I recently acquired a used 66-5. It had some issues....and among those issues I believe that the previous owner trimmed the strain screw. By any chance, does anyone know what length it should be? If not, is there a way to measure the mainspring to ensure the new screw is short/long enough?

Not sure what the length is supposed to be, but are you having problems with light primer strikes?

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John

I had a few light hits today using a Wilson Combat mainspring and 14lb return spring. I have a stock mainspring on hand, but I assume it will not return things to factory spec with a shortened screw. Just trying to properly diagnose rather than throw parts at it. I know I could just put in an extended firing pin but I have read that you can't dry fire. I would prefer to be able to dry fire.

Was the light strikes issue apparent before replacing parts? Order a new screw and test. Screw is not expensive and there should be several sources of acquiring it including directly from s&w.

I will answer you to the best of my ability.

I'm NOT referring to the original Factory length here but the length I have finalized them to. Factory Strain Screw lengths are 100% reliable but to me they are just way to excessive and make the pull too heavy. There IS a happy medium IMHO! Off the top of my head I do not know the Factory length because I have never measured them and at this point I doubt I have any revolvers that have the factory length screw still there as they are usually too long for a smooth action as I like it - especially the newer ones. It all depends on several things and just so you know I never even bother to measure it. I go strictly by "feel" and performance but if one was to measure it, a Calipers reading in thousandths of an inch would need to be the tool to do so, NOT a rule.

Each Revolver is a bit different due to the specific springs in the gun, how old and worn in the gun actually is, what spring weights they installed, how your Revolver's specifications are (end-shake & tolerances) and exactly how far your Hammer retracts in DA mode.

In general, I know by feel just where to set the length to but that is very hard to convey on-line. Your best bet is to buy a few extra screws and experiment. They are cheap enough to do so and you can custom tune them for the smoothest or lightest pull you can but still have 100% reliability & good primer hits. I always use CCI primers as a "litmus test" as they are generally known to be the hardest of the primer brands. If your Revolver functions 100% with CCI's then you should be GTG. Once you have done this to a few Revolvers, you will sort of get the feel and know in short order within a few thousandths where to stop - without even measuring. If you do go to far and your action is not reliably setting off all primers, just toss the screw and start over.

When I work on Strain Screws I also round off the end just as they are form the Factory, and sometimes even re-blue them making the job look professionally done. NEVER just leave a Strain Screw full length and backed out. This leaves the screw loose and will work itself out under recoil. Always make sure it is tightened all the way. Loosening it under fire CAN be TEMPORARILY done JUST so you get a sense of how much to safely remove. Once you get that sense, remove your metal, reshape tip and try. If OK, you can re-blue it if you like. I also suggest leaving the originals alone and place them in the box with a note that they are the Factory original Strain Screw and do your thing on a replacement screw. AGAIN....... buy a handful so you never have to scrounge for one if you go too far!

If you simply want 100% reliability and are amicable to s stiff pull, just install a new Factory screw and you are GTG. If you want both reliability and performance, the procedures above are the way to go IMHO.

ADDED: I have also noticed over the years that Factory Strain Screws are NOT all exactly the same from year to year and from S&Ws supplier to supplier. They have varied a bit and so has the shape and length to some extent. Another reason to fit it to your individual gun.

Don't. If your 66-5 has the original firing pin it will be steel (not titanium) and be .495" long. It's just fine. There is a small advantage to replacing it with the newer titanium S&W firing pin but the advantage is SMALL and will only show up if you're setting your gun up with a really light DA pull.

There is NO advantage in going to a C&S (or other) extended firing pin if your S&W firing pin measures .492" - .495".

The strain screw should measure .605" OAL and .480" under the head for sq. butt and .505" OAL and .380" for rd. butt

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Stang1911

Srgvaz, I'm not sure whether it was a pre existing condition. I bought the 66 off of gunbroker, and these issues happend at the first range test.

Chief38-awesome write up, thanks. Tomcatt51, duly noted. Stang1911- thanks so much...I measured mine and it has indeed been shortened. THANKS TO EVERYONE FOR THEIR SUGGESTIONS AND HELP!!!!!

I experienced some light strikes today (about 1 every 10 rounds) with my 66-2 using CCI no. 500 small pistol primers. I have had some light strikes in the past, but never so consistently and I noticed that the screw had backed out a bit, so I didn't think anything of it at the time. The strain screw now measures .570" overall and .436" under the head. Adding a primer cup brings it to just a few thou under spec and should do the trick on my next trip to the range if indeed the short screw length is the problem. The tip of the screw appears to be very slightly mushroomed. Will they flatten on their own through wear and tear over time, or is the shorter length more likely due to a previous owner shortening the screw length?

Strain screws do wear over time and the tip will mushroom enough to require using a needle file on the tip to remove it from the frame without damaging the threads in the frame. The Stainless steel screws seem to be worse for this than the blued strain screws so I've started replacing every stainless strain screw in my revolvers with blued ones when I take a particular gun out of the safe to do a bit of shooting with it. Fortunately I like to shoot with rubber grips that cover the front strap so this isn't visible but even if I were using a wood grip with an open front strap I would still use the blued screws.

Do you know of a source for either blued or stainless square butt strain screws that isn't ridiculously overpriced? The ones I've seen online are like $5 or more per screw, plus shipping in some cases. I wonder if my local gunsmith might keep some on hand...

Midway sells them for $1.99 each and Brownells for around $3 bucks.

Smith & Wesson sells them. 1 (800) 331-0852 extension 2.

Stu

He want's square butt not round!

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Stang1911

S&W is $1.58 for Round or Square Butt, Blue or Nickel or Stainless or whatever they have in stock at any given time.

Stu

+1, this is correct and the reason I will NEVER grind the ends off strain screws. Add a little wear and you get light strikes. If I want to reduce main spring force I just bend the mainspring which is 100% reversible.

bountyhunter, how about some info on main spring bending??

thanks,

Stu

The simplest thing to do would be to:
1. Put the factory mainspring back in the gun, assuming that it has not been altered.
2. Get some 0.010" (approx.) shim stock and cut a piece about 1/8" square. Automotive style feeler gauges are good sources for shim stock. Slip this shim between the tip of the screw and the mainspring. Tighten the screw all the way.
3. If this resolves the light strikes, you are done. If not, try a thicker shim.

Thanks! I was under the impression that S&W had stopped making strain screws for square butts. As long as the primer cup does the trick, a dedicated shim won't be necessary, nor an OEM strain screw for that matter. But I would eventually like to put the proper screw in there "just because".

Also, thanks Andy!, the workaround with the shim is also good information for people who don't reload any may not have ready access to a used primer.

I do a mandrel bend over a screwdriver handle with my panavise to make sure it's a progressive curve. I center the point of bend about 1/3 of the way up from the base (widest part) of the spring. Sometimes I lay the spring on a piece of paper and trace the curve so I can check after bending. To reduce spring force you just increase the curve. To get stronger spring, you flatten it.

You're welcome. But I don't think of the shim as a workaround.
A primer cup is...how thick? Do you have a selection of primer cups of varied thicknesses, so you can get the right resistance? Seems to me that the primer cup is what you do if you can't make a shim.

SeamasterSig, two weeks ago I received an order from S&W with 10 square butt and 4 round butt strain screws. They still have them.

Stu

I have a pre-model 15 4" Blued that had issues.
I called S&W Customer Service and ordered a new mainspring and screw.
On arrival I compared the replacements to new modern ones and found there is a very slight differences in length between the replacements and the on hand new ones.
I had been advised that there were the same no matter what the vintage, but there is enough to make a slight differences.
The ones for my older revolver had a different stock number than the more modern ones per S&W rep.

To me they are both workaround solutions, i.e. as opposed to replacement with the original part. But I understand what you mean. A selection of shims would certainly allow for more precise control of the overall screw length. In this case, the first primer I grabbed brought the length to within a few thousandths of the size indicated above in post #7, so I should be good to go for the time being. The trigger is back to "heavy but smooth".

That's strange, I just got off the phone with S&W and the service rep I spoke to looked it up and swore the "old school" square butt screw was out of production. Do you have a part number that would work with an early mid-80s model 66?

Before doing any strain screw work, you should first check the double action trigger pull using a scale or weights. If the trigger pull is within factory specs of about 12 pounds, the screw should be left alone. If heavier than spec, you may take it down a bit, constantly checking trigger pull.

Some people think that screw is for action adjustment. Once loose, it unravels on its own and eventually the gun no longer fires. An SD handgun should always be tuned for reliability over accuracy.

SeamasterSig, the screws I ordered were blued with 05035B000 for the RB and 05064B000 for the SB since almost all of my guns are blued. What is an "old school" strain screw anyway???

Stu

Thanks, Stu! I called up S&W again today and gave them the part no. for the SB screw and the rep said they have plenty in stock and offered to send me a couple free of charge. Not a bad deal!

Since we're talking strain screw length when an un-altered screw is fully tightened should the screw head be flush with the back strap of the grip .
I have a model 10 where the screw head sticks out some even though it's fully tightened . It came that way when the gun was bought new back in the 80's . Doesn't look very good .

I believe that the older version of these screws protruded like you said, The wooden grips were relieved, if necessary, to clear the screw head. Later versions shortened the screw head so it would be flush with the grip frame.

S&W is sending one for my 13-2 NO CHARGE !!!

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ΜΟΛΩΝ ΛΑΒΕ

The problem is that once you install the new factory strain screw, you will find out why the previous owner had shortened the original one.

The strain screws I recently received from S&W measure 0.6045 inch, which makes the trigger a wee bit stiff But it's still very smooth, so I can live with it.

Jimmy's solution:

Call Smith and Wesson Customer Service at 1-800-331-0852.
Give the Rep the model number and serial number and request a mainspring spring and strain screw. Despite popular belief there is a minor differences in the lengths between older and newer models.

Was having light strikes with my 66-2. Strain screw look like it was never touched but I ordered a new one anyway. It was 8 thousand longer than the one in the gun. I didn't think it was enough to make a difference. But it was no more light strikes.