I assume you mean the large diameter lower hose. As I recall, it is 16 mm ID. I bought ~3 ft length of silicone hose on ebay and used in both my 300D's. Should last longer than the factory rubber.

I removed the T-stat housing. Don't know if you could force the hose in that small gap otherwise. With the housing off, I checked the timing chain tensioner while easier access. If pressed for time, have a new tensioner spring on hand (~$7 @ P-P) since the spring coils wear a flat on one side (both mine). Also, need a new gasket.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans