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Joe,

I haven't done it personally but have talked to others that have. There is a tool called the flare tool which is available from various sources. Some people also use a ballpein hammer to lightly round the edge of the tube.

Some people use 4200 inside the hole before putting the tube in so it seals the wood in case the "O" rings crack with age... others use expoxy to seal the wood before putting in the drain tube but letting the expoxy dry first.

Maybe someone that has done this task will elaborate more for you.
Good Luck