M&P 340 Preconditioning

[OlongJohnson]

I said a long time ago I’d write up my adventures in J frame rehabilitation. I finally got around to it this week.

Preamble:
I am not a gunsmith. I am not a gunsmith instructor. I am not consulting as an engineer. This is just my experience, highlights of what I did to my gun, and some interesting things I learned along the way. I have acquired my own tools, techniques and experience for working on guns from a variety of sources, including my decades of working on and fabricating other kinds of machines. I hope you enjoy seeing some of what may be in your S&W. If you choose to mess with your own stuff, that’s on you.


Further preamble, since we like to talk about tools here:
There are some basics that I’d be lost without.

I’ll start by saying I do all this work by hand. No power rotary tools. I have them, but I don’t use them for this kind of work.

I use stones, not sandpaper, unless I’m working on a compound curve. Most of the surfaces we’re interested in conditioning are, if not flat, at least linear in some axis (simple curves). Sandpaper is only linear if backed up with (or carefully glued to) something flat. It’s rare that that can work out better than a stone, which is rigid by itself. A few links:

• https://www.brownells.com/gunsmith-t...618-24084.aspx
• https://www.brownells.com/gunsmith-t...627-24152.aspx
• https://www.brownells.com/gunsmith-t...-625-2574.aspx

One might notice that those are all Norton products:

• https://www.nortonabrasives.com/sga-...-Norton200.pdf

You can always look up the part numbers in the Norton catalog and go searching for them through your favorite sources for industrial supply. I started with the "gunsmith" set of Arkansas stones, but almost immediately duplicated the shapes in India stones. Since then, I've supplemented here and there, and replaced the ones that I stupidly dropped on hard surfaces. They're pretty fragile, so handle them with care.

When stoning parts, I use baby oil. Did a bunch of research on what oil to use, and finally decided the one that’s safely applied to babies’ skin and is cheap at Walmart was probably a good call. It’s worked well for me so far.

I have an assortment of small files, but rarely use them due to the roughness of a filed surface vs. a stoned one. They are capable of getting into tighter nooks and crannies than the stones, and are also good in the very rare situation that there’s substantial material to be removed (i.e., multiple thousandths of an inch from a surface larger than a burr). But I don’t use them often. Most of the time, it works out that slower is safer and less is more. A quality, fine-toothed, small pillar file is a really good thing when it’s needed and a good set of needle files is a good way to start. Good quality small files are surprisingly expensive, but poor quality files are not worth the money.

I don’t polish anything with felt. Felt smooths a surface but leaves it lumpy until waaay too much material has been removed. Stones level a surface, knocking down the high points without even touching the low points, leaving it flat and smooth. I find the surface left by the Arkansas stones is smooth enough for the results I like. I also have a hypothesis that, much like the cylinder walls of an engine, a little bit of surface texture for oil to live in is probably a good thing.

I solvent clean everything before final lube and assembly. When I wipe off a part that’s been stoned as well as I can so it looks and feels clean, dropping it in solvent and swirling it still raises a cloud of fine particulates of stoning swarf. The swarf is a mixture of steel and abrasive material from the stones themselves, which fractures microscopically as they are used. This material would make a lapping compound that would wear the gun at an accelerated rate if not cleaned off. So everybody gets The Dip.

One of the most important tools I use is a 10x loupe. I have pretty good eyes for my age, but the loupe makes a massive difference. It lets me see what’s really going on with an edge, a nick, a burr, the condition of a surface. I can do exactly what and as much as is needed, and no more. Now that I’ve used it for a while, I wouldn’t do this stuff without it.

I have three gauge pin sets covering the range from 0.010 to 0.500 inch. The sets all together cost several hundred dollars (I shopped around and took advantage of sales), but I find them to be worth it. Being able to figure out reasonably precisely how large a hole actually is, or the width of a slot or the clearance between two things when there’s no way to get calipers into the space is extremely useful. It’s one of those game changers that didn’t just expand my physical capabilities, it changed the way I think about approaching problems. I don’t just use them for measuring. I use them as tools, and occasionally even as materials. Also, individual pins are available from Amazon at reasonable prices.

We like vises. I have mentioned all of mine before. Two of them get 99 percent of the use. I have an 8-inch Wilton bullet style that weighs ~100 lb and was worth every penny of what I paid for it used 12-15 years ago. There’s no reason it shouldn’t remain in service well into the 22nd century. The other is an old Panavise I got off fleabay. It’s made much nicer than recent production Panavises. It says “Long Beach” on it, so a historian could figure out how old that makes it. I only got it a few years ago, but I definitely recommend it. On this project, I only used the Wilton when making the hammer and trigger shims, but it’s obviously the go-to for torquing barrel nuts and receiver extensions.

With that out of the way, here's the story.

---------------------------

Picked up the M&P 340 this afternoon. Front sight pin was driven too far, so it sticks out on the right and the barrel housing is marred on the left. I can at least center the pin. Maybe a dab of flat black will survive cleaning. The crown is non-concentric with the bore and has a burr on one of the lands. Some burrs on the ratchet, but not the worst I've seen. Trigger press is not rough, but not silky. The reset feels like pulling your boot out of the mud: less force than it took to drive it into the mud, but still not a nice clean return, like it kinda wants to stay where it is. Barrel does not appear canted at all (at least not visually detectable by me so far), and the sideplate fit is exceptionally good. Lockup is nice and tight, timing is good. Crane fit is correct. Overall, I'm keeping it. Digging the sights over standard J sights. I expect that touching up the crown and a thorough deburring/cleanup of the action will make it right.

Unlike other defects I've seen in S&Ws, nothing makes this one obviously non-functional. It cycles, everything is "OK." I would expect them to send it back to me with no changes other than some handling damage, and tell me it meets specifications. Having the PC do an action tune leads to variable results, according to some who have many samples to evaluate, and I have an almost supernatural knack for finding the "Monday morning/Friday afternoon" quality surprises. I suspect an independent who will take the time to get it right is a better option.

I clearly wasn’t looking closely enough. Or maybe it was that the defect I would find next was never in any revolver checkout list I have ever seen.

So, in spite of the paragraph above, as I inspected the little bugger more carefully, I found a serious dimensional error. Nothing I've seen or heard of before, but totally obvious once you realized what you were looking at.

It looks like I never did post pics of this. Since an aspect of this post will be a litany of S&W’s shortcomings in building revolvers, I might as well include them. The yoke was mismachined with the hole for the ejector out of position. It appears the wall got so thin that it was deformed outward by the machining forces. (It might have happened another way, but my imagination isn’t coming up with anything.) That outward deformation was enough to mar the inside of the crane section of the frame.

Shipped it back to S&W. Got it back today from FedEx with a new crane fitted, as well as the expected new handling damage. Nothing all the way through the anodizing, thankfully...

Unfortunately, they also took either a file or a grinder to the extractor, so I'll be asking S&W to send me a new one, and having it fitted locally by someone who is familiar with the concepts of "skill" and "care."

It's really reinforcing the statement I've read here (BBI?) that new guns are an expensive, time-consuming PITA.

The paperwork says they "repaired" the extractor. This is the result of their repair. Before they repaired it, it was just a normal, brand new extractor in moderate (on the scale of what I've seen) need of deburring.

If I was to undertake that deburring myself, there would have been ultra-fine stones and a 10x loupe involved.

Just to reiterate, this is the work done by the S&W factory warranty shop.

This has gotten pretty long already for the first post. We're all caught up on preambles and old posts, so I'll make a new post for new info.

[OlongJohnson]

S&W did send me a new extractor when I asked. But I was somewhat daunted at the prospect of fitting it, so the whole mess sat around in the safe for a couple years while I did other things. Last spring, I read Kuhnhausen on extractor fitting and decided it didn't look that bad. I took the M&P all the way apart.

I’ve noticed that MIM parts all have a raised lip around the perimeter of any flat surface with square edges. This can range from minor to constituting a notable burr. From my understanding of how MIM parts are made, I think the lip forms similarly to the raised, hard edges of a brownie baked in a pan. The corners cool and harden first, then when the rest of the part shrinks, the hard corner can’t quite shrink with the rest and ends up proud. In both cases, you also end up with a slightly depressed center if the surface is large enough. I could be full of it on that, but it makes sense to me. It seems to be inherent to the process, as it’s present in MIM parts from many different manufacturers. In any case, I clean off the raised lip from all the parts where it matters so that flat surfaces are flat.

In this picture, you can see where the MIM burrs on the sides of the hammer were dragging on the inside of the side plate and the frame and wearing through the anodizing. Anything that’s doing that is adding a ton of drag, so it had to go. Sides of the hammer got deburred and flattened. This photo was taken when the gun was going back together, so the slot in the frame for the bolt and all other details that needed it have also been deburred.



The rebound slide bearing surface was interesting. I have seen a “how the master gunsmith does it” video on YT where the “master gunsmith” stoned right through the anodizing on this surface, until the swarf came up silver aluminum. DON’T DO THAT! The anodizing is the low-friction, wear resistant surface. Once it’s gone, wear will accelerate. I gently pass an extra-fine white Arkansas stone across the surface (with oil) until the drag of the stone decreases slightly. That’s when I know the highest points are knocked flat, but the majority of the anodizing is still present.

In this case, there was a raised transverse ridge at the front of the bearing surface (bright line). Making the surface flat required knocking it down into bare metal. The bearing surface doesn’t go all the way back to the rebound slide spring pin. It ends where the less-severe but much larger patch of visible wear ends. This wear was generated in the minimal use the gun had seen prior to disassembly. It was due to a dimensional issue on the rebound slide itself that I’ll explain when I get there.



The forward end of the bolt was fitted for length at the factory by grinding, but the end was left rough and burred. I broke the edge around the perimeter, knocked down the peak roughness on the flat of the end, and touched it up with cold blue, which you can see wrapping around the corners in the photo below.

There’s a pronounced gouge in the hand window. I broke off a curled chip that was still attached at the upper (left-most in the photo) end of the gouge. I have no idea how this gouge got there, as the hand never even contacts that part of the surface. Had to have been made by a tool of some kind.

The mark from the center pin moving across the recoil shield was developing faster than I thought it should. Magnification showed the rounded tip of the center pin had circular tool (lathe) marks on it, and it was acting like a file when rubbing the aluminum. So I sanded that (compound curved) surface smooth with increasing grit numbers. The cylinder swings in and out more smoothly with less drag and reduced wear rate.



The yoke had a raised burr around the edges that was concentrating recoil forces and peening through the anodizing into the thrust surface of the frame. There was a similar situation on the leg where the stem enters the frame. I made both surfaces flat, so the loads would be distributed over a larger area. However, the gun was originally fitted to the burrs, not the flat surfaces, so this caused the yoke to sit too far into the frame. Clearance between the thrust surface of the yoke and the frame where the peening is shown was excessive. I ordered shim kits for the yoke to frame and for cylinder end shake from https://triggershims.com/. Power Custom also makes them and sells them direct or through Brownells, but TriggerShims is cheaper, the quality is excellent, and they have coverage of many more guns. I played with different shims until I had just the right clearance between the yoke and frame, then adjusted the cylinder end shake until I could hear rattling but not feel or reliably measure it. Close to zero, but not zero.



The cylinder notches had burrs at the bottom of each ramp. When firing fast, that notch provides a little kick outward to the cylinder stop, moving it toward reduced engagement with the notch just when it should be moving into the notch as quickly as possible. Reduced stop engagement at the moment the cylinder is stopped increases stress on the steel of the notches, promoting peening. I have had good results stoning that burr out of the notches on my GP100, so I did it here, too. The blackened cylinder is still stainless underneath and won’t take any kind of cold bluing, so I was careful to minimize the bright surface left by the operation, but they are all smooth now. On the cylinder stop itself, I (just barely) broke the sharp edges and lightly polished it so it doesn’t aggressively wear a turn line into the finish on the cylinder.



I thought further about adjusting the engagement of the cylinder stop in the cylinder notch, and even started this thread about it:

https://pistol-forum.com/showthread....tch-engagement

In this photo, you can see the witness of the cylinder stop in the frame window already.

There was a full discussion over on the blue forum.

At the end of the day, as I got into checking everything more carefully, raising the stop at all would have reduced the engagement of the lower portion of the stop in the frame window. Since the frame is aluminum alloy and more subject to peening than the cylinder, I didn't change anything.

The ejector rod was a small learning experience of its own. I noticed the rod that came with the gun was pretty bent, and assumed I’d f’ed it up and bent or twisted it by overtorquing it. Ordered a new ejector rod from Midway, nice and straight. And assembled it, only to find that there was now an alignment problem. The straight ejector rod pointed sideways once it was tightened, enough that the cylinder didn’t spin completely freely when inserted in the yoke. Thinking about how to fix it, I read Kuhnhausen’s discussion of bending ejector rods. Realized maybe I didn’t f up the one that was in the gun. Reinstalled the original and found it was bent exactly how it needed to be to point straight and make everything work.

This photo illustrates using gauge pins (.381-.382 fits perfectly) to hold the extractor in the cylinder when loosening or tightening the ejector rod. It is a much lower-cost option compared to the tools that are sold for the purpose. I found the Brownells ejector rod tool/wrench worked a lot better with a wrap or two of aluminum foil on the ejector rod to more closely match the diameters. The foil is also softer than the anodized surface of the tool; both reducing the chance of slippage and reducing the chance of marring the blued or polished stainless finish if it does slip.



The hammer spring strut was extremely rough as it came from S&W. I cleaned it up, focusing on the edges because the curved spring never touches the flats. Now, when the spring and spring seat rub the strut, they rub smooth surfaces. The “inherent” roughness that a coil hammer spring on a strut is supposed to add doesn’t have to be very much at all. This photo also shows the lightweight substitute hammer spring I will talk about later, when I use it to reduce forces when checking function and timing of the action.



I trued up the head of the strut where it rides in the hammer. This started off very rough, basically the as-stamped surface. I used first a fine red India stone, then an extra-fine white Arkansas stone, held against the side of a steel parallel, and rubbed the strut against it to clean it up and make it nice and square. Then I cleaned up all the edges so they are burr-free and not sharp. Due to the fracture surface and rough blanking, it didn’t clean up all the way, but there’s enough flat and smooth there to do the job. This surface is where most of the force resisting trigger movement is applied to the action. Making it smooth, square and round, without removing too much material, is absolutely necessary for a smooth trigger press.

[OlongJohnson]

Below is the trigger system installed. Notice the hammer, trigger and rebound slide sides (as well as those of the smaller parts) have been flattened and smoothed. The mold parting lines on the sides of the trigger shoe were knocked down, leaving just a witness that you don’t feel. The lower corners were smoothed, too. The trigger needed to look right. I first tried Brownells Oxpho Blue cream. That just left it medium blue, not blackened. I then tried Birchwood Casey Super Blue liquid over the Oxpho. The combination darkened the trigger and gave an irregular effect, kinda sorta like a case hardened part. Good enough.

I mentioned the rebound slide earlier. The rear part is notched where it slides rearward, passing above and below the rebound slide stud. The lower part of the slide was too thick, causing it to rub against the lower surface of the stud and lever the forward part of the rebound slide upward away from the bearing surface of the frame. This meant that the rebound slide was riding primarily on the rearmost edge of the bearing surface machined into the frame, leading to the excessive wear seen in the earlier photo. I filed the notch in the rebound slide wider, thinning the lower arm of the fork, until the slide could slide freely in both directions, making full, flat contact with its bearing surface on the frame and not dragging at all on the stud. Of course, I cleaned up all the roughness and burred edges when I was done filing. I also flattened and smoothed the lower rubbing surface of the rebound slide.

I made shims from K&S brass shim stock for the hammer and trigger. This keeps them from rubbing the inside of the frame except through the shim, which is very small diameter, so there’s very little drag resulting from that rubbing. I measure the side clearance with feeler gauges, subtract 0.002”, divide by 2, and round down to get the shim thickness. This gives either 0.002” or 0.003” side play, which is plenty of allowance for mung buildup. It’s amazing how much more solid, precise, consistent and confidence-inspiring a trigger feels when it doesn’t rattle excessively side to side. (This works on classic Sigs and Buck Marks, too.) You can buy such shims for most guns from https://triggershims.com/. I will probably start doing that in the future instead of making them.



One irony of this project is that it started off with an agenda to fit the extractor and I did that almost last. Here’s a refresher on what my extractor looked like when returned from being “repaired” by S&W factory warranty service.



For all the angst and vapors people get about it, the geometry requirements of the ratchet teeth are simple enough. The surface that the hand bears against in lockup should be parallel to a vertical plane defined by the center axis of the cylinder and the barrel bore centerline. It should be positioned laterally so that there is as near as possible zero lateral play of the hand between the right side of the frame window and the fitted surface of the ratchet. Any looseness and the gun risks being out of time, with the cylinder not rotated far enough before the hammer drops. Any tightness, and there will be binding, causing excess drag in the trigger press, wear of the hand, frame window and ratchet, and forcing the cylinder toward the left side of the frame, pressing the center pin against and wearing the bushing that is pressed into the frame.

Rather than fitting the new extractor from scratch, I ended up cleaning up and properly refitting the extractor that was fubared by S&W. First I deburred and cleaned up the rough surfaces, and deburred and smoothed the hand. I assembled the trigger, hand, cylinder stop, cylinder/yoke assembly, trigger lever and rebound slide into the frame with a very light spring in place of the rebound slide spring, just stiff enough to control the movement of the trigger. (I took the stock hammer spring and rebound slide spring to my local Ace Hardware and picked out springs that had the necessary geometry but were very light.) No hammer or other parts. Then I checked the lockup and cycling of the cylinder with minimal other force. I discovered that three of the ratchets were binding as the hand rose into lockup, due to being too long. With the gun fully assembled and standard-weight springs in place, the bind couldn’t be felt, but it was clear when all that resistance was taken away.

This provided an opportunity to correct the crappy ratchet fitting done by S&W. The surfaces they filed were tapered, or “wedge fit” so the lower, or more forward when installed, part of the surface did all the work. Not only that, the basic angle wasn’t even correct. Each fitted surface should be parallel to the line (shown in blue) through the center axis of the extractor and the center of the chamber that is aligned with the bore when the cylinder is locked up. The S&W fit angle is indicated in red. Because they were left long, I was able to refit them as indicated in green.



I used a small pillar file and stones, doing just a little at a time and using a red Sharpie to show me where the binding contact was occurring. The binding I could feel went away at the same time the Sharpie ink stopped being rubbed off the steel in one or two cycles, so I figured that was just right. And I ended up with correctly-oriented surfaces. The extractor has already been touched up with cold blue here, but I left the fitted surface white for the photo. You can see the taper and the roughness of the surface as S&W left it.



When I was pretty sure I had it right, I installed the hammer and associated rest of the action parts, but with a very light substitute hammer spring. This allowed me to cycle the entire action and check timing very carefully with the side plate off and without overstressing the studs. All was good. (If you cycle the action with the side plate off and the factory springs, you are likely to damage the studs, either breaking them or disturbing their fit in the frame.)

The final piece was the crown. The original crown, shown below, was not concentric to the bore. It also had obvious burrs on each land, making the last contact of the bullet with the barrel irregular. Not ideal.



I recut the crown, using Dykem to indicate where I’d cut and cleaning up the minimum amount. I stop while there’s still a sliver of Dykem left indicating the old crown surface, so I don’t do any more than absolutely necessary to clean it up. I use a PTG piloted crown cutter (also sold by Brownells for twice the price) and follow by lapping it in each direction with a brass lap ($8 on fleabay) and 320 grit Clover paste to eliminate any burrs. With a cleanly cut crown, you can see how uneven and irregular the rifling is in this barrel. It’s not just an irregularly lit or angled shot. It is what it looks like. Each land and groove has different height/depth and different angle/shape to the sides of the land. Concentricity of the crown is ambiguous, because the center of the land surfaces is not aligned with the center of the groove surfaces. For all the PR there’s been about the precision and accuracy of S&W’s electrochemically machined barrels, I’m not impressed. I’m unlikely to ever be a good enough shot (especially with a snubby) for it to matter, but being able to see the irregularity of the barrel with the naked eye is still pretty lame. If my middle aged eyes can see the dimensional differences with unaided vision, it’s not precision.



I was pretty disgusted with all this when I was going through it. That happened to be the time when the Beretta/MR73 revolver thread kicked off, and this J frame had me thinking that paying that much for real quality might actually make sense. Every single part of the S&W, except the springs, required work to be what it should the best it could be. (Polishing the usual surfaces of the rebound slide and nothing else, as is often recommended, would have accomplished probably in the low single digits percentage of the improvement I achieved.) I couldn’t begin to say how many hours I spent doing this, but it was too many. Nobody would pay for it at shop rates and you couldn’t run a business doing this.

Once I got it back together, all that annoyance kind of melted away. The trigger is truly sweet. It’s smoooooth. With the massive reduction of friction, the weight of the trigger press with the stock springs is completely reasonable.

As it came, I was not a fan of the reset:

The reset feels like pulling your boot out of the mud: less force than it took to drive it into the mud, but still not a nice clean return, like it kinda wants to stay where it is.

The biggest factor was probably the binding between the rebound slide and the rebound slide stud. Eliminating that was huge. In addition, I smoothed out the sear where the SA edge of the trigger drags against it and slightly broke the sharp SA edge of the trigger (safe to do because it has no sear function in a DAO Centennial – DO NOT do that in a gun that can be cocked to SA). I detailed out all the surfaces where the trigger and cylinder stop interact, and took care smoothing the ratchets and the surfaces of the hand that drag across the ratchets during reset. The cylinder stop spring had been misinstalled by S&W, too, which might have been additionally screwing things up. The reset is now nice and crisp and smooth.

I really, really like this little revolver today. I have my boot version of hacked Sile grips on it. It looks good, it feels good, the trigger is just what it should be for a defensive tool. And I have less than zero interest in acquiring any more S&W revolvers for a very long time to come.

[Doc_Glock]

This is gold!

[JAH 3rd]

All of a sudden I have a new appreciation for the 1911.

[Joe Mac]

Well done, sir. You're a patient man.

[03RN]

[Elwin]

All of a sudden I have a new appreciation for the 1911.

All of a sudden I feel like a caveman when it comes to the work I do on my 1911s with paper and files I bought at Home Depot. Fortunately the stainless gun I screw with the most is a Monday or Friday Kimber that was already ugly and then well used and worn on top of that, so I don’t care, but point stands that if this is just “hobby gunsmithing,” I have no idea what we call what I do.

[JAH 3rd]

Thanks for the effort in addressing areas of concern with that revolver. And then photographing and commenting on the photos. Just goes to show the time and labor involved in going through that revolver. I'm going through my S&W 340PD to see if I have similar markings.
No doubt I will find some. Thanks again!!

[PNWTO]

Fantastic details and post!

So... do you have a waitlist for revolver work or can I just start mailing them your way?