Just wanted to share some experience on fitting brand new hand to 1851 navy, after the original one had spring broken. It is quite easy and only took me about 30 minutes to get a perfect fit and timing with absolutely zero previous experience with it.
If you have the original hand, it makes it easier because you can get a visual of how the new one should be shaped, but if not - you should still be able to do it. New hand does fit through the hand slot in the frame, but will get stuck if you try to lower the hammer. Also keep in mind that you will have to use a tooth pick to press the hand back into slot, because spring will cause unfit hand to be stuck snug in the frame slot if you try to take hammer assembly out with brand new hand.
You will see that you need to use file to taper off top left side of the hand with a file, it's quite safe to do so as long as you leave about 1/3 of the right top side intact. You will also find that after you tapered off top left, you will have to taper off top front a bit, again leaving 1/3 top right untouched. You simply put the hand in the hammer and try how the fit works in the frame after each file modification. You can see what modifications to make to the hand purely based on how hand gets stuck in the frame when you try to lower the hammer. You keep shaping it till hammer lowers relatively effortlessly. Relatively is because when you shape hand enough to stop hard jam in the frame slot, there will still be some resistance from the hand spring when you lower the hammer.
After you are able to lower hammer fully, that's when it's time to adjust the top of the hand so the timing of the cylinder is correct. Easiest way that I found - look for the marks on the hammer face that old hand left. Put the old hand back in to see how it measures against those marks, and carefully shape the top of the new hand to fit the same. If you don't have the old hand, you will have to adjust hand manually based on how cylinder locks in the frame. New hand is always a tad too long, and will cause the hammer to either not lock all the way back at all, or lock with substantial force. You can fit it either based on marks from old hand, or just by filing very little at the time and checking how hammer and cylinder lock together to get a perfect fit. This is all really just makes sense when you try it yourself.
If you have the original hand, it makes it easier because you can get a visual of how the new one should be shaped, but if not - you should still be able to do it. New hand does fit through the hand slot in the frame, but will get stuck if you try to lower the hammer. Also keep in mind that you will have to use a tooth pick to press the hand back into slot, because spring will cause unfit hand to be stuck snug in the frame slot if you try to take hammer assembly out with brand new hand.
You will see that you need to use file to taper off top left side of the hand with a file, it's quite safe to do so as long as you leave about 1/3 of the right top side intact. You will also find that after you tapered off top left, you will have to taper off top front a bit, again leaving 1/3 top right untouched. You simply put the hand in the hammer and try how the fit works in the frame after each file modification. You can see what modifications to make to the hand purely based on how hand gets stuck in the frame when you try to lower the hammer. You keep shaping it till hammer lowers relatively effortlessly. Relatively is because when you shape hand enough to stop hard jam in the frame slot, there will still be some resistance from the hand spring when you lower the hammer.
After you are able to lower hammer fully, that's when it's time to adjust the top of the hand so the timing of the cylinder is correct. Easiest way that I found - look for the marks on the hammer face that old hand left. Put the old hand back in to see how it measures against those marks, and carefully shape the top of the new hand to fit the same. If you don't have the old hand, you will have to adjust hand manually based on how cylinder locks in the frame. New hand is always a tad too long, and will cause the hammer to either not lock all the way back at all, or lock with substantial force. You can fit it either based on marks from old hand, or just by filing very little at the time and checking how hammer and cylinder lock together to get a perfect fit. This is all really just makes sense when you try it yourself.