Due to the inconsistency of molding of the tank threads it can be difficult to put the cap on without cross-threading. The threads are inconsistent in both dimension and shape. However, once the threads have been properly engaged, the cap screws on fairly easily. Even then, it's not a perfect fit and I don't believe it will seal sufficiently to contain the contents should the tank be in a position other than upright. Perhaps a thicker gasket in the cap might help.