Strain screw length

[BC38]

I picked up this nice old M&P from the "roaring 20's"

http://smith-wessonforum.com/s-w-ha...613709-can-original-finish.html#post140704935

It had a pushoff issue which was easily resolved, but when I took it to the range the other day it was producing a light strike or two almost every cylinder full.

I'm trying to figure out if Bubba also thinned (or replaced) the mainspring, or possibly shortened the strain screw.

I took the strain screw out and put my caliper on it the length appears to be right at 0.63" long, and it is flat on the end.

Can anyone tell me if that is the correct length, or has it been shortened? Should it be flat on the end or pointed?

Of course I could always just try the old "primer cup over the tip of the screw" trick to see if that helps, but if the screw is stock length, then I'd prefer to fix it right by replacing the mainspring.

 

[armorer951]

Strain screw length sounds correct. The screw should have a very slight crown on the end. (slightly rounded)

What about endshake/headspace problems? Too much headspace can cause reliability issues. Shooting debris in the cylinder assembly or under the extractor?

 

[BC38]

Strain screw length sounds correct. The screw should have a very slight crown on the end. (slightly rounded)

What about endshake/headspace problems? Too much headspace can cause reliability issues. Shooting debris in the cylinder assembly or under the extractor?

Nope. This one is very nearly pristine - other than the Bubba "trigger job". I'm thinking it probably has a reduced power main spring in it.

So now I guess I'm in the market for a stock mainspring.

 

[Seven High]

I would suggest that you tear down the weapon and then carefully clean all the internal parts. You can use carb. cleaner to clean it. Carb cleaner is not a lubricant. The internals will need to be relubricated.

 

[BC38]

I would suggest that you tear down the weapon and then carefully clean all the internal parts. You can use carb. cleaner to clean it. Carb cleaner is not a lubricant. The internals will need to be relubricated.

Thanks for the suggestion - that's already been done. It was clean as a whistle inside - not gunked up at all. But good thought & suggestion.

 

[GyMac]

Try putting a fired primer under the screw while you get a new mainspring. It will increase the weight and may temporarily fix the problem.

 

[ken158]

The strain screw should not be flat on the end but slightly rounded. If it is flat that indicates someone likely filed it.

 

[BC38]

Try putting a fired primer under the screw while you get a new mainspring. It will increase the weight and may temporarily fix the problem.

Yup, that is what I'm going to try.

The strain screw should not be flat on the end but slightly rounded. If it is flat that indicates someone likely filed it.

Yeah, it isn't perfectly flat, just not pointy. It is very slightly domed. Since others have confirmed the length as correct I'm thinking it has a reduced power spring.

 

[zonker5]

Is the main spring flat or does it have a ridge? As I recall the factory spring is flat and the Wolff springs are ridged.

 

[BC38]

Is the main spring flat or does it have a ridge? As I recall the factory spring is flat and the Wolff springs are ridged.

I'm pretty sure it is flat, but I'll have to take another look to confirm that.

 

[stu1ritter]

Why don't you weigh the mainspring weight and see what you have. See below.

Stu

 

[Protocall_Design]

I agree with stu1ritter. If you're not measuring, you're just guessing and hoping something works. You can do this measurement with a trigger pull gage and know where you're at above or below the desired result, and what effect any changes you made had on the hammer fall.

You have a way of getting the same hammer fall if you change springs or strain screws. You have a way to figure out if it's the hammer fall or something else.

Different guns need a different minimum hammer fall to be reliable on a given brand of primer, or all brands of primer. If you record what that number on the scale is for each gun, you can always check back if a problem develops. You can use this information to get the lightest reliable trigger pull instead of just going overboard to make sure it's enough.

 

[BC38]

Why don't you weigh the mainspring weight and see what you have. See below.

Stu

Why? A couple of reasons:
1) I haven't been to Armorer's School, so I wasn't even familiar with this procedure (thanks for sharing it BTW) and
2) Since I haven't been to Armorer's School, I don't have a trigger weight set lying around. Got one I could borrow?

Seriously though, I have something I may be able to make work - see below.

I agree with stu1ritter. If you're not measuring, you're just guessing and hoping something works. You can do this measurement with a trigger pull gage and know where you're at above or below the desired result, and what effect any changes you made had on the hammer fall.

You have a way of getting the same hammer fall if you change springs or strain screws. You have a way to figure out if it's the hammer fall or something else.

Different guns need a different minimum hammer fall to be reliable on a given brand of primer, or all brands of primer. If you record what that number on the scale is for each gun, you can always check back if a problem develops. You can use this information to get the lightest reliable trigger pull instead of just going overboard to make sure it's enough.

LOL, here is a picture of my "trigger gauge" - it's a luggage scale.

But it will probably work. I'll give it a try. Thanks for the suggestion guys.

 

[stu1ritter]

BC38, I should take a photo but my trigger weight set is a little bucket, a bent stiff wire and some lead ingots for course adjustment and some cast bullets for fine adjustment on a postage scale.

Stu

 

[BC38]

BC38, I should take a photo but my trigger weight set is a little bucket, a bent stiff wire and some lead ingots for course adjustment and some cast bullets for fine adjustment on a postage scale.

Stu

LOL, well, then I recon that my improvised version with the luggage scale is down right sophisticated.

I appreciate you posting the info about how to measure the spring weight. That is very helpful info - and a process I was completely unaware of prior to you sharing it.

 

[BC38]

After I got home today I pulled the grips and looked at the main spring. It isn't ridged, so I'm thinking it probably isn't aftermarket - or at least not a Wolf spring.

Next I used my luggage scale to test the spring weight, and it measured right around 2.5 pounds average. Obviously just a little too light. I'm thinking that in addition to taking a file to the single action notch on the hammer, Bubba also took his file to the spring and thinned it down, then maybe cold-blued it.

Anyway, with a small primer cup on the end of the strain screw the weight came up to right about 3 pounds. So I put the small primer cup inside a large primer cup, and put them both on the strain screw. That brought the weight up to almost 3.5 pounds.

I'll be taking it to the range Friday to confirm consistent solid strikes and ignition. I expect the problem to be solved, and then my next purchase will be a replacement spring.

Thanks for the advice and info guys.

 

[BC38]

Well, I took it out Friday and it was better. Only had one FTF in single action, but a few more than that in double action.
I'm going to replace the mainspring.
Does an old M&P from the late 1920's like this one use the same mainspring as newer versions like M10's and such? Newer mainsprings are easier to find and less expensive, not to mention that brand new ones are available, whereas finding a brand new one that is nearly 100 years old is going to be a virtual impossibility.

 

[stu1ritter]

I've used new springs in 1920's guns without any problem. The hooks on the new springs are not as nicely made as the olde springs (what else is new) but they work well.

Stu

 

[Protocall_Design]

You can also bend the spring you have straight and it will have more preload when you tighten the strain screw. A properly tempered spring can be adjusted like this and will hold it's new shape. OEM S&W mainsprings are very good quality. I've reshaped dozens of them over the years, even a lot more than just a bit of straightening.

 

[BC38]

I also compared the trigger pull on this one to another lightly newer M&P in the safe. It is still noticeably lighter,

I've used new springs in 1920's guns without any problem. The hooks on the new springs are not as nicely made as the olde springs (what else is new) but they work well.

Stu

EXCELLENT! That is just what I wanted to hear - THANKS!

You can also bend the spring you have straight and it will have more preload when you tighten the strain screw. A properly tempered spring can be adjusted like this and will hold it's new shape. OEM S&W mainsprings are very good quality. I've reshaped dozens of them over the years, even a lot more than just a bit of straightening.

Yeah the only problem with that idea is that the spring is already flat. To preload it by bending it would require it to have a reverse-arch. I'm afraid this one is just so light that no amount of preloading or strain adjustment is going to make it reliable.

I'll be replacing the spring. At this point, I'm pretty well certain that a $10-$15 dollar new spring will be the thing that will fix the problem once and for all.