1984 Other Nissan Models water in oil

Leaking head gasket, crack cyclinder head. If it is a head gasket you need to run a compression check to see if you have low compression on one cyclinder. If it is a head gasket, you will need to take the head off and have it resurfaced and put on new gaskets. If it is a cracked cyclinder head, you will need to replace the head, with a recondition head or a new head.