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You can measure the hammer strength using a trigger scale. You pull the trigger and hold it back, and then you hook your trigger scale under the hammer. Then you let the hammer down at rest against the trigger scale and the back of the frame. Then you pull back on the trigger scale until the hammer just starts to move, and you read the weight. Note that the trigger must be held back the whole time so that the hammer moves freely except for the strength of the mainspring.

There are pictures of this method in the S&W Armorer's manual, and with the older machined hammers there is a standard for the hammer strength. it should take 3.25 lbs to move the hammer on a .38, and 3.5 lbs for a .357. With the newer MIM hammers, the front of the hammer is shaped differently, so the trigger scale will sit in a different place, and those weights don't apply. But you can still use this method to compare different springs or different strain screws and get a measurement for how much spring tension is present.

The above info, as well as lots of other info about springs and strain screws is covered in more detail in an article I wrote:

Improving the trigger on Smith & Wesson Revolvers
 

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How heavy is the rebound slide spring you installed? If I recall correctly the Wilson kit has a 13# spring. I think that is too light. I have had good luck with 14# - 16# rebound springs
I installed a Wolff standard power, power rib and a 16# rebound spring in my 617, along with some polishing. Trigger pull was just over 11#. I am using the allen screw instead of a strain screw so I don't know if I am exerting more or less pressure on the spring than a tight strain screw would but now the pull is about 9.8# and smooth as silk. I ran about 150 rounds through it with no light strikes
On my 2.5" 686-6, which had a 12#+ trigger pull, I installed a Wolff, reduced power main spring, 14# rebound spring, polished internals. Now I am at 10# for a DA trigger pull, very smooth. Ran 100 reloads with CCI primers and had no light strikes
 

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Discussion Starter #24
How heavy is the rebound slide spring you installed? If I recall correctly the Wilson kit has a 13# spring. I think that is too light. I have had good luck with 14# - 16# rebound springs
I installed a Wolff standard power, power rib and a 16# rebound spring in my 617, along with some polishing. Trigger pull was just over 11#. I am using the allen screw instead of a strain screw so I don't know if I am exerting more or less pressure on the spring than a tight strain screw would but now the pull is about 9.8# and smooth as silk. I ran about 150 rounds through it with no light strikes
On my 2.5" 686-6, which had a 12#+ trigger pull, I installed a Wolff, reduced power main spring, 14# rebound spring, polished internals. Now I am at 10# for a DA trigger pull, very smooth. Ran 100 reloads with CCI primers and had no light strikes
I was going to invest in a trigger pull scale but .. after my last outing I am very satisfied with the gun and more importantly my performance with it so I think I am done playing with it. I had gotten the wilson kit and the strongest rebound spring was 14lbs. That is what is still installed. I will put the wolf in the spare parts bin and maybe revisit it but .. I am pretty happy now with the stock main spring.

Those CCI primers are HARD! My 3 inch ruger which had a 10# main spring and NEVER had a light strike in a thousand+ rounds this weekend had a couple with my latest reloads using the CCI primers.
 
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