Removing the cylinder studs can be a trying experience. You'll want to run a search on methods, which can include heat, penetrating oil, and using two nuts on the free end to create an anchor to wrench them out. Many times they go down with a good fight (twisting, etc.). You want to be careful to avoid them shearing off. Otherwise, you get to drill the broken off portions and then helicoil the threads in the case. Take you time, and do some research before just jumping in with a vice grips.