Gasket between housing and engine block. This is not the water pump gasket
This new water pump housing gasket fits all 5 cylinder diesel engines from 1978 to 1985 including the 300SD 300D 300CD and 300TD.
Diesel Water Pump Housing to Engine Block Gasket
$1.65
SKU:
DSL-COMOHG 
Product Details
Recommend you use of non-hardening Permatex as the correct sealant for this gasket.
DO NOT USE SILICONE OR RTV. I can get in your cooling system and ruin your climate control valving.