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My First Part: G41(W) Gas Piston

I’ve been working on the lathe for a couple days at school, and been doing turning and drilling (no threading yet). I finished the sample piece my instructor wanted (a cylinder with several different diameters and grooves, which we will be using for threading instruction tomorrow), and I was looking for something to practice these operations on today. After flipping through some old copies of Home Shop Machinist, I realized that the perfect item to make would be a gas piston from the Walther G41(W) that I was looking at last week over at Forgotten Weapons. It’s a very simple piece, which slides over the barrel and inside a thin cover. It is pushed by gas briefly trapped by the muzzle cone of the rifle, and acts on a connecting rod to move the bolt. The upshot is that it’s a simple cylinder with three gas seal grooves cut on the outside surface and two more on the inside surface.

Walther G41(W) piston

Walther G41(W) piston (original on the left, mine on the right, piston cover piece below)

So, I broke out the gas piston from our G41(W), and began by doing a basic reverse engineering of it. I drew out the part and used  series of tools to measure all the important dimensions. A basic dial micrometer for the outside measurements, a micrometer with pointed contacts for the groove diameters, and a digital caliper for the inside diameter (I don’t have a good tool for precise measurement of inside diameters, as it’s larger than the set of plug gauges we have).

Once the part was drawn out, I pulled a piece of 1″ diameter stainless stock from the steel room (the original is made of stainless, since it is exposed to hot, high pressure gasses on every shot), and chucked it up in the lathe. I first faced it off, and then turned down the OD with a series of cuts until I had the proper 0.904″ diameter. Next I used a cutoff tool to cut the exterior grooves, followed by a series of drilling operations to cut the ID. We don’t have a reamer of the specific 0.650″ diameter necessary for this part, so I instead used a boring tool to cut the final inside diameter. Not nearly as fine a finish as a reamer, but it will work. A second boring cutter allowed me to cut the interior gas relief grooves, and then back to the cutoff tool to, well, cut off the part from the stock.

This took me about 4 hours from start to finish, and it does have several problems. My speeds and feed rates on the first two grooves were not ideal (and I was using an HSS cutting tool on a stainless part, which is not recommended), and that led to a lot of chatter. The inside diameter should be reamed for a smoother finish, and the inside gas relief grooves are not deep enough (we would be best served to make a specific tool for that operation if we were to make a bunch of these parts).

Once I had the final piece in hand, I did a little buffing to the outside to clean up some burred edges, and tested it out on the rifle. It actually fits! :) My first actual gun part, and it fits both over the rifle barrel and inside the cover with the same feel as the original part.

I expect we will be revisiting this piece down the road when I start running the CNC lathe – it could be done in a single program on the CNC, and will be a good practice job.

12 comments to My First Part: G41(W) Gas Piston

  • Russ Mularz

    Nice work!

    You got the first two of the “F’s” down, form and fit, how about a function test? Good reason to go out and expend some ammo, in a purely “instructive” manner.

  • Will Cushman

    You might want to look into getting a set of snap gauges (inexpensive) for measuring inside diameters. I have an inexpensive set I use for revolver cylinder chamber throats and it works fine.
    They work by snapping out compressed pins against the walls of the to be measured, locking them in place, withdrawing the gauge and then using a micrometer to measure across the extended pins.

  • Hrachya Hayrapetyan

    What if make this piston with not grooves but threads? The threads can have same depth and width as original grooves. Threads will start from the position of first original groove, to leave nice and even working (front) surface, which directly interacts with hot gases. Threads will be open at rear of piston. The idea is to guide the carbon buildup out (back) from the system and add some reliability to the gas mechanism. If what I am talking about doesn’t make sence, I can try to draw it on paper.

    Well, of cource it’s not an ingenious idea. The gun itself is a historical piece and such changes must not be very desirable, but what if make it and try it to see if it could make the gun more reliable, if it was done by the designers back in 1941.

    I would appreciate to hear some comments by Ian and blog readers concerning this little idea.

    • Ian

      I’m not sure that the gasses would make it all the way back through the threads – they would probably start to build up carbon at the front, and once there was a decent it there it would seal off the rest of the threading. In addition, the threads would take a lot longer to cut than a couple simple grooves. It’s an interesting idea, though.

      • Hrachya Hayrapetyan

        Then maybe it would be better to make kind of barrel rifling style grooves instead of threads to form smooth track for the carbon buidup? Sure, it would be harder to make than simple grooves.

        • Ownerus

          The purpose of the grooves isn’t just for carbon buildup. They serve to extract energy from the escaping gas and serve as a seal. That’s why hydraulic valve spools usually have similar grooves. A thread wouldn’t do as well since it would provide a continuous (though long)direct path for leakage.

          • Hrachya Hayrapetyan

            To me, carbon buildup is not a purpose at all …
            Theory always sounds perfect, but I am for trying the stuff. Although this is not the gun to modify and improve, I think such experiments worth to take place.

  • THIS is far cooler than my brute force gnawing at AR lower forgings.

    Re:the steels. Of course, metallurgy has evolved a good bit in the last 70 years. The metals used in WWII German weapons conformed to the then-current DIN norms and may or may not be a good match for today’s steels. In my experience with aluminum, there are alloys that are substitutable because they’re structurally similar, but they’re not the same.

    Despite the evolution of metallurgy, plenty of metals and alloys that were useful many years ago are still useful and widely available.

  • Brian

    I got pointed this way via Tam at view from the porch. You’re in my “gun and prep” folder of bookmarks now, and I’ll be checking in every few days to see progress. I’m currently active Army, on the MEB route, and once I’m done, I’ll be trying to use my GI Bill to either go to the CO school of trades or Murry state college for a gunsmithing program. Thanks for this interesting educational resource.

    On a side note, is the commenter will cushman the gentleman who ran a gunsmithing blog a while back?

  • Will Cushman

    @Brian- No, Brian, I’m not the same guy. My only ‘smithing experience comes from doing some careful tuning of my 1911’s, replacing the hammer components in a Ruger .45 Colt Single Action to get a more “Colt-like” function, and removing an internal lock from my .38 S&W 442 J Frame carry piece. All projects have worked out well; I took my time, used Jerry Kuhnhausen’s excellent Shop Manuals for the 1911 and S&W revolver. a good DVD for the Ruger work, and an excellent YouTube video for the internal safety removal. Also, and very important, a real set of gunsmithing screwdrivers!

  • Awesome. I was hoping someone would start a blog with this subject matter. I salute you.

    – T Bolt

  • […] In fact, Ian is undertaking a career change of sorts, and from being a marketing guy he's taking classes to learn to be a builder of … stuff. (Frankly, it sounds a bit like funemployment to us, but it takes all kinds to make a world). He has, in fact, already wrangled his first scratch-built metal part, a stainless-steel gas piston for a German G41(w) rifle, a rare and short-lived experimental weapon. The G41(w) was a milestone on the path to the much more successful and common G/K43 semiauto rifle. But rather than hear the story from us, get thee hence to a nunnery to GunLab and read it on his pages. […]

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