Too big in diameter for the drain plug. Approx. 1" ID. It's round in cross section. Not flat like a "crush" washer. The filters come with an Aluminium "crush" washer that goes under the head of the canister lid bolt.

I'm thinking it may be a seal between the canister assembly and the engine block. I've never had this off. Do you know if it should be removed periodicly for cleaning. It's no big deal for me as I have a lift and a full shop.

Thanks for your help.
Dave