Align the center axis of the nipple's thread with the hole's thread axis with a constraint.
Then apply a tangent constraint to the outer curve and the lower face of the hex on the grease nipple.
The tangent may only work if the lid's curvature is circular or conical. If the curvature is some other function, don't expect the constraint to work.
If it fails, then determine the distance between a point on the axis of the nipple (the center of the ball, for example) and the centerline axis of the lid itself. Constrain the point to that axis at that distance. Adjust by small increments until the hex touches the top of the lid. That kind of constraint is admittedly a poor one, but it will work until you do the last step:
When it's in the perfect position - GROUND it.
STF
