[DoD]

How does the Blish lock work in a M1921 Thompson SMG? This is what I figured out from an explanation I found.
http://www.arcor.de/palb/alben/26/863426/1024_3736633561313862.jpg
When the bolt (1) moves backwards, it presses the lock's (2) extensions (3) against the receiver (4). Due to the different angles the lock is forced upward. When the extensions reach the top, the bolt is freed. Am I right with it? I know the proportions are false, it's the principle that counts.

[dfariswheel]

A little more to it.

The Blish lock supposedly works on "the Blish adhesion principle".
This is the idea that two dissimilar metals will tend to stick together momentarily under pressure.

James Blish was a Naval officer, who noticed that when firing blank practice rounds from big navy guns, the interrupted breech guns would often spontaneously open the breech block and the case would fall out.

He believed this "adhesion principle" could be used in a small arm.

When General John Thompson formed Auto Ordnance, the original plan was to develop a 30/06 semi-auto or automatic rifle.

After much experimentation, they came to the conclusion that the Blish lock just couldn't handle rifle pressures, so the development turned to a pistol caliber "trench broom" concept, and the rest is history.

The Thompson-Blish lock in the Thompson gun has the angled surfaces, and an aluminum-bronze "H" lock.
The lock sticks to the angled surfaces and slows the rate of fire, more than the light bolt would allow in a straight blow-back system, or if a steel "H" lock was used.

Today, there is some doubt whether the Blish Principle as stated by Blish, is actually valid.

[Jim K]

The Blish principle does seem to have some validity. It is often said that it never worked because the M1 and M1A1 Thompsons did not use it and showed it was not necessary. But those guns had a heavier bolt and were beefed up at the rear because they had problems with straight blowback.

It is also often written that the British "took out the block and threw it away". That is nonsense. Without the block, the bolt cannot be cocked, since the block provides the connection between the actuator (cocking handle) and the bolt. It would be possible to screw or weld the actuator to the bolt, but it would not be something the troops would be able to do themselves and might well cause other problems. The story should be taken with a grain of salt.

In either case, the bolt is relatively light and has a light spring. This is possible due to what is called advanced primer ignition, or API. With an open bolt gun, the forward movement of the bolt provides the firing pin blow. Whether the gun has a hammer* like the M1921/27/28/M1 Thompson, or a firing pin made as part of the breech face like the M1A1 Thompson/STEN/MP.38/M3, the primer is fired BEFORE the bolt fully closes and loses its forward momentum. This means that the blowback of the cartridge has to overcome not only the mass of the bolt but the residual momentum of its forward movement. In other words, API causes the bolt to act like it is much heavier than it really is.

But the current semi-auto Thompsons fire from a closed bolt (mandated by BATFE) so it must have a heavier bolt and a heavy spring, since they are not able to use API. This is why it takes two men, three small boys, and a tow truck to cock the darned things.

*The TSMG does not have a "hammer" in the conventional sense; it has a rocker, called the hammer, which is shaped like a triangle. It is pinned in the bolt and when the bolt closes, its bottom part strikes the frame, causing it to pivot and drive its top part forward. The top is in contact with the rear of the firing pin and drives it forward to fire the primer. The M1A1 is the only TSMG that does not use this system.

Jim

[dfariswheel]

I still occasionally hear that old gag about the British Commandos in North Africa "Throwing away the Blish lock, and the gun not only worked, it fired even faster".

Not without a connection between the actuator and the bolt it didn't.

When I bring that up, the story teller stops and stares at me for a moment.

In the late 1920's Mr. Paine, the actual inventor of the Thompson, developed a simplified Thompson gun that used fewer parts, including doing away with the Blish lock.

In a charming display of a day long gone by, General Thompson refused to allow the gun to be manufactured, stating that he'd shaken Mr. Blish's hand and agreed to use his lock.
END OF STORY, per the General.
A gentleman had given his word to another, and that's all that counted.

Every Auto Ordnance gun up until Savage developed the M1, used the Blish lock.

[DoD]

Thank you for the replies, but does the drawing show the proper way the parts interact with each other?

[BigG]

I think you have the right idea but the surfaces are cut at opposing angles to retard the movement of the block and thus the bolt. It's an expensive solution to a problem that others solved by making the bolt more massive.

[Matt G]

Without the block, the bolt cannot be cocked, since the block provides the connection between the actuator (cocking handle) and the bolt.

I believe I've fired a '28 Thompson with the Blish lock removed. Lemme check the man who took it out.

[Johnny Guest]

I. too, had read the gun would function w/o the B Lock, and decided to try it once. Turned out, without the articulation provided between the actuator and bolt, it is VERY difficult to make it run. If one lost the lock or something it is sort of possible to make it run without it. Holding the gun muzzle up, the actuator just slides rearward. You CAN take a screwdriver and use that to withdraw the bolt to full back position. (Don't actually try this, unless you have a beater gun. You stand a very good chance of marring the bolt.)

THEN, with the bolt cocked, put the safety ON, insert a magazine, and proceed as normal. When the box mag goes dry, the bolt SHOULD lock back, and so forth. If you let the bolt run foward, you need to go thrrough the whole thing again.

The actuator would now NOT be part of the assembly, and the bolt alone, with the lesser mass, would undoubtedly yield a LOT higher cyclic rate.

Have I tried all this? Yea and nay. I DID do all this stuff with my 1928, EXCEPT shoot it. I saw no point in endangering a good piece of machinery to prove a no-account point.

I did some checking, and there is ONE account where the writer says you can grind the ears or "wings" off the Blish lock and the gun will work normally. Yeah, right. ;)

So, I kinda, sorta, believe the "run without the Blish Lock" legend - - It just wouldn't run well. Don't lose your B Lock! :p

Best,
Johnny

[DoD]

So, if you own a 1928 Thomson, can you tell me if it's bolt works the way the above drawing shows?

[Johnny Guest]

Well, KINDA that way. Let me see if I can locate a drawing for you. My talents do not extend to posting digital photos - - Or even taking them very well. :(

If I can remember, I'll bring my gun to the office tomorrow and see if one of the crime scene guys can shoot a couple of images that depict the relationship of the parts. Trouble is, once it is assembled, it is hard to tell what's happening inside. ;)

'Til later, I hope - -
Johnny

PS - - Your original question mentioned speficially the 1921 TSMG. Mine , as I said, is the '28 model. While I fully expect they operate the same, I must admit that I've never taken a '21 apart.

JPG

[DoD]

If I can remember, I'll bring my gun to the office tomorrow and see if one of the crime scene guys can shoot a couple of images that depict the relationship of the parts.
Looking forward to it. :) Any good pictures, especially those of the bolt with the lock inside would be greatly appreciated. All I could find are these two pictures of the lock itself
http://img20.exs.cx/img20/7580/blishlock.jpg http://www.blish.org/gens/pics/blish_lock_brass.jpg
plus some microscopic scale photos of the bolt which isn't much helpful to understand the function.