Marlin jam

[Brad]

Waco mentioned in a different thread that he was having issues with the Marlin jam.


The problem starts with the cam right in front of the hole for the lever screw. Notice how the cam on this lever has a slight radius? That is good. If it has a sharp corner it tends to cut a groove I to the bottom of the carrier. That lets the carrier drop too low and it doesn't cut off the magazine properly. A second round tries to feed and since it is 1/2 in the action and 1/2 in the tube it binds the carrier. Carrier can't rise so the lever is jammed in the open position. Either remove the lever and take out the offending round or use a small dowel to shove the offending round into the magazine enough to let the carrier rise.

This is the bottom of the carrier where the damage is done. At the rear end of the flat shown is where the groove starts. No groove here because of the radius on the cam.

Solutions is to add metal back to the carrier. My 32-20 has a small section of razor saw blade on the bottom. I soft soldered it in place as it merely acts as a surface for the cam to slide on. Some people use a small weld to add metal then file it back down. Make sure to cut the radius on the cam or it will come back in time!

I can assure you that a 2 shot lever action sucks. So does a rifle that jams every few magazines full. Makes a guy leave a fine shooting rifle at home a lot. Fix it and the fun returns.

 

[waco]

Thanks Brad.

 

[smokeywolf]

Thanks for the gunsmithing lesson Brad. This is the kind of thing I'm always hungry to learn. Aside from the opportunity to share this hobby with like minded folks, this is where I see the greatest value in a forum like this.

And from the pics, I have to agree with you. Looks like a casting from here.

 

[Ian]

Smokey, I think it's a machined part (all mine are machined but are likely older guns) but finished in a vibratory tumbler that gives a finish that appears like a cast part. I will say that the carriers probably have the best machined finish of any part in the rifles, the rest typically look like they were hewn out with a chain saw, but that's ok because Marlin leaves a lot of metal to work with in the critical places when tuning and smoothing an action.

 

[smokeywolf]

I've never shot, much less worked on a Marlin, so this is all new to me. I've seen parts vibrated in media to produce a work-hardened finish. Perhaps that was an intention.

 

[fiver]

they shot peen the parts to harden them.
it's the same principle as peening a crank shaft so it will take more combustion pressure.

there is another marlin jam where the cartridge comes out under the lifter and locks everything up on the down stroke. [up stroke of the lever]

 

[Pistolero]

Shot peening with steel shot puts a billion microdents in the surface. This leaves a
residual net compressive stress on the surface. In any loading which puts the surface in
tension (like bending), the compressive stress has to be cancelled out before there is any net
tensile stress, so effective surface stress (where cracks start) is lower. Say there is a net
15,000 psi compressive stress residual from shot peening. If the loading stress during
operation is 40,000 psi, then the first 15,000 is just bringing the surface up to zero stress,
and the overall final loaded surface stress is only 25,000 psi --- much less likely to cause
a crack. And, yes, the residual compressive stresses INCREASE the net stress on the surface
on the compressive side of the part during bending loading. However, cracks do not open up
in compression, so no problem.

AND the shot peening work hardens the surface, increasing the surface yield stress. Cracking from fatigue
occurs if repetitive stresses exceed 50% of yield stress (steel only, does not work for aluminum).
So if you 1) raise the yield stress by shot peening from maybe 60,000 psi to 75,000 psi and 2) leave
a residual compressive stress to lower the effective surface stress -- you have a really nice two-fer
on reducing the chance of long term failure from repetitive loads. Yield is up to 75 Ksi and effective
stress is down to 25 Ksi, so now fatigue life is infinite. If the previous numbers of 60 Ksi yield and
40 Ksi stress were held, the part would eventually crack - 40 Ksi is well above 50% of 60 Ksi yield.

And shot peening isn't very expensive, either. Most connecting rods are shot peened, as are a LOT
of highly stressed auto engine parts, especially in hotrod situations. Makes good sense to do
this to gun parts, too. It does leave a pebbly looking surface which can't be disturbed or you lose
the effect, so no good for finely finished surfaces. Look at a connecting rod (steel only) if
you want to see a shot peened surface.

Tumbling just lightly abrades the surface, no particular stress improvements other than polishing
off scratches which can be stress concentrators if in a very highly stressed location.

Bill