If you file the window you file the lh side. Thats the side by the cylinder release.
You never file the hand. First the hand is hardened and the file won't cut it. Second, the hand thickness is what turns the cylinder to the proper carry up.
I posted this in the past. Copy and pasted for your pleasure.
Hand fitting does violate "file the cheapest part" rule. Before a wider hand will do any good, it must catch the ratchets closer to the center of the cylinder. This means you must file the hand's window only on the left side and only enough to allow the hand to move freely in the window. If you take metal off of the right side of the window or from the hand itself, you won't accomplish a thing, in fact you might make timing worse. In other words, you have to move the hand's point of contact to the left.
Installing a wide hand compensates for wear on the ratchets outer edge. The idea is to force the cylinder to turn a little farther and get a good lock-up. If the left tip of the hand is worn, it will not rotate the cylinder far enough. In this case, you can install a standard width hand to restore timing.
Hands are made of some hard stuff. It is a good idea to use some 400 grit sandpaper to break the sharp front edges of the hand so it won't bite into the window.
DCU or timing
DCU (doesn't carry up) or basic timing is easy to check.
How I check basic timing is pretty easy to do. Without touching the cylinder or holding back the cylinder just gently cock the hammer slowly in single action, (on all stations) and see if the cylinder stop snaps in place before the hammer falls. When the trigger is pulled to the rear (MIM triggers will rotate the cylinder further when the trigger is pulled) the cylinder stop must snap into the cylinder notch, if not, then you have a problem.
One thing I must insist, before swapping the parts make sure it's not a loose fitting ejector causing your dry fire and poor carry up problems. This is true with non-pinned cylinders. Drop empty cases into the chambers then check your carry up. Sometimes the gun is fixed when cases are inserted in the chambers.
As the extractor ratchets, hand, and hand window wear, they get to a point where the hand won't rotate the cylinder far enough to allow the cylinder stop to snap in before the hammer drops (carry up condition in DA mode). This is not a hand length issue, it's a hand width problem. If you take the side plate off and watch carefully in the hand window while pulling the trigger, you will see the hand cams off of the ratchets and the left side of the hand tip positions against the right section of the ratchets to rotate the cylinder those last few degrees.
Unless a gun has been fired a zillion times, the hand and the window seldom show wear, except for the left tip of the hand. Most times when you see a hand loose in the window, it's because it was shipped that way.
Factory hands run from about .093 to .095" and wide hands run about .098 to .100" There shouldn't be more than .001 side play between the hand and window.
If the hand has worn thinner, or the hand window has worn wider, or the edges of the ratchets have worn thinner, you will start loosing carry up. Without fail, the ratchets are the most usual cause.
There are several solutions. The absolute best one is to replace the extractor (ratchets). Unfortunately, that's a factory fitted part and you can't buy one without sending the gun back to the factory.
The next best option is to fit a wider hand. This will have nearly the same effect as a new extractor because it will take up the slack from ratchet wear and make the cylinder rotate a few more degrees. When the wider hand is fitted, you must widen the left edge of the hand window so the hand will be positioned a few thousands to the left.
An undesirable option is to lay a fine bead of weld on the right edge of the hand window then dress some material from the left edge of the window. This will position the hand closer to the ratchet and make the cylinder rotate further. Welding that thin of metal is sure to do some damage to the finish and could ruin the frame.
You can bend the tip of the hand slightly to the left. This will also cause the cylinder to rotate a bit father. The hand is made of some very hard material and will fracture if you try to bend it without first softening it. You have to heat the tip of the hand until it is red hot then let it cool to room temperature. It will then be soft enough to bend without breaking. Once you get the hand tip bent, you must re-harden the hand. Herein lies the problem. Most of us don't have the proper metallurgy skills to re-harden the hand. If it is soft, it will soon wear out.
One of the very precision specs in an S&W revolver is the hand-to-ratchet clearance. A few thousandths makes the difference between good and bad carry up. You want the hand tip to be touching the ratchet but not binding on it when the trigger is pulled. Basically a zero clearance.
If you insert the tip of the hand into the window, it should move freely with minimal side play. The tighter the better, as long as it doesn't bind.
The left side of the window creates a reference point in respect to the ratchets. You must move the reference point to the left before the wide hand will make the cylinder rotate more. That means you must take metal off of the left side of the window. Use a "safe" window file for this. Taking metal off the hand or the right side of the window moves the reference point the wrong way.
Once your wide hand is installed, the tip of the hand may bind on some of the ratchets. This will cause a bad gag in DA trigger pull. With the side plate off, watch through the hand window as you cycle the gun in DA. You will see where the hand cams off of the ratchet and the point of contact between the hand tip and each ratchet. Dress the ratchets at the point of contact so the hand is touching but not binding.
Go slowly changing the hand. It's easy to do and will fix your timing (DCU) problems.
