I have a basement fiberglass shower unit probably installed in the mid 1970's. The drain pipe is 2" I.D. ABS which extends up into a 3" I.D. bronze / brass fitting in the pan of the shower. The ABS pipe ends flush with the fitting in the pan. The approximate 1/4" gap between these had been packed with oakum, with no other apparent sealant. The drain now leaks. I have some concern about minor flexing of the shower floor causing movement between the ABS and bronze / brass. What is the best way to fix this seal?

I know these drains with fat rubber washer that fits around ABS pipe and slides into the opening in brass drain. Then you put over it wedged plastic washer and pack it into the rubber washer.

I personally don't think oakum should have been used to seal the drain. Plastic pipe is flexible and will get loose from oakum grip.