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I am at the stage of fitting the recuperator and am confused:

1) Is the barrel stopped by the trunnion or the recuperator once it travels aft in the unlocking cycle?

2) What is the normal rearward barrel travel distance?

3) What is the normal distance the recuperator piston is allowed to travel before reaching a mechanical stop?

I am extremely grateful to anyone willing to find the time to answer these questions!

(2) I remember reading that the barrel will travel about 8mm before the trunnion unlocks the bolt. I imagine all your other questions will result in number greater than 8mm or you have serious problems because you want it to unlock before anything comes to a stop.

I have no idea about what stops the rearward movement or the barrel. Just a guess but the spring should absorb enough of the force so even it it does hit the trunion it will not break anything.

VMA wrote:I am at the stage of fitting the recuperator and am confused:

1) Is the barrel stopped by the trunnion or the recuperator once it travels aft in the unlocking cycle?

2) What is the normal rearward barrel travel distance?

3) What is the normal distance the recuperator piston is allowed to travel before reaching a mechanical stop?

I am extremely grateful to anyone willing to find the time to answer these questions!

Interesting questions

1) The barrel is stopped by the recuperator it seems,
before you asked I'd always assumed the CAM (not trunnion) was there to stop the barrels rearward movement. So I decided to rip my working SA42 apart to check. I was surprised when the action on it showed the recuperator being fully compressed while there is still a 1/4" gap between the cam and barrel extension.
A quick survey of barrel extensions at hand showed bright spots where the barrel must have contacted the cam at some point on quite a few. This brings up the question of.....
how did so many of those barrel extensions ever manage to hit the cam in the first place ?

2) About 1/2 "

3) The front of the recuperator plunger aligns perfectly with the rear face of the cam lobes when fully compressed.


P.S. Make sure the area in the receiver where the recuperator plunger moves is perfectly smooth and free of slag, & put a slight bevel on your plunger where the barrel extension hits it when the barrel is inserted to make it easier to POP it into place.

BTW, I think the proper name for what we are referring to as the trunnion is called the camming piece, at least that's what I've read.

I greatly appreciate all of the responses in such a quick amount of time. I was carefully inspecting the barrel extension for marks which would indicate impact. As stated in a post earlier it does apprear that there are impact marks which would indicate that the barrel was stoped by the camming piece which I incorrectly refered to as the trunion. With that in mind my barrel travels 1.9 cm to the stop which I guess is about a half of an inch. I can only get the recuperator to move about 1 cm. I have taken it apart about 15 times and have studied all of the diagrams in the stickies section. Weird! When I remove the spacers which hold each section I can get the required travel which of course would indicate that they are holding up the rearward motion. If I screw with this enough I know I can ultimately fix the problem, but why? This brings up an idea of leaving them out. The springs would be under the same tension with another spacer which is the same diameter as all of the spacers put together. As far as I have read if the recuperator has enough spring compression force to support the weight of the weapon then it is strong enough. Any thoughts?

The "spacers" are needed. The springs in the recuperator act in parallel and not in series. When the barrel presses on the recuperator in compresses each spring 8mm (or whatever distance) because of the pins/guides. If the springs are in the tube without the spacers then each spring is only compressed 2 mm (8mm/4 springs). The spring load is created by spring rate (lbs/inch, kilogram/mm) and the distance compressed. Because the spring rate wouldn't change by removing the spacers (spring rate is determined by wire diameter, coil diameter and # of coils) the load placed on the barrel would be 1/4 of the load (pounds) with the spacers. It would actually less without the spacers because the springs would not have as much preload (pre-compression). Remember, the Germans weren't known for adding extra parts into their weapons just for fun, if the parts are in there then there was a reason.

VMA,
The "recouperator"....slows the rearward movement of the barrel-Assembly (so that it doesn't beat against the front of the "Camming-piece") and returns the barrel-assembly back into "battery" (forward) so that the next round of ammo can be loaded and bolt able to be locked into the camming-piece. It does not stop the barrel-assembly.
NOTES: Allowable movement before parts hit is approximately:
1: .835" movement for the barrel-extension vs. camming-piece.
2: .940" movement of front-plunger in recuperator vs. front of recuperator housing.
3: Also note that heavier-duty springs may even keep the hitting down to a light touching....and maybe even not touch at all.

If you take an MG42 "recuperator" out of the gun.........and measure the slots that are cut into it....the rear two slots are not as wide (from front-to-rear) at the third-slot. The front-slot is only wide enough for the front-bolt-head w/tang....."but" the front plunger is machined so that it can have more movement. What this means is that the recuperator is given more allowable movement at the front and center-slots, then the rear two slotted-holes...and allows the spring-pressure to build up as the parts are moved rearward within the recuperator.

The barrel may hit (or even just touch) the front of the camming-piece......and the amount of force when it "hits" will be determined by such things as (but not limited to):
1: Condition and assembly-order of Recuperator.....with attention to the direction of the front mounting-bolt's tang/tab.
2: Condition of "unlocking-cams" inside the "camming-piece"...which determine the timed un-locking of the bolt-assembly from the camming-piece.
3: The size hole in the front of the Booster-Cup/"Nozzle" which determines hole much pressure is allowed to remain within the Booster-Cup/"Nozzle" as the bullet passes through it.
4: The length of the barrel-Bearing....this acts in conjuction with the rear-slots of the Booster-cup/"Nozzle" to have a timed-release of "high-pressure" within the Booster-Cup/"Nozzle"....the longer the bearing...the longer the high pressure is allowed to remain....and the longer that the "harder" force is allowed to exert pressure onto the front of the barrel-bearing as it pushes the barrel-assembly to the rear. This higher-pressure also speeds up the gun....but has the added problem of wear/breakage of parts.
5: Caliber and "LOT" of ammunition used.

My suggestion at this point.....because there are a few variables to work with......please post a clear digital-photo of the recuperator layed out in the order that you are trying to assemble it...paying particular attention to laying out the direction of the tang/tab on the front mounting bolt as you would be assembling it in the recuperator.
Maybe if we can "see" what parts you have....and "see" the order that you are trying to asemble them......maybe we can sort this out for you. Maybe the order is incorrect (?)...or parts (?)....or ...(?).
Maybe a second photo showing the location of the front-mounting-hole in your receiver so that we cas see location vs. the camming-piece cut-outs would help for us to see of "location" is good (?).

Last question(s) would be for you to give us a good discription of what you have to work with (?).......
1: SA42...or MG42 (?)
2: If an SA42.....were the original-mounting-holes used to mount the recuperator (?)....or new holes drilled (?).
3: If using a DEWAT receiver......was the original location for the camming-piece in the receiver vs. the front of the recuperator maintained (?)....or could something have been changed/moved slightly (?).

To answer your questions (using my C&R MG42 to take measurements) is:
1: The Camming-Piece (what you are calling the trunion) stops the rearward movement of the barrel-assembly......this is assisted/slowed by the recuperator....and the recuperator returns the barrel-assembly back into "battery"/forward.
2: My C&R MG42 moves:
A: .835" (total) at which ponit it will hit the cmming-piece in my MG42.
B: .470" at this point...the forward recuperator-plunger and first spring have been compressed...and the other three-springs start to work in adding additional spring-pressure.
3: My recuperator front-plunger can travel approximately .940" and at that point it is at "mechanical-stop"/head inside the recess-cut in front of recuperator-housing.

Regards, RichardS in MI.
Blanksguy2001@chartermi.net

"The Barrel strikes on the front side of the Unlocking Cams and is pushed again forward by the Recuperator."

That's a line from the MG3 manual that I've almost finished translating. It's on page 241/2, second paragraph from the ZDv 3/14 manual, which is available for free download in its original German on my website Links page ( http://www.GermanManuals.com/Links.html ). The English translation of this manual should be done in about a week.

The recuperator slows the barrel, the barrel extension hits the cam piece, which stops the barrel's rearward travel. The recuperator pushes the barrel again forward. One of the symptom's for having a bad recuperator is that the cam piece is loose on the receiver, a result of the repeated severe impact being more than the cam piece can take. That's because the barrel beats it till it's loose when the recuperator is too weak to slow the barrel sufficiently.

After 60 years of production, the recuperator has remained essentially the same design. I wouldn't re-engineer 60 years of success by taking out a few spacers.

Since I don't have access to edit my above message:

The barrel and bolt release their interlocking after 12 mm of rearward travel. The manuals don't say what the overall rearward travel of the barrel is supposed to be before the barrel extension hits the cam piece.

jbaum,
I won't change the MG42 except for some of the "up-dates" such as:
spring-loader roller for top-rear of Bolt....maybe the lever-spring from and MG3 top-cover....and the new spring-set for the recuperator.

I would suggest a set on newer/stronger recuperator springs as springs do get old and take a "set"....especially after 60+ years of being in a gun. Also install "new" recuperator bolts and nuts as they are "staked" at the top to keep them from coming loose during operation of the gun.
(NOTE: Clean and lubricate the recuperator while it is apart.).

It would also be a good time to get familiar with the inside of your gun while you are cleaning it....checking for "wear" and using jbaum's books/manuals while doing all of this (great books).

Regards, RichardS in MI.
Blanksguy2001@chartermi.net
PS: Measurements that I took/posted earlier are from my C&R MG42 manufactured by "cra".

Blanksguy wrote:VMA,
Allowable movement before parts hit is approximately:
1: .835" movement for the barrel-extension vs. camming-piece.
2: .940" movement of front-plunger in recuperator vs. front of recuperator housing.
3: Also note that heavier-duty springs may even keep the hitting down to a light touching....and maybe even not touch at all.

To answer your questions (using my C&R MG42 to take measurements) is:
B: .470" at this point...the forward recuperator-plunger and first spring have been compressed...and the other three-springs start to work in adding additional spring-pressure.
3: My recuperator front-plunger can travel approximately .940" and at that point it is at "mechanical-stop"/head inside the recess-cut in front of recuperator-housing.

Regards, RichardS in MI.
Blanksguy2001@chartermi.net

Interesting ,
I also have
1) .835" of space between the front of the cam and rear of the barrel extension, when the barrel is fully forward.
2) .940" of space between the inside of the plunger head and body of recuperator.
My barrel bearing only sticks out .68" past the receiver

but my recuperators will only compress .55" . At that point all the retainers have bottomed out I don't know if theres even enough space for all the rods to compress more or I'm not using enough force.
Also they all move at once but at slightly different rates. Not in sequence like you described. pretty much once the retainers reach the bottom of their windows I can't compress them much more.

Does your plunger have a rod attached thats 2.47 " long?
I wonder if there are different variants? It has been 60 yrs.
I hate to disassemble the spare recuperator to get all the rod measurements.
12MM = .472"

drooling idiot,

When you push back on your barrel-bearing....to the point of .55".....this is the point where you start to get more/added spring-pressure from the other springs.

If you turn you gun muzzle-down (on a block of wood) and push the gun downward.....you will get more movement (although you will have more spring-pressure). ...at .68" on your gun.....you the barrel-bearing will be flush with the front of your receiver.

The barrel and barrel-bearing will move back a total of .835"....so it will be able to push it back even more if you can find something smaller to push the barrel-bearing deaper inside the receiver.....until it makes contact with the camming-piece as in "live-fire".

Regards, RichardS in MI.
Blanksguy2001@chartermi.net