Go to the dealer and give them your vin# and get the correct cap and be done with is. I think the service manual gives a "range" 18-22, but they use 21. DOesn't matter just go and get the right one.

+1 shaps, I forget about the ATF+4 factory fill on the trans, well for those that have ATF+4 standard. Regardless, I would change the ATF+4 standard fluid filled OEM transmissions at least at 60k, but that's just me.

 

DESCRIPTION
All cooling systems are equipped with a pressure cap (Radiator Pressure Cap—Typical) in the radiator upper hose. This cap releases pressure at some point within a range of 131-to-158 kPa (19-to-23 psi). The pressure relief point (in pounds) is engraved on top of the cap

The cooling system will operate at pressures slightly above atmospheric pressure. This results in a higher coolant boiling point allowing increased radiator cooling capacity. The cap contains a spring-loaded pressure relief valve. This valve opens when system pressure reaches the release range of 131-to-158 kPa (19-to-23 psi).

A rubber gasket seals the radiator filler neck. This is done to maintain vacuum during coolant cool-down and to prevent leakage when system is under pressure.

 

Well not all Durango's, but yea I have to agree with the rest. The 1998-1999 radiator cap came on the radiator it's self, the in line version came on the 2000-2003 when the redesigned the cooling system.