nagy's right, they need to be dead on. There can't be any room for error.
Be careful when you loosen the bolt. Try not to let the gear spin; the valves may hit the piston. It may also be extremely difficult to remove, so be prepared for that. Make sure your tools are up to par, and whatever you do, don't let the hex key slip. My friend has rounded two of these bolts and now he can't get them out.