K-Frame Main & Rebound Spring Replacement Question

[roo_ster]

Howdy:

Wondering if there is a way to shim up a k-frame mainspring to get a smidge more power without having to move back to a factory power mainspring.

Wife's S&W M64 Snubby .38spl
Used Wolff's lightest ribbed mainspring and trigger return spring (Power Rib Shooters Pak Type 2 with reduced power mainspring) to produce the sweetest DA trigger pull. Strain screw fully screwed in.

Problems:
* Does not light off cheap magtec brand .38spl. At all.
* Had one misfire out of 50 with my wife's preferred load of Federal 148gr HBWC match.
* One misfire with 50 rounds of Rem FBI load.

I really care only about the Federal Match Gold 148gr HBWC load my wife likes in hte M64. Getting the Remington FBI load to light off 100% would be nice, too, but not required.

Mitigation Options:
* Shim/something I have not thought of.
* Move up to next-higher weight mainspring
* Try other brands of .38spl match 148gr HBWC

- S&B
- Fiocci
- Remington
- Black Hills
- Winchester
- Buffalo
- Old Western Scrounger​

FTR, this 2" bbl revo is accurate. Take your time and punch out the X-ring all day long. I would not mind having a half-dozen of these stashed about the place.

 

[rcmodel]

I have heard of people picking the anvil out of a fired primer and putting the cup on the end of the strain screw to increase spring flex preload slightly.

But, that stresses the spring further then it was designed for??

rc

 

[BBBBill]

What else has been done to the gun (trigger job)? How many rounds through it?

How much end shake? What is the headspace? How much firing pin protrusion? Lighter springs demand continuous attention to those to ensure reliability.

 

[MrBorland]

If you really do need just a smidgen more oompf, rcmodel's advice to place a shim between the mainspring and the tension screw usually works. I've used a piece of brass cut from a case, myself, though an anvil-less primers seems popular.

BTW, what are the current pull weights of the DA and SA triggers? The main and rebound springs should be balanced, and the relative trigger pull weights are a quick way to tell if they're way off or not. This is particularly important when one goes in and starts swapping springs for a lighter action.

If the rebound spring is too strong for the main, or the main too weak for the rebound, you could regain reliability without affecting your net pull weight if you get them balanced correctly.

 

[Jim Watson]

What I see is that the tip of the strain screw drops into the groove in the Wolff "Power Rib" mainspring which reduces the amount of preload even with the screw all the way in. Does Wolff take that into consideration? I don't think so, and I don't think a primer cup or other shim to level out that groove will overload the spring.

I have two such with primer cups under the tip of the screw to add preload. One installed by me, one by the gunsmith who did the work to start with.

That would be a zero cost, simple fix.
If you want something more substantial on a defense weapon, you could get the standard power Wolff, or just put the factory spring back in.
Be sure you have enough rebound spring to balance the mainspring.

The next time I get the urge to tinker with a .38, I will try the Wilson reduced power spring set which has a conventional flat spring. Or maybe just try the Skeeter Skelton grind.

 

[roo_ster]

rcmodel:

Thanks for the tip.

==========

What else has been done to the gun (trigger job)? How many rounds through it?

Bought it used. In its former life it belonged to a security guard. Zero mods to gun except grips I could detect. It is a DAO M64 with the DAO hammer, not a spur grind-off deal.

I would say no more than a thousand rounds, absolute tops. Wife and I have dry-fired it a few thousand times, though. Pretty much enough smooth out the parts interfaces as much as work-in wear is going to get you.

How much end shake? What is the headspace? How much firing pin protrusion? Lighter springs demand continuous attention to those to ensure reliability.

Did the Jim March revolver checkout on a cardboard box full of used snubby M64s. I picked the tightest one. [Wish I had bought another and a couple 4" M64s, too.]

No or minimum end shake. Eyeballed headspace, did not use feeler gauges when I selected it. Headspace is not minimum, but not excessive. It is no Bullseye tuned revolver, pretty much carried a lot with external dings and minimal internal wear.

Did not check FP protrusion.

======================

If you really do need just a smidgen more oompf, rcmodel's advice to place a shim between the mainspring and the tension screw usually works. I've used a piece of brass cut from a case, myself, though an anvil-less primers seems popular.

I really like the bit of brass from a case idea, brass being softer than the steels and kinda "grippy" under pressure.

BTW, what are the current pull weights of the DA and SA triggers? The main and rebound springs should be balanced, and the relative trigger pull weights are a quick way to tell if they're way off or not. This is particularly important when one goes in and starts swapping springs for a lighter action.

If the rebound spring is too strong for the main, or the main too weak for the rebound, you could regain reliability without affecting your net pull weight if you get them balanced correctly.

No SA option; DAO with DAO hammer with no SA sear.

Do not know pull weights, except in a relative fashion, as I have no trigger pull gauge. Probably ought to get one. DA before was OK. Smooth from wearing the bits together, but not as light as the trigger on my S&W M686 I had a smith work on some years back. I'd guess 10-12lbs before and 7-8lbs after, and somewhat lighter than my 686.

==============

What I see is that the tip of the strain screw drops into the groove in the Wolff "Power Rib" mainspring which reduces the amount of preload even with the screw all the way in. Does Wolff take that into consideration? I don't think so, and I don't think a primer cup or other shim to level out that groove will overload the spring.

I have two such with primer cups under the tip of the screw to add preload. One installed by me, one by the gunsmith who did the work to start with.

That would be a zero cost, simple fix.
If you want something more substantial on a defense weapon, you could get the standard power Wolff, or just put the factory spring back in.
Be sure you have enough rebound spring to balance the mainspring.

The next time I get the urge to tinker with a .38, I will try the Wilson reduced power spring set which has a conventional flat spring. Or maybe just try the Skeeter Skelton grind.

Hot dog, I think you're right.

I have had the plate off before to clean my S&Ws, but had not done a trigger replacement. Watching the Jerry Miculek youtube video, I expected to uh, strain, a bit more as I turned in the screw. And I expected more a difference in trigger pull as I turned it in. It feels like the strain screw is more holding the mainspring in place than loading it.

I think I will cut a bit of brass and fit it between the two and then check ignition.

 

[rcmodel]

Use a spent primer cup.

It is also brass under the silver nickel plating.

And unlike a brass shim you make yourself?
Once it's over the end of the screw and tightened, it can never fall off.

rc

 

[roo_ster]

Use a spent primer cup.

It is also brass under the silver nickel plating.

And unlike a brass shim you make yourself?
Once it's over the end of the screw and tightened, it can never fall off.

rc

Didn't know that. Thought it was steel or something else.

Too late for the moment, as I cut up a .38spl case and wrapped two thicknesses around the mainspring before driving the strain screw home. Will try it out and see if it ignites Fed Match HBWC.

Even if it does, I still may go up to the heavier Wolff mainspring.

 

[rcmodel]

That right there sounds like a bad idea.

Two wraps of .38 Special brass is likely to be uneven, and even tapered more on one side to the other, depending on which part of the case it came from.

You may put a kink in your get-along spring!!

The beauty of using a primer cup is that it becomes captive on the strain screw.
But more importantly, is going to be uniformly Flat against the spring.

rc

 

[ljnowell]

That right there sounds like a bad idea.

Two wraps of .38 Special brass is likely to be uneven, and even tapered more on one side to the other, depending on which part of the case it came from.

You may put a kink in your get-along spring!!

The beauty of using a primer cup is that it becomes captive on the strain screw.
But more importantly, is going to be uniformly Flat against the spring.

rc

I have used a primer cup on several l frames that have wolf springs.

 

[MrBorland]

There's not a whole lot of room between the grips and the mainspring, so wrapping the mainspring with a brass shim might also cause some binding. From the top of a .38spl or .357mag case (where it's more uniform), I just cut a square-ish piece of brass that's the same width as the mainspring. I've not yet had one that moved when the strain screw's tightened down.

The anvil-less primer's a good alternative, though I'm thinking the brass shim might (or might not) be a wee bit thicker and offer a wee bit more oompf to the DA strike.

Even if it does, I still may go up to the heavier Wolff mainspring.

If you do go with a heavier mainspring, and find it's now a bit too heavy for your taste, you might be tempted to adjust the tension down by unscrewing the strain screw a bit. Don't do it! It's not an adjustment screw. It's meant to be tightened down (a little blue loctite on the threads wouldn't be a bad idea). A tension screw that's not fully set will give you all kinds of headaches.

 

[roo_ster]

The more i look at it the more i think i need to go the primer cup route or--more likely--install the stouter mainspring.

As installed the brass shims work fine and do not shift or bind on my 100 or so dry fires. And i suppose with them being held in place by the strain scew they really are not going anywhere.

If it were a target gun i would declare victory and move on. But since it is my wifes car gun i think a clever hack is not going to cut the muster.

As for the strain screw the miculek video suggests using it to tune the trigger pull to get the lightest pull while getting reliable ignition. And if it pokes out the front of the grip to grind the pokey end against the mainspring down by the amount exposed on the front. It looks like that is a debatable approach.

 

[MrBorland]

As for the strain screw the miculek video suggests using it to tune the trigger pull to get the lightest pull while getting reliable ignition. And if it pokes out the front of the grip to grind the pokey end against the mainspring down by the amount exposed on the front. It looks like that is a debatable approach.

Very debatable. Believe it or not, JM's word shouldn't always be taken as Gospel. Jerry is known to do a number of things that're unique and go against conventional wisdom. Nearly every other knowledgeable revolver shooter and/or gunsmith will tell you to keep the screw tightened down, and adjust pull weight via the springs.

BTW, JM doesn't lighten the springs on his guns, so it's possible he's never had to personally deal with the downstream consequences of his own advice.

 

[roo_ster]

BTW, JM doesn't lighten the springs on his guns, so it's possible he's never had to personally deal with the downstream consequences of his own advice.

What sort of consequences? Does the mainspring come unmoored from its little shelf at the bottom of the frame or something similar where it mates with the hammer?

 

[Wireman]

MrBorland is telling you the truth. So far, the discussion is headed down the dead end of messing with the mainspring, assuming that you can dial in the energy of the primer strike and trigger pull weight at the same time. Let's do some reality checks first.

The most critical thing about an S&W mainspring is its installed shape. The strain screw is NOT a convenient trigger pull adjustment screw. It is there to set the spring to the correct shape so that it works smoothly without bumping into the rebound slide or binding at the stirrup. Once the correct shape is obtained, the tip of the strain screw - properly done - is ground so the shoulder of the strain screw bottoms out at that point. All the way in is the correct way to install the strain screw.

In my opinion it is wise to stay with the factory weight spring. The single action pull is not that sensitive to the weight of the mainspring anyway. I would also have a good look at your hammer, firing pin, firing pin spring, and hammer block to make sure they are all in good shape.

Wire

 

[Wireman]

The consequences are as stated. Backing out the strain screw changes the shape of the spring and can result in the spring binding against the rebound slide and/or pinching the hammer stirrup. Both of these symptoms can put you out of business but minimally will result in a very ugly trigger pull and erratic hammer cocking, not to mention inadequate primer strike energy.

I'm not saying that the operation can't be improved, but take it to someone with lots of S&W experience. Not everything can be fixed by changing springs.

Wire

 

[MrBorland]

What sort of consequences?

If it's not fully tightened, the strain screw has a tendency to unscrew even more and negatively affect reliability. It's irritating when it happens at the range, but in a serious situation, it's bad ju ju indeed.

In my opinion it is wise to stay with the factory weight spring. The single action pull is not that sensitive to the weight of the mainspring anyway. I would also have a good look at your hammer, firing pin, firing pin spring, and hammer block to make sure they are all in good shape.

If this is a SD gun, I'd agree that the wisest option are stock springs. I also agree that if springs are to be swapped out for a lighter pull, it's important to be sure everything else is in spec. Parts that aren't properly centered, true, plumb, straight, smooth, clean, and lubed can cause reliability issues when the springs are lightened.

If you do want a lighter-than-stock action, and the gun is for SD, it's important that 1) the innards are all in spec, clean and lubed, 2) you have reasonable expectations regarding the balance between pull weight and reliability, 3) the main and rebound springs are appropriately balanced, and 4) you've thoroughly tested it's reliability with the SD ammo you intent to rely on. If one's not really familiar with the workings of a S&W revolver, it's best to have a good 'smith do the work. A good action job is more than just swapping springs.

 

[roo_ster]

Thanks for the heads-up. Guess I won;t be using the strain screw to adjust trigger pull.

 

[Jim Watson]

Ron Power used to sell a kit with a set screw to keep the strain screw from walking around. Which it can do even if turned all the way in. Had to be drilled and tapped for, of course.

Teddy Jacobson had a headless strain screw with flats ground on the tip so it would detent in the groove of the Wolff Power Rib mainspring. It would click-stop in quarter turn adjustments.

I have seen several remarks on the S&W board about soft strain screws that peened on the end and reduced tension. Best replaced with a hardened set screw.