied with the thermostat. Im guessing the 603 does also, and that is the seal for it. However the thermostat housing to block is a paper gasket (if its like the 617)

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1981 300SD 512k OM603

There are several seals around a 603 thermostat - a paper gasket between the thermostat block and the engine block, a paper gasket between the water pump and the thermostat block, an o-ring between the radiator hose fitting and the thermostat block, an o-ring between the cabin heater pipe and the thermostat block, and a washer between the transmission VSV temp sensor and the thermostat block. The part in the link looks like the o-ring for the radiator fitting. But I'm still curious - what's leaking?

edit - image pilfered from ************************

Sixto
87 300D

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Thank you for the pic, very helpfull, I am pretty sure the thermostat rubber O ring is leaking and the coolant is coming through the housing that covers the thermostat... I will be getting a new t stat and o ring

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1987 300D
2005 E320 CDI