I have read all the post and still can't find the answer to my question so here goes. I am installing a new toilet and due to a 10" rough in will install an offset flange so I can use the Toto Aquia toilet. My drain pipe is 4" lead (straight) tapering into PVC approx 18" into my concrete slab. All the plumbing is in good condition at 18yrs old. The plumbing supply company provided me with a new brass offset flange and a piece of 4" dia lead pipe that also tapers approx 10" long. I understand how to install the flange but have one question. When I insert this lead sleeve into the existing drain pipe (which would be cut below grade) do I need to seal the connection or is the friction fit tight enough. After reading the posts I am concerned about gases.