With a standard "bayonet" cap you need NO adaptor with most cooling system pressure testors.
The replacement reservoir for that W202 chassis requires replacement of the cap as well, to the screw on style.
MERCEDES Benz Master Guild Technician (6 TIMES)
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190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)