I'll state the obvious Andy: do not allow the anvil or the hammer to touch the aluminium crankcase when you do this. You may need a wide, flat steel packer in between the anvil and the boss to make sure this happens, depending on how you set it up. Never use anything in between the hammer and the boss - it dulls the impact. Hammer speed is just as important as hammer mass, so hit it sharply. And, of course, wear safety glasses. Occasionally bits fly off things, even hammers.
