MG 42 recuperators and firing pins

[MGMichael]

I posted this on another MG forum a few weeks ago but for the information of others who may not have seen it, I'll repeat it here.

For those to like to shoot their MG 42s and want to minimize the risk of breakage and/or dangerous malfunctions by trying to ensure, insofar as is possible (or known), that the parts are within specification, the following data comes courtesy of then-Colonel Friedrich Kittel, der Oberbefehlshaber des Heeres. Over his name was published H. Div 181/7, a technical manual entitled Untersuchung und Instandsetzung des Infantreriegerates (Inspection and Repair of Infantry Equipment), Tiel 7: Waffentechnisches Handbuch fur MG 42, dated 3 May 1944.

In a section devoted to the Laufvorholer i.e., barrel recuperator, a dimensioned drawing is presented for a clamping tool which the Waffenmeister was supposed in fabricate in his vast leisure to compress the springs so that the little winged-donut retainers could be removed. (It's more complicated than the tool I made, but its use does not require the simultaneous use of a bench vise.)

The manual discloses that the Germans used two different types of springs during the war, conventional coil-type and, at some point after September 1943, a new version (neu Ausfuhrung) of braided-wire type ( "gedrillt" ). These two had different specifications, particularly length:

Single wire: 75.5mm long, 25 coils, 9mm dia., 1.7mm dia wire.

Braided wire: 85 mm long, 18 coils, neatly closed and ground, 8.8mm dia., wire 4x 0.95mm, mit Seele (included space?) 0.4mm.

Springs shorter than the above were to be replaced. The importance of these springs is greatly stressed in the manual, including --as some have learned the hard way-- warnings about breakage of the receiver trunnion, or cam yoke.

The diameter of the shanks of the push rods for each type spring also is specified: 4.8mm for the solid wire type, and 4.0mm for the braided. For reasons unknown, none of the rods that I have measured, either WWII or MG3, are actually that diameter. One set, believed to be original, found with solid wire springs, measured 3.90mm. Another set, either German or possibly Yugoslav replacements (found with braided springs) averaged 3.78mm. The little winged-donut retainers also come in two different sizes, with different diameter holes, 4mm and 5mm. OAL of pushrods is 63mm for the first (headless) rod, then 65.5mm for the next two (headed); the rearmost headed rod is, according to the manual, supposed to be 20.5mm shorter than the other two (and it is, exactly 40mm).

Early production MG 42s used a recuperator head (i.e., the plunger in front) combined with an integral (first) pushrod, all in one piece; this was soon replaced by 2 pieces: a short head, plus a plain headless pushrod. Either type is subjected to a lot of stress, and when the recuperator housing is dismounted from the gun, the front pushrod slides out and can be readily inspected without further disassembly. It should be checked to be sure it is straight, not bent, and not mushroomed on the ends. The recuperator head is the same in the MG 42 and MG 3.

In the Rheinmetall MG 3, due to its longer recuperator tube, the springs and rods are correspondingly longer. For use in an MG 42 they need to be precisely cut. The braided wire springs average in length, when new but presumably after taking a set in the tube, about 100mm, with 21-1/2 coils (indifferently closed and ground on the few I've examined), 8.8 mm coil diameter, using 4 x 0.95mm wire. The rods measure 3.75mm in diameter, and use retainers with 4mm holes. MG 3 retainers have a noticeably deep chamfer around the hole on the convex side, apparently to ease passage of the pushrod in front, and are installed accordingly, convex side forward.

The manual states that the "4mm" pushrods also could be used with the single-wire coil springs. Obviously the larger (4.8mm) diameter pushrods would not work with the braided springs, or with the smaller retainers. But there is enough variation that when replacing any of these components, manually test to make certain that the hole in each retainer is big enough to allow its pushrod to pass through freely.

The manual advises that if any springs need replacement, they should be replaced, if possible, with a new full set.

It should be noted that the actual piston travel of the recuperator is the same in the MG 42 and the MG 3. The only difference is the length, hence capacity, of the springs to absorb the energy; the longer springs are not working as hard. In the MG 3 each spring proportionately compresses only 86% of the amount by which its counterpart in the MG42 is compressed. Whether this change was made to absorb the extra energy imparted by the optional heavy 950g. bolt carrier, or the sharper impulse of the 7.62x51mm NATO cartridge, or to smooth out the operating cycle, or simply to extend the service life of the springs, I cannot say. Engineers who can work out the quadratic equations will have to answer that one.

Firing pin protrusion also is addressed in the WWII manual. A gauge is illustrated showing a max of 1.65mm (.065") and min. of 1.35mm (.053"). The manual advised favoring the lower dimension in order to avoid perforating primers. If shortening was required, correction was to be made by removing material from the rear of the firing pin, not the tip.

Postwar, this was further reduced. I have a Rheinmetall G3 manual that specifies firing pin protrusion at 1.45mm( -0.31mm), (max. .057", min. .045").

While the manuals don't say so, the specification for firing pin protrusion obviously is that which is obtained mechanically with the pin vertically upright, fully supported by the firing pin holder (wedge) pressed forward (upward) as far as it will go, and with the rollers fully extended outward. While the pin holder has a positive stop, the pin itself does not. It is free to move forward a considerable additional distance by inertia, as can be seen by simply inverting the bolt head so that the pin drops forward by gravity. The pin probably is too light to function solely as an inertial-type pin, but obviously its capability for additional unsupported travel has some augmenting percussive effect as the prescribed mechanical protrusion is significantly less than that specified for the purely striker-fired MG 34 (1.8mm min. to 2.0mm max; .071"-.079"); likely this also helps to prevent breakage by cushioning the last increment of movement.

From the design theory perspective, the firing pin of the MG 42 is unusual if not unique in that it combines a mechanically positive operating principle with supplemental inertial effect.

M

[12thWaffenSS]

Heck of a first post! I wonder if those MG-42 manuals are around anywhere. If they are, they're probably in German.

[JBaum]

And,

if anybody wanted to read the either the MG42 HDv 181/7 manual, or the MG3 repair manual in its entirety, I hear that they're both in English and available at some guy's website who translates those sorts of things.

And the firing pin protrusion gauges are still available, too.

[www.Prussia.us]

Sure Jbaum

Like there is some magical place that sells easy to read manuals in English translated from German for the: Mg34, Mg42, Mg08, Mg 08/15, Mg13, Mg15, MP44, STG58, RPK, Mp38, AK-47, AK-74, and MANY more, and where might that magical place be selling those "German Manuals" ?

Welcome aboard MgMichael, that is helpful information and that is why as BIl has said, before someone starts shooting their expensive gun, picking up some of these manuals is common sense, as many pitfalls abound

[MGMichael]

John, I have purchased many of your translated manuals but I did not realize that you had completed a translation of H. Dv. 181/7. It would have saved me a lot of time, as I struggled with translating from the German the parts that concerned me. I'll be sending you an order soon for that one and others.

I was compelled to study this matter after chipping off the four corners of an MG42 receiver yoke -- thereby becoming dimly aware after nearly 30 years of shooting this gun that the recuperator springs (very likely the originals) probably needed replacement.

I earlier had purchased an MG3 recuperator tube assembly, complete, understanding from the get-go that I'd also have to install an MG3 bolt buffer. I had been told --erroneously--that the complete assembly would fit into an MG 42. At worst I figured I could rob the MG3 recuperator springs from the tube. Well, of course it was not that simple.

There is a heavy-gauge piece of sheet metal welded into the floor of the receiver that acts as a buttress for the rear end of the recuperator tube, and it effectively blocks the installation of the MG3 tube, which is longer at the rear.

About that time I also discovered that the springs in the MG3 tube are longer and the slots in the tube that govern the progressive compression of the springs are spaced further apart than in an MG42. The rods inside the springs also are correspondingly longer, and the only parts that appeared to be interchangeable with those in the MG 42 were the front plunger and the little winged donut retainers. (Even the latter are not quite, as they need to be matched to the o.d. of the rods.)

This led to comparative measurements, and I found that everything in the MG3 recuperator was about 116% longer (or further apart) than in an MG42.

As it happened I also had a spare MG42 recuperator tube that came with a kit, presumably from Yugoslavia. It appeared to have been rebuilt with new stranded wire springs. These were removed and measured, and sure enough they were not only considerably longer than my original tired German springs, but they measured just about 86% of the length of the new MG3 stranded springs--which is to say that the MG3 springs were, as expected, about 116% longer.

So I set about to install the stranded-wire springs in my original tube, figuring I could always go back and cut and end-grind the MG3 springs to the same length if I needed to. Of course after several hours of frustration I had to stop and make a strange-looking tool to compress the plungers sufficiently to install the retainers but eventually it all went together. The hardest part was the actual installation of the tube into its niche in the receiver--coaxing the little screw to come through the hole in the left receiver wall without popping loose from the recuperator tube and sending the plunger flying.

Anyhow, I did not realize the recuperator would make such a big difference in the way the gun shoots. It changed the entire behavior of the gun from a violent and unpleasant jackhammer to a remarkably smooth and even-cadenced gun, a bit slower --but not by much.

M

[JBaum]

HDv 181/7 has been in English for a few years, and I just finished the MG3 repair manual about 3 weeks ago.

The recuperator is a much more important part of the MG than most people realize. Don't forget to keep it oiled inside.

For you RPD lovers, I'm nearing completion of the RPD armorer's manual.... some really neat stuff in there.

It will be my 90th translation.

[MGMichael]

...The recuperator is a much more important part of the MG than most people realize. Don't forget to keep it oiled inside....

John: You deserve enormous congratulations for such an ambitious and prodigious output. I have some appreciation of the effort necessary, as some 40 years ago I spent many months struggling to informally translate portions of four sections of bildheft 131a (apparently redesignated 68/9 a,b, d and e) for the MG 34, and even prepared my own glossary of terms specific to that gun. It wasn't easy.

Indeed, making sense out of any translation depends a lot on establishing a logical and consistent assignment of parts nomenclature; otherwise you go crazy trying figure out, for example, what a "trigger lever" is.

As for oiling the MG 42 recuperator, I am not sure I would agree with that. Certainly a light film of oil should be applied during assembly, but I haven't found anything in the manuals to suggest that further oiling is necessary. If copiously oiled it will become a magnet for the dirt and powder residue that is concentrated in that area anyway, and the recuperator tube is not an assembly that can be routinely dismounted or disassembled for routine cleaning.

M

[JBaum]

Regular oiling and standing the gun in a corner lets the oil carry out any residue with it when it drains. It's in one of the manuals that it should be oiled, but with 10 (or so) manuals on the MG42/MG3 translated, I couldn't say which one.

Do you still have the MG34 manuals??? I recently translated 131/c, but would certainly like to find the rest of the set. According to the manual list, there was an /e volume, too.

[MGMichael]

John: What I have are photocopies shot many decades ago (by whom I am unsure) apparently from microfilm images of Bildheft 131 a, b, d and e, which have been re-labeled by hand to 68/9a, b, d and e. At least one version of this set of manuals might well have existed in the form of instructional slides for a projector. Some of mine show stamps and evidence of inked-in corrections made during the war. They look like editor's copies. They are clear enough for me, but I don't think they would reproduce very well for publication.

Bildheft 131a is a general basic description of the MG 34 (and to a minor degree, the MG 42) and its operation and employment. 131b is detailed parts description and most useful nomenclature; 131c (which I don't have) covered procedures for loading and firing. Bildheft 131d covers functional inspection by the operator and preparation for shooting; 131e deals with specific stoppages and their remedies. All of them were profusely illustrated, and the text --thank God-- is not in that awful gothic script that makes your eyes glaze over.

M

[42rocker]

Thanks for the great posts of info on the recuperators spring and rod sizes..

Thanks again

Later 42rocker

[Michael J]

If i do recall, i have a HDv 181/7 manual on it's way to my house !

Thanks again John for your hard work, i don't even know any other sources i could go to for manuals like yours. I'd be SOL .

- Michael

[JBaum]

The 131 series for the MG34, and the 142 series for the MG42 were the paper versions of the glass slides that were used in the days before 35mm film slides in the little cardboard squares.

The pictures were the same in the manuals as in the slides. I translated 142 a, b, and c, and have them compiled into one volume. So far, I've managed to get an original 131 c, but would love to get hold of originals of a, b, d, and e, and with a little luck, 142 d, which I've never seen. SO many manuals, so little time.

John