There is no thicker seal. You do not use the same wax seal as in a House. The seals are rubber. Usually there may be up to 1/2 inch extra down pipe. If NOT, then you will build a new down pipe with a new flange. Early 80's the down pipe may be glued to the entry on top the holding tank. Hopefully not, it will either be threaded or have a rubber slip gasket on top the tank and the flange/down pipe will just pull out. If threaded and after 30 years, you may have a devil of a time unscrewing that 3 inch down pipe. Doug
