I looked at my m54 expansion tank and I do not see a nipple hear the cap... Do you mean on the actual cap? If it is leaking at one of the hoses just replace the oring in the hose and lube it with some silicone grease and throw it back on. Not replacing orings usually causes leaks.

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2004 E60 545i Sport 6MT. Black/Black Build Date 6-2004 102k miles