What is the thread on a S&W mainspring strain screw? I'm playing around with with bending the mainspring and shortening the strain screw to get the best trigger pull. It would be nice to be able to determine the exact length before I chop the OEM strain screw.

The old trick to weaken a flat main spring was to gradually, and equally trim metal off both sides. Not the flat sides, but the narrow sides, as in right side, left side.

I used to do this with a fine machinist file. Then smooth it out with emery cloth after. You want to be careful not to leave any score marks in the metal that could develop into a crack.

According to most of what I have read shortening the strain screw is a poor idea.

Narrowing the main spring is probably preferable though I would have an unmodified spare one on hand before doing so in case you go too far.

Wolff and Wilson Combat offer reduced tension spring kits that can be used. Quite reasonably priced but they do increase the chance of a FTF incident.

If a defense or hunting gun I would leave the main spring and strain screw stock. A FTF could be fatal.

Always read that trigger pulls can be reduced by 25% or so with stock spring and strain screw via a good action job from an experienced gunsmith, possibly along with a minor rebound spring strength change.

I recently got the Jerry Mikulek action job video and found it to be quite informative if you want to try your own action job and have some mechanical ability.

quote:

Originally posted by Stillwater788:
What is the thread on a S&W mainspring strain screw? I'm playing around with with bending the mainspring and shortening the strain screw to get the best trigger pull. It would be nice to be able to determine the exact length before I chop the OEM strain screw.

Two worst ideas for "improving" trigger pull:
1 Shortening screw (messes up geometry)
2. Bending stretching, or otherwise damaging an already tempered spring. (seriously damages spring)

Better to deburr action and try lighter replacement Wolff springs. Don't be surprised if you need stock mainspring to keep reliability.

The strain screw thread is 8-32. I replace them with a socket set screw (with 290 Loctite to hold it in place). Using a set screw you can adjust it "longer than stock" to go with lighter mainsprings, like the Wolff reduced power. If you haven't noticed, on this forum the strain screw is considered a sacred object.

This message has been edited. Last edited by: tomcatt51,

I second the motion. While you can get buy with shortening the main screw A TINY BIT it does drastically alter the geometry of what is going on inside the gun. You are MUCH MUCH better off getting a set of springs from Wolff and a couple of good stones. Smooth out the inside of the gun and the rebound slide mating surfaces. Make sure the hammer boss and trigger boss are smooth. It is surprising what you can do that way without shortening the strain screw.

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on this forum the strain screw is considered a sacred object

Okay, I did not know that or I wouldn't be messing with mine .

Here is my 2c worth. Sorry if anyone thinks is a blasphemy.

Standard L-frame strain screw (for flat mainsprings - not Wolf or "bossed") is 0.500-0.505". With polished action and slightly reduced rebound spring, the pull of my 686 was awful. Way, way too heavy. I shortened strain screw to .455". Had no FTF's after 3,000+ rounds of various factory ammo and handloads primed with WSP, WSPM, Remington's and KVB-9 primers. Not a single FTF. Trigger pull was very light and silky smooth. IMO the pull was too light for SD carry and I replaced the strain screw with one cut at 0.475”.

Tried S&W bossed mainspring too. In my 686 it required LONGER than standard strain screw – minimum for reliable ignition was 0.525”.

The method I use to adjust strain screw is as follows. I prime some empty brass (no powder or bullets) with hardest magnum primers I have. Then I gradually back the strain screw out and fire the empties while pointing gun in the safe direction. I do this until I either start getting FTF’s (1 FTF out of 6-12 rounds) or the primer dents look too shallow. Then I turn the screw 1-1.5 full turns in. That’s the minimal length. To find out how much to cut off, I turn the screw all the way in while counting the turns. Knowing step of the thread I calculate the length.

Mike

Thanks for the advice gents. Tomcatt51 – I’ll pick up some 8-32 socket screws and have at it. Hoptob – I’ve been using live rounds, but the primer only idea is excellent – thanks for the info. I should have stated that these modifications are for game guns only – IDPA – using Federal primers. That said, a game gun has to go boom every time if you want to be competitive, so 100% reliability is a must. I do slick up the actions and I have used Wolff springs with good effect. However, as I have talked to knowledgeable, competitive revo shooters I’ve come to discover that they change the geometry on the OEM mainsprings and alter the length of the strain screw to get the best possible trigger pull – none of them use aftermarket springs. They run their guns for tens of thousands of live rounds (and maybe 5X that dry firing) with zero FTF due to light strikes and I’ve never heard of them having to readjust the mainspring/ screw setting once it’s set. I’ve got a new to me 10-8 that looks beat all to hell but is crazy accurate that will make a great game gun once it gets tuned up. I’ll see if I can’t try adjusting the OEM mainspring and strain screw to get where I want it to be.

Stillwater:
There is a "plus or minus" tolerance that can be useful on the strain screw AFTER you have done everything else, which you failed to mention you had done. The strain screw is never the place to start.
The advice to go in a couple turns if you get light hits or feel a "wobble" in the spring as you cock the hammer is excellent.

Just to complete the record, I'm an IDPA match director and have shot revolver (mostly) in IDPA practically since it started.

I've seen several "modified" revolver mainsprings break in competition; bending a tempered spring can create a stress crack.

Many more than that have had serious failure to fire problems. One of our members spent months and hundreds of dollars "improving" his 610 and finally quit, and sold the gun cheap to another member. Next week the gun was back with a Wolff mainspring and a new strain screw, and ran flawlessly.

The last state level match I shot, I won my SSR class by 0.97 sec because the #2 guy had 4 failure to fire. My double action pull on the 686 is maybe 2# heavier than the "ultimate" light pull, but I have never had a failure to fire in competition.

YMMV.

I shoot USPSA with a 625-8. I run Wolff reduced power mainsprings that I narrow to further lighten them, then use the socket set screw (strain screw) as the final adjustment. The stock strain screws are usually not long enough for my spring setup and have a bad habit of mushrooming/deforming their tips.

Very interesting! OKFC05 – I’m a newbie to IDPA (any competitive shooting for that matter) – only 2 years and just 9 months with a revo. The best revo guys I shoot with - like you – don’t go for the lightest pull, but for the overall smoothness of the pull. I’m trying to find my “sweet spot” and I’m trying any suggestions that sound reasonable. Which leads me to your method tomcatt51 – I’ve never heard of anyone thinning their spring! I’ve got several Wolff reduced power springs to play with – how much do you thin them down?

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Originally posted by Stillwater788:
Which leads me to your method tomcatt51 – I’ve never heard of anyone thinning their spring! I’ve got several Wolff reduced power springs to play with – how much do you thin them down?

I don't thin them, semantics here, I narrow them, the wide dimension. They are widest where they engage the frame and narrower where the hooks engage the link or stirrup. I grind a slight curve into the sides of the spring. From nothing at the hooks, to approx .060" narrower, to nothing at the frame end. Center of the .060" narrower area being approx. 1/3 of the way down from the hooks and approx 1/2" long. I just grind (gently) the sides of the spring then polish. Harder to describe than do. You end up with a lghter rate spring and you preload (strain screw adjustment) slightly more. I think it gives a more consistent pull through the trigger stroke. Can you PM someone thru this forum?

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The stock strain screws are usually not long enough for my spring setup and have a bad habit of mushrooming/deforming their tips.

Tomcat,

You can get longer strain screws from S&W. I believe their K-frame screw has same thread as L-frame screw but it is 0.6" long. S&W sells them for a few pennies.

Mike

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Originally posted by Hoptob:
Tomcat,

You can get longer strain screws from S&W. I believe their K-frame screw has same thread as L-frame screw but it is 0.6" long. S&W sells them for a few pennies.

Mike

The socket set screws are hardened so the tips don't deform against the mainspring and by having no shouldered head allow more adjustment latitude. 290 LocTite keeps them from moving. You have to LocTite the stock strain screw anyway and it's a lousy alloy/heat treat with a soft tip that deforms so it's self shortening. No advantage I can see in favor of the stock screw.

That's a good tip. Thank you, Tomcat.