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AusPPC

Strain Screw Length vs Trigger Weight

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I thought I'd share a tuning observation with the group that might help somebody out.

I've recently been tuning my new PPC revolver to get rid of the dreaded light-strike while also minimising trigger weight (i.e. finding that happy medium).

Ultimately, it appears shimming the hammer was the key to solving the light strike issues (trial and error is involved in determining the shim number and left/right order), but once the hammer was free of binding, I needed to find the optimal strain screw setting.

I did this by making up several different strain screws (using 8-32 hex head machine screws) in 0.010" increments, and then experimenting with them at the range to find the shortest one that gave 100% reliability.

OBSERVATION: For every 10 thousands of an inch (0.010") you add to a strain screws length, you add about 1 pound of weight.

After repeated tests (with all other variables remaining unchanged) and using a Lyman trigger gauge I was surprised to find this relationship to be quite consistent. It also highlights that very small changes in a strain screws length (almost imperceptible to the naked eye) can have a significant reduction in strike power.

As reference, this was with a K-frame, Bang-Inc mainspring, Apex Evo hammer, clipped rebound spring, hand cut & tapered 8-32 strain screws, and fully smoothed action.

Hope this helps someone :)

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OBSERVATION: For every 10 thousands of an inch (0.010") you add to a strain screws length, you add about 1 pound of weight.

It also highlights that very small changes in a strain screws length (almost imperceptible to the naked eye) can have a significant reduction in strike power.

Hence the reason I don't shave the tips of strain screws to "tune" trigger pulls. You get it just right and later after the tip wears slightly, you have misfires and need a new strain screw.

Edited by bountyhunter

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OBSERVATION: For every 10 thousands of an inch (0.010") you add to a strain screws length, you add about 1 pound of weight.

It also highlights that very small changes in a strain screws length (almost imperceptible to the naked eye) can have a significant reduction in strike power.

Hence the reason I don't shave the tips of strain screws to "tune" trigger pulls. You get it just right and later after the tip wears slightly, you have misfires and need a new strain screw.

The factory strain screws are a bit soft and can wear as you described. The machine screws I used seem very hard, and given that I've recorded the exact length I will be able to monitor over time to check for any wear. This is all based on the strain screw being bottomed out and torqued down to enable repeatability

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AusPPC, you say 'machine screw' but do you really mean a hex socket head cap screw? not to be picky but there is a difference. Most machine screws have a head that has a much larger diameter than the body of the thread. That aside, a yes most cap screw will not mushroom like the stock strain screw (which I'm guessing is made of stainless). Stainless has a yield strength of about 30ksi compared to an alloy steel hex cap screw which has a proof strength of about 140ksi. Of course I would still give a little safety margin and once you find what length works give it an extra 1/4 or 1/2 turn for extra insurance.

Also based on the 32 thread pitch one revolution is 0.03125 in. So based on your number of 0.01" per pounc that equates to about 1/3rd turn of the screw. That sounds about right from when I was playing with my 929 2 weeks ago when adjusting the strain screw with the Miculek main spring.

Edited by Fordfan485

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If the strain screw wears a little, you can still make adjustments by bending the spring. A properly heat treated spring will stay wherever you bend it to. All the S&W springs (except the ones maked with an S for stainless) will and most of the aftermarket springs will. You can bend it forward for less tension and backward for more tension. I just set the top end on a table with thumb in the middle, pulling up on the bottom end and bend them like that. That gives a large radius curve that won't develop a crack.

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If the strain screw wears a little, you can still make adjustments by bending the spring.

Bending the spring is how I adjust pull weight, I always leave the strain screws unmolested.

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OBSERVATION: For every 10 thousands of an inch (0.010") you add to a strain screws length, you add about 1 pound of weight.

It also highlights that very small changes in a strain screws length (almost imperceptible to the naked eye) can have a significant reduction in strike power.

Hence the reason I don't shave the tips of strain screws to "tune" trigger pulls. You get it just right and later after the tip wears slightly, you have misfires and need a new strain screw.

The factory strain screws are a bit soft and can wear as you described. The machine screws I used seem very hard, and given that I've recorded the exact length I will be able to monitor over time to check for any wear. This is all based on the strain screw being bottomed out and torqued down to enable repeatability

Are the strain screws soft for a reason? Might not a harder screw burrow into the spring

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AusPPC, you say 'machine screw' but do you really mean a hex socket head cap screw? not to be picky but there is a difference. Most machine screws have a head that has a much larger diameter than the body of the thread. That aside, a yes most cap screw will not mushroom like the stock strain screw (which I'm guessing is made of stainless). Stainless has a yield strength of about 30ksi compared to an alloy steel hex cap screw which has a proof strength of about 140ksi. Of course I would still give a little safety margin and once you find what length works give it an extra 1/4 or 1/2 turn for extra insurance.

Also based on the 32 thread pitch one revolution is 0.03125 in. So based on your number of 0.01" per pounc that equates to about 1/3rd turn of the screw. That sounds about right from when I was playing with my 929 2 weeks ago when adjusting the strain screw with the Miculek main spring.

You know, you are right - they aren't machine screws, they are socket head cap screws.

The proof strength sounds right, after multiple removals they have not mushroomed, the cold blueing I applied is even still intact. They are a pain to hand grind to length and shape using a Dremel.

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I just relieve the back of the head on a standard screw in the lathe and install a locking screw to stop the strain screw from moving.

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New guy here. 

 

Have read though numerous threads regarding the Smith and Wesson Strain screw and it’s nuances. 

 

Of the last 4 guns I have purchased, 3 came from the factory with the screw not bottomed out.

 

Frequently, these screws will randomly loosen during a range session despite my tightening the morning of the range day. 

 

My play guns (TRR8, 327pc and 627 pro) are not for street use and I don’t compete so I have no qualms with non-factory specs/parts. 

 

My question as a lurker here, is related to the TYPE of socket cap screw. There are at least half a dozen different ends for these including the knurled one pictured below (8-32 x half inch).  Allegedly, this prevents vibration from backing it out. I assume the numerous posts on the forum relate to these with a flat tip. 

 

Have purchased 5 to experiment with as well as some blue Loctite. 

 

Anyone have any experience with these?

 

Doug Ericson, MD

Neophyte Revolver Guy

NSSA Master Skeet Instructor

E2D79C98-9561-4171-BC5F-56631B311F3A.jpeg

Edited by DougE
Fix

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The knurled point won't keep it from working loose. The blue LocTite will. I use a half dog point or flat point for mine. The SS screws are soft. The black carbon steel ones are hard Grade 8 and don't wear down. The way to be consistent is to use the setscrew to adjust the tension and measure the tension on the hammer with a trigger pull gage hooked on the hammer, not the trigger. Start low, adjust till you have 100% ignition, measure and record the hammer pull. Then if you work on the gun and have to undo the mainspring or you want to see if it's changed, you can re check the hammer weight and know exactly where you're at. That takes all the guessing out of it. No 2 guns are exactly alike. You will likely have a different setting for each gun. You can also dial the weight up or down to adjust for different primers.

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Toolguy;

 

Thank you so much for the help.  Will move straight to your recommendations which will save me a week of experimentation. 

 

Blue LocTite to the rescue.  

 

Doug

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I use ones with a nylon patch on them ... I don't know how much the little patch helps because I use loctite on them anyhow. Set 'em and forget 'em. 

520872653_mainspringscrew.jpg.531a250a1c172a2efa04e2dc88e1b2b6.jpg627scale2.thumb.jpg.bfa9f3063f04f0a6eaa69985087be699.jpg

edit: large pistol primer cups are .020 thick ... if you get in a bind put one of those over a regular strain screw tip ... it won't go anywhere (wedged between the screw tip and the spring).

Edited by pete627

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You're welcome DougE! The nylon patches wear out quickly. The liquid Loctite stays good for many adjustments.

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Gents;

 

Thank you for the help.  This forum is a fantastic resource.  

 

 

Doug

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