carver wter heater?

Gas & elec tie bar broke tank came off problem is removing the broken bit from the housing drilled out about 3/4 inch deep. How far should I drill as I assume the obvious the hole does not go all the way through or it would defeat the purpose of not having to seal the housing end. So I need to know so I don't mess it up and drill right through.

Thanks in anticipation I don't know why they don't have photo posting facility.