Intermittent ftf for k frame

[Cazmont]

I'm currently stymied in my attempt to cure a 1950 K38 Target Masterpiece ftf issue. I bought it from a lgs with this problem. The gun has had very little use quite possibly because of it. It has strong strikes but for every 10 rounds or so it will produce light strikes which won't fire a standard primer. The problem is not specific to any cylinder chamber.
I've been following Jerry K's shop manual very carefully and so far have reduced cylinder end play to practically zero.
The trigger and hammer have both been centered with shims and their respective bosses have been trued and polished. All friction areas on parts and frame have been deburred and polished.
The factory springs have remained in place. There is no drag noticeable. The da pressure is very smooth at 9 to 10 lbs. Sa break is crisp at 3 lbs. per Jerry K I have chamfered the front edge of the da sear. Push off is very strong. But for the intermittent ignition the action is textbook and a pleasure to shoot. Something must occasionally be interfering with the otherwise drag free hammer fall but I've run out of ideas of what it could be. I would be pleased to entertain your suggestions.

 

[VikingDude]

Are you using the same ammo or have you tried different brands?
Is the strain screw screwed all the way in?

 

[D Brown]

Something is hitting where it shouldn't. I can think of no other explanation.

My suggestion; coat every surface on every part on the inside of the action with Dykem Dykem 80396 $6.38 Layout Fluid, Steel Red(TM), 4 oz | Zoro.com (Red or Blue, your choice) work the action through about 30 single action trigger pulls, then take out the parts and look for the indications of parts interference.

Best of luck.

 

[D Brown]

By the way, do you really have "push off"? That's a really bad thing, and should be addressed immediately.

 

[ken158]

You either do or do not have push off... what do you mean by strong? How are you measuring trigger weight? Sounds like your strain screw is either not tight or maybe too short.

 

[armorer951]

This may not be the problem at all, but double check the very top of the canopy in the frame where the hammer nose passes under the opening. This clearance can be very close, so close that the hammer nose can strike the top of the tunnel opening as it attempts to pass under. You'll have to take a close look there with a magnifier at the location to see if there is evidence of contact. There will be a "shiny" contact spot on the frame.

If this is the case, the intermittent contact between the frame and the hammer nose can damage or break the tip of the nose, or cause light strikes every so often.....for no apparent reason to the shooter.

You can replicate this errant contact, if it is present, by holding down on the back of the hammer nose with your finger as you slowly let the hammer down.

Not a common issue, and this may not be the problem, but it is worth checking. The fix is to install a hammer nose spring.

 

[oldgranddad]

Thanks armour951 (post #6). Learned something new today....and how to fix it.

 

[SWOOP01]

I noticed that you didn't say anything about the strain screw. I had a friend give me his 66-1 to look at because he was getting light primer strike every once and awhile, then it was all of the time. When I took the grips off, the strain screw was backed off. The previous owner did this to lighten the trigger pull. I replaced the main spring and snugged the strain screw. Works like a dream.

 

[Targets Guy]

My FTF issue with my 1951 K-38 was also the strain screw. Once tightened the FTF's went away.

 

[Cazmont]

Thanks for the some great feedback and info. To answer some questions brought up in replies. Yes the strain screw is all in on a factory main spring that hasn't seen much use indicated by the lack of overall wear and bore condition, lack of residue or cleaning marks. Again, other than the intermittent ftf's the spent primers show strong impact marks. I use a scale for the weights I've stated. Push off is over 15lbs. and very safe. Thankyou for your concern. I agree, something is intermittently hitting while the hammer drops. I like the dicum concept. I used it for eliminating hammer and trigger drag but didn't carry through to the internal action surfaces...yet! I've had an eye on the canopy, great tip! The clearance is very tight. I've used 2 hammers, one the original service and also a later target. The noses are quite different. The later got a contemporary extended nose that was relieved to the originals contour. The original doesn't use a spring as the targets does. No difference in ftf! I'll look at the clearance again with good light and magnification.
Thanks very much to all for the tips, info and support. When hopefully I have made progress I'll report back as I think (hope) this is an uncommon issue.

 

[Protocall_Design]

Check to make sure the hammer isn't hitting the rebound slide when it's all the way down. You can check this by pulling the trigger, holding the hammer down against the frame, then move the trigger front to back and see if there is a bit of movement between all the way back and where it goes forward to rebound the hammer.

If the hammer touches the rebound slide, it will dampen the forward momentum of the hammer.

 

[WR Moore]

About that strain screw......I've noticed that, at least lately, a lot of variation in the length of new, factory strain screws. You might check the length against a new one.

Also, you don't mention if the misfires are in SA or DA. If you're trying to be extra precise in squeezing off a DA shot, you may not pin the trigger all the way back. If that happens, the secondary DA sear on the hammer can contact the trigger on the way down and rob energy from the hammer.

 

[smithra_66]

2 things:

1. Check for a shortened strain screw. A lot of people used to do that. Even though it's all the way in tight, it still may not be long enough.

2. Check for bends in the mainspring. That was also part of the home trigger job back in the 50s and 60s. Even a tiny bend will make a difference.

I've owned a lot of used guns that had various trigger work done that resulted in spotty ignition.

I'd recommend buying a replacement mainspring and strain screw and seeing if the problem goes away.

Also: on a 19-3 that I have, the previous owner(s) must have lightened the mainspring to the point that it was right on the edge. It is spotty with CCI primers but 100% with Federal or Winchester. CCI's are known to be hard.

You may want to try softer primers if you reload.

Good luck.

 

[lefty_jake]

Again, other than the intermittent ftf's the spent primers show strong impact marks.

The primer imprint on a fired round does not accurately show the strength of the hammer strike. When a round fires, the brass and the primer are pressed back against the firing pin with the full pressure of the powder charge. So the primer strikes on fired rounds usually look good, but that appearance can be deceptive.

It is possible that something is causing intermittent variations in the hammer strike. But I have more often seen cases where the hammer strike was consistent, but had marginal energy. So the primer imprint on the misfired rounds actually showed what the hammer did every time. And that small strike was enough to set off most of the primers, but not quite all of them.

 

[VikingDude]

Push off is over 15lbs. and very safe.

Please clarify by what you mean push-off and how you measured it. Push-off is something that you really don't want in your revolver and is a sign that someone have been messing with the SA sear engagement.

 

[sheppard]

There should be no push off.

Are you using your own reloads or factory ammo. If reloads some primers may not be seated properly causing a failure to fire.

 

[Cazmont]

Wow, what great comments from this thread. So far I've only had a chance to follow up with Armorer951's suggestion to check the frame/hammer canopy for sign of contact. Sure nuff, as shown on images, such as they are, shiny spots from contact. I will recenter the hammer with additional shim magic and relieve the hammer nose where it is proud of the hammer. Thanks Armorer951!
Thanks to lefty jake for reminding me that the fired case recoils into the firing pin re imprinting impact dent on the primer. Good call!
Thanks to Protocall Design for directing attention to proper rebound/hammer seat fitting to ensure a clean hammer strike. I will!
Haven't had a chance to pop the hood to check all action contact areas with dicum but I will soon. Thanks for the suggestion D Brown. Anything untoward I find I'll document and post.
I'll have to be careful to cover as much troubleshooting as possible before my next range session as I'm down to 40 rounds of .38s with no small pistol primers in sight. Sigh...
Now...let's address the push off issue. I'm referencing Jerry Kuhnhausen's S&W Revolver shop manual, page 59. My understanding is that push off is always present. Why? If you push a cocked in sa hammer hard enough it will always slip the sear and fall. Yes?
You want the push off to only occur after a predetermined minimum amount of pressure is applied. Above that amount(Mr.K mentions 16 and 15 lbs. on that page,) you're good. Below that minimum amount is considered unsafe.

 

[Cazmont]

Here are a couple of images of the old timer I'm fussing with. I'm a wannabe Bullseye shooter and at 25 and 50 yds. this old boy works great when he works at all. Yeah, I know I should be focussing on.45 and .22 but...

 

[armorer951]

You should not push the hammer with increasing pressure until push off occurs. This (using too much force) can cause damage to hammer notch or the bevel on the trigger. This is one place in the shop manual where J. Kuhnhausen is wrong. That's way too much forward pressure (16 pounds) on the hammer for testing this problem. If you are unable to determine what 'nominal' pressure is, welcome to the club. This test should be done with a trigger pull gauge hooked behind the hammer spur, so you are able to quantify the force used in a reliable way.......not a bathroom scale. For crying out loud, a bathroom scale, is he kidding? Mr. K missed the boat on this one.

The armorer's manual says to use "nominal pressure" when testing for this issue, which to me would be a bit more than 'normal' trigger pull weight, which is 3-4 pounds. I would keep the push weight for testing at 10 pounds or less. More pressure and you may cause damage. By testing you are trying to determine the integrity of the trigger bevel and hammer SA notch. Using more than nominal pressure can literally damage the parts and surfaces you are testing.

 

[Cazmont]

Good to know. Where can I get an armorer's manual? That must be handy.