The cylinder ratchet holds the hand further to the rear and it is pulled back from the frame window as you cock the gun. The hand, without the cylinder, is pushed too far forward by the spring without the cylinder in place, and it gets tangled up with the top of the frame window, if I recall, with the spring behind and the hand in front of the slot,. so it can't allow the hammer to go all the way back.
With the cylinder out, cock the hammer, but hold the nose of the hand rearward with a finger, you will find that it will cock fully. Don't cut on any metsal until you know there really is a problem,,,