I recently upgraded my 1980 H33 holding tank to 25 Gallons. For those of you contemplating such an upgrade please learn from my mistake. After carefully measuring the available dimensions within the old sail locker, with consideration given to the irregular shaped hatch opening, I ordered a custom built tank that holds 25 gallons. The tank arrived and I attempted the installation. Somewhere I mis-copied a dimension and the tank would not fit through the hatch opening. My mistake. In total depression, I put that project aside and moved onto rebuilding the head. While so engaged, I began to study the wall behind the head, between the head and the sail locker, and discovered that this wall is removable. When I say removable, please note that I did not say easily removable. You must remove the cabinet above the sink, the wooden trim in the front of the sink, and several other pieces of wooden trim before you can remove the plywood wall. You can do this without any permanent damage and the original parts go back into the same places. Once this wall is removed you may slide in any size tank that fits the interior dimensions. Happy sailing.