The sear spring serves multiple roles in the functioning of the 1911. The original JMB design of the sear spring has three tines/leafs/fingers but there are some later designs that have four. I will only consider the original design here. I'm unaware of any official terminology to describe the three parts that apply force to the various fire control parts. I will use the term "tine" here.
Below is a drawing of an EGW sear spring. Note that EGW designs its sear spring with a relief machined into the left tine to prevent contact with the frame. If your sear spring doesn't have this relief and you're skilled with files, you can create one. "Do not make a square inside corner. Note that quality sear springs have a radius at the inside corners of the cuts. Use a round needle file to start at the base of the tine, then transition to a flat file to finish the relief. Make your strokes lengthwise and finish with sandpaper or a stone." - @BBBBill
It does not always happen but if the left tine gets hung up on the frame it will not apply force against the sear which can result in it not engaging the hammer which means the pistol will not fire.
In addition to creating a relief on the left tine you should also remove the sharp edge often found on the middle tine. That 90° corner (thin orange line) will damage the disconnector's ramp (yellow area) as the disconnector moves up and down. It can actually wear a gouge in the disconnector that will prevent it from moving and will cause malfunctions.
Below are pictures identifying the fire control parts that depend on the sear spring for proper functioning. In the pictures you can see the trigger bow does not contact the sear spring. Instead, the sear spring pushes on the disconnector which, in turn, pushes against the trigger bow.
Now we add the sear spring to the pistol and hold it in place with the mainspring housing. You can see how the left tine rests on the sear and how the middle tine rests on the disconnector. The right tine controls the grip safety. If the right tine doesn't apply enough force to the grip safety, the grip safety may not fulfill its job of blocking unintentional movement of the sear which is potentially dangerous.
T U N I N G
Next is how to tune each tine of the sear spring for proper and safe functioning of the fire control parts.
There are several ways to go about tuning the sear spring. Here's the easiest and most foolproof way I've found.
Remove the grip safety and the thumb safety. Leave the hammer, leaf spring, and all other parts in the pistol. Do not remove the slide. Leave the hammer strut hanging freely outside the frame. Fully seat the mainspring housing to retain the sear spring properly. Don't bother reinserting the mainspring housing pin until the tuning is finished.
Setting the middle tine (disconnector)
Thumb cock the hammer. Rotate the hammer strut up against the hammer and apply a little upward pressure to keep the sear and hammer engaged.
Hold the pistol with the muzzle resting on the work bench with the palm of your weak hand over the top of the slide and the thumb holding the hammer strut up against the hammer.
Engage the trigger with the trigger pull gauge. While observing the disconnector pull the trigger gauge upward until the disconnector moves upward. Don't pull it upward so much that it releases the sear. Ideally, the trigger gauge will read 16oz (1lb). Usually more than that is unnecessary so you can bend the middle tine backwards bit by bit until you get the 16oz reading. If the gauge reads less than 16oz, bend the middle tine forward bit by bit until you get the 16oz reading. Once you've achieved the 16oz tension on the middle tine, you're done and can move on to the left tine.
To bend a tine remove the sear spring first. Then hold the spring using the thumb and forefinger of your your weak hand about a finger's width above the the point where the tines merge into the main base of the sear spring. Use your strong hand thumb and forefinger to bend just the specific tine you're working on. You do not want to create a hard bend in the tine, just a gentle curve. Exactly how much to bend a tine is unknowable ahead of time. It is all trial and error.
Here are a couple of pics showing how I hold the pistol as described above. Although you can't see it I'm holding the trigger pull gauge with my right hand.
Setting the left tine (sear)
The next step will likely require very good illumination and the use of a magnifier (I use a 10x jewelers loupe). Pull the trigger and push the hammer fully forward against the firing pin stop. Then while observing the left tine and sear slowly pull the hammer back until it engages with the half cock notch. You want to see the left tine maintaining contact with the sear leg. If it does not maintain contact, bend the left tine forward bit by bit until contact is established with the sear leg when the hammer is at the half cock position. There is no specific weight requirement. All you need is just enough pressure to keep the left tine in contact with the sear leg when the hammer is at half cock. Very little pressure is needed.
How much is very little? It depends on what you want the final trigger pull weight to be. A good place to start is 8oz then reassemble the pistol and pull the trigger enough times to determine if it suits you. Rather than trying to measure the weight of the left tine by itself simply pull the trigger back until both the disconnector and sear move. If you know the disconnector tine is 16oz and the total of both tines is 24oz then the sear tine is 8oz.
Better yet is to live fire test the pistol. I do not try to hit a specific trigger pull weight. I want to be confident that the pistol will only fire when I want it to. I'm not a high-speed, low-drag, super-ninja national champion level competitor and while I've tweaked a couple of 1911s to have safe 2lb trigger pulls they did not suit me at all and I reset them to something noticeably heavier. Just because it's 2lbs doesn't make it a nice trigger. A nice trigger is much more than just sear spring tension.
The feel of the trigger is much more important than hitting a specific number. A correctly fit hammer/sear will feel much lighter than it measures on a scale.
Setting the right tine (grip safety)
This is accomplished in the same way as the first two: bend bit by bit until you like the way the grip safety feels. As with the left tine, there is no specific weight requirement. I prefer my grip safeties to be quite firmly pushed outward so I bend the right tine to achieve that.
Testing for hammer follow
After all three tines have been adjusted you should run the hammer follow test. We've all had the mantra "Never drop the slide on an empty chamber" drummed into our heads. I cringe even writing the words. Here's the exception. With the pistol now fully assembled and unloaded lock the slide open then push down the slide stop suddenly allowing the slide to move forward as fast as the recoil spring will push it.
Repeat this three or four times. If the hammer does follow, even once, you've got a problem to solve. Check that the middle tine does, in fact, have 16oz of tension on it and check that the sear makes firm contact with the hammer at the half cock position. If both of these are good, it's time to take a close look at the sear/hammer contact surfaces. There are other possibilities but they are outside the scope of this post.
These two YouTube videos may be helpful to you. You may well hear contrary opinions in these. Keep in mind that my objective is to create a sear spring that is reliable and safe in all scenarios, not to make a race gun.
https://youtu.be/OARYyFlbI3Y?t=294
https://www.youtube.com/watch?v=J_UAQ4SzQUI
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Originally Posted by niemi24s
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