safest way to do it with the heads on is in the firing order with a breaker bar on the crank so it won't move. Pressurize the cylinder with air and change #1 and #6 seals intake and exhaust. Rotate the crank clockwise 1/4 turn do #8 & #5, 1/4 turn #4 & #7, 1/4 turn do #3 & 2. This way if you lose air pressure for whatever reason your valve won' fall completely out of the guide into the engine.