MP40 dumb question

[Rick C]

Just a dumb question or two for the pros - Stole the images below from a web site. My questions are:

1. How high is the “breach pin” pictured below supposed to come ‘up’ to lock in the receiver?
2. Is it’s height (how far it penetrates) adjustable?
3. Are spares available?
4. How the heck is it removed?

Thanx, RC

[Gran Sasso]

2. Is it’s height (how far it penetrates) adjustable?

No - when you pull it, you have to turn it a quarter, so it stays up. Now the MP could be taken apart.

[Rick C]

Understand how to get it apart. Problem appears to be that the pin doesn't go 'in' far enough to hold it all together.

Pull back the bolt, pull the trigger & the action comes apart...

[armorrich]

Hi RC....i get your point. Starting with simple stuff...make sure there's not a buildup of grease, dirt or something else under the head of the knurled nob thats preventing it from seating fully up inside the handguard thus keeping the pin from penetrating deeply into the cavity for which its intended. Probably not the problem but ya never know.....cheers....rich

[Rick C]

Hey Rich, Always nice to hear from the Army on such matters as "grease", "dirt" or "penetrating deeply into the cavity" which, of course, makes me ponder if your talking from the perspective of the 'old Army' or the 'new Army'

Seriously though (I know your "old Army"), your other comment though: "see if there's a drift pin that runs horizintally through the side of the knurled nob that holding it to the shaft" - there is 'something' (below). Would that be a drift pin?

[John Hodgin]

Understand how to get it apart. Problem appears to be that the pin doesn't go 'in' far enough to hold it all together.

Pull back the bolt, pull the trigger & the action comes apart...

Something is clearly amiss.

The pin is spring loaded, and when the knurled cap is turned so that the pin seats, it should strongly do so. If yours is missing it's spring, or the spring is weak, therein is the problem. If this is not the case, there may be problems where the pin seats in the receiver, or the receiver end of the pin is worn.

[Rick C]

The pin does seat "strongly". "there may be problems where the pin seats in the receiver, or the receiver end of the pin is worn." That was my thought as well which is why I wondered how 'high' the pin is supposed to be...

[John Hodgin]

Then there may be a larger problem. The receiver locks into the end cap above the pistol grip when the magazine housing is rotated inline with the pistol grip. Is the receiver separating from the end cap without rotating the mag housing outward in a clockwise direction?

[Rick C]

That 'end' doesn't seem to be the issue. The reaction of the bolt slamming forward disengages the receiver at the ejection port end. Thus, I don't think the pin or whatever it's called, is making good upward contact locking everything together.

At first I thought the oval hole to receive the pin might be too small but my caliper says otherwise. Then I thought the pin wasn't going 'up' enough to get into the oval opening. Therefore my query about adjusting the pin or how high the bugger should actually be.

[Alan Smith]

Rick,

When you get the receiver tube assembly back into the frame assembly, line everything up like you think it goes together. Then, turn the knurled knob a quarter turn until it is released to pop back into place. At that point, gently twist the receiver tube around just a little bit by grabbing the mag housing and twisting it slightly back-and-forth to each side of the gun.

It may be that the pin is not lining up with the hole in the receiver tube, so that playing around with it just a bit will permit the pin to snap in place. You will for sure hear/feel the pin securely snap into place and you will no longer be able to twist the receiver tube back and forth.

I hope this suggestion is of some help to you. Good luck.

Alan

[John Hodgin]

That 'end' doesn't seem to be the issue. The reaction of the bolt slamming forward disengages the receiver at the ejection port end. Thus, I don't think the pin or whatever it's called, is making good upward contact locking everything together.

The pin does not hold the receiver to the lower, the pin simply keeps the receiver from rotating clockwise and then disconnecting. The receiver and lower are held together at the front just behind the magazine release button. A narrow portion of the curved lower fits tightly within a curved "ledge" on the rear of the mag. housing area. See dark red arrow. Perhaps this is the problem.

Even with the breech pin extended the weapon should not come apart without rotating the receiver via the mag. housing!
Or put another way, the weapon is held together at the red and green points, not by the breech pin.

[Alan Smith]

Here are some photos of what (I think) is the pin in question. Maybe these photos will help (as the gun shown fits together just fine and does not have the problems Rick describes).

Here is an external view of the spring-loaded pin in the "down" position (as if the gun were fully assembled and ready to use).

[Alan Smith]

Here is an internal view of the pin, as it is sticking up and would be protruding into the oval slot that it mates with (the oval slot being in the underside of the receiver tube).

[Alan Smith]

Another view of the protruding pin.

[Alan Smith]

Yet another view of the protruding pin.