[NaCl-H2O]

On most SCH80 bulkheads the gasket included has two sides. One side is smooth and the other has ridges or ribs.

The ribs help create a water tight seal around the glass. The smooth side should face the plastic of the bulkhead so that pressure is evenly distributed across the gasket.

If your does not have ribs one side, it really does not matter! Just make sure it is on the wet side as Brett suggested.

Another good tip is make sure all surfaces that meet the glass should be clean of all debris so that all the surfaces mate up correctly.