Go slow first off. Start using some penetrating oil such as PB Blaster and let it soak a day or two. Remove anything in the way preventing you from getting a good bite with a socket.

If it starts getting serious, you can heat the cyl head up and then hit the GP with a spray "freeze" product to contract/shrink it.

__________________
Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)