well if the cases are apart I just use a socket the same size or a tad smaller than the seal and gently tap it in with a hammer, or use a pin punch working your way around the out side very gently and very gradually or you can wreck the seal by making it out of round, if using a punch try to work opersite sides like clock numbers working your way around, most of the time it is getting it started that is the hardest, just dont get too frustrated, take your time and a swig of beer when it starts to get the better of you.

some of the seals have rubber outside and can be a pain, others with the steel outer are a lot easier to get started, a bit of rubber grease on the rubber outer ones sometimes helps.