WH Thompson Running Too Good..

[Ryo]

I picked a WH Thompson 1928 and I've only shot it in full auto, complete mag/drum dumps. It runs really good. However I let my friend shoot it in Single mode, and it decided to go full auto on him. He knows how to shoot SMG so he just held on for the ride, finger off the trigger. I tried it in Full Auto mode to do bursts, and same result, uncontrolled runaway.

 

I believe it has something to do with the rocker not 100% contacting the bolt. Looking at the lower it looks like when the rocker trips, it allows the sear to lock the bolt back when trigger is released or locks back immediately when in single mode.

I don't believe the bolt is 100% of the time contacting the rocker since I don't hear that solid click I hear when I try to trigger the rocker by pulling the trigger and cycling the bolt. I forgot to take a photo of the bolt underside, but it looked like it was in good condition.

 

My rocker does look beat up. I don't know it was from usage or someone doing a modification to it when they replaced parts in the WH. My guess is the bolt is sliding over the rocker since there is enough of a gap to do so. I'm hoping it is just a worn rocker, if not, then I'll just work on that part when I cross that bridge.

 

 

 

 

 

Assuming I need to replace the rocker..

1. What is the easiest way to remove the pivot plate? I remember someone making a tool for this, rod with a half circle or more tip. I have 3D printer I can just print out a similar part.

 

2. Are all rockers the same.. ie if I buy one from RTG or Numrich.. they all will work in my lower?

https://www.gunpartscorp.com/products/517290

https://www.robertrtg.com/store/pc/viewPrd.asp?idproduct=3483&idOptionArray=

 

3. Anything else I should change out as mess around repairing it? ie springs?

 

Maybe some instructions would be helpful, otherwise I'm going to be disassembling it part by part.

 

Thanks!

Edited June 3, 2021 by Ryo

[TD.]

I cannot speak for the functioning problem. However, if you decide to purchase another rocker, I would reach out to Board member Phil Askew to see if he has a New-Old-Stock (NOS) rocker. The rockers you are going to find at the part houses are likely used. I would want a new rocker to address this issue. I would also ask Phil about NOS springs for the frame.

 

Phil can be reached at: philfordparts@yahoo.com

[gijive]

It is more likely the sear, sear lever, or disconnector spring, not the rocker. I would start first with the sear lever spring. There are a couple of good videos available on You Tube that show how to disassemble the grip frame and remove the parts. Sarco has Thompson spring kits available for a reasonable price. While you are in there I would replace all three of the springs I mentioned.

[timkel]

Take care when replacing the rocker. If it is installed backwards, the lower can slide on but not off..

[Ryo]

I cannot speak for the functioning problem. However, if you decide to purchase another rocker, I would reach out to Board member Phil Askew to see if he has a New-Old-Stock (NOS) rocker. The rockers you are going to find at the part houses are likely used. I would want a new rocker to address this issue. I would also ask Phil about NOS springs for the frame.

 

Phil can be reached at: philfordparts@yahoo.com

Thanks, I'll contact him for the parts once I decide what the main issue are.

 

It is more likely the sear, sear lever, or disconnector spring, not the rocker. I would start first with the sear lever spring. There are a couple of good videos available on You Tube that show how to disassemble the grip frame and remove the parts. Sarco has Thompson spring kits available for a reasonable price. While you are in there I would replace all three of the springs I mentioned.

Only reason why I thought the rocker is that it sits pretty low. and if the bolt rides over that it would easily be a run away SMG.

Other reason I thought that was when I pull the trigger and slowly let the bolt slide forward, I don't hear the rocker (or what the rocker is connected to click). It was a very distinct sound when I manually press the rocker with my finger it allows the sear to pop up.

I'll take a look at it again just to make sure. Also thanks for the suggestions on the springs.

 

Take care when replacing the rocker. If it is installed backwards, the lower can slide on but not off..

Thanks for the warning.

Edited June 3, 2021 by Ryo

[deerslayer]

Some of the bolt cavities are milled deep on West Hurley's.. so that could be part of your problem. The rocker doesn't get tripped.

[Ryo]

Thank you deerslayer for your input about the depth.. So with that in mind, I took some measurements.

 

Having the receiver upside down i measure from the bottom of the rail to the bottom of the bolt to have a gap of 0.035". So taking some paper, I folded it to be 3"x0.25(ish)" with a depth of 0.3" measured on my Mitutoyo calipers.I taped them on both sides of the lower's slide so the bolt can glide on it. I found that the bolt wouldn't touch the rocker unless I pulled the trigger very hard. Pulling the trigger with middle weight which releases the bolt will cause the bolt to not touch the rocker. Reason why pulling lightly vs pulling hard would not touch the rocker has something to do with the sear being a bit higher when the trigger is pulled, and lower (out of the way) when the pulled hard. That angle was enough to make the bolt miss the rocker!

 

So I took measurements of the rocker:

* Rocker Hole diameter: 0.243"

* Top of hole to tip of rocker: 0.445"

* From center of hole to tip of rocker: approx 0.5665"

 

Other measurements I took:

Receiver internal depth of channel: 1.327"-1.33" (I measured 1.33" at the location of where the rocker would reside)

 

Does someone know what the the dimensions of the rocker are suppose to be? I just want to see if my rocker is worn down.

If my measurements are normal for the rocker, then is my receiver depth to deep, or maybe a combination of both.

 

Looking at photos of a rocker, it looks like mine still has it full length where it counts but that's just eyeballing it. Possibly the back side of rocker worn down a bit as well so it's not pushing the disconnector?

[Ryo]

I just found some specification drawing.. Looks like my rocker might be in spec based on some drawings I found on the Unofficial Thompson site. (http://www.nfatoys.com/tsmg/)

However looking at the depth of the receiver, I measured 1.33", the spec looks like 1.300 +/- 0.006 which means I'm about 0.03" (or 0.024") from the drawing spec.

 

I know 0.03" lower would always trip my rocker (that's when put my bolt direct contact to the lower slide it to trip the rocker). That length seems so minute to cause an issue.. but it's enough.

If I can find a rocker on the upper end of the limit.. could possibly work, bandaiding the possible issue. Another way I can think of is hammering the top part of the rocker, causing to bulge higher allowing it to contact the bolt..

I'm not sure of the hardness of the rocker, but likely I would need to anneal and then reharden it..

 

Another idea is add material in the receiver which would push the bolt down a bit... but it's such a small amount not sure how to do it reliably yet. Probably has to be something like thin steel but thin steel can flex more easier..

 

What do you guys think? Yes maybe they are stupid ideas.. but I'm trying to come up with a solution.

Edited June 4, 2021 by Ryo

[StrangeRanger]

...Another idea is add material in the receiver which would push the bolt down a bit... but it's such a small amount not sure how to do it reliably yet. Probably has to be something like thin steel but thin steel can flex more easier..

 

What do you guys think? Yes maybe they are stupid ideas.. but I'm trying to come up with a solution.

 

Maybe braze a couple of pieces of brass shim stock to the underside of the top of the receiver, one on either side of the actuator slot?

That might discolor the blueing on the receiver but it should solve the problem.

[full auto 45]

My WWII M1 did the same a couple of years ago. I took the lower, submerged it upside down in my ultrasonic cleaner with simple green. I let it sit for a couple of minutes and then let the ultrasonic run for a couple minutes. While the lower "looked" clean, there was enough grit coming out of it to fill a sand box. I was surprised. It has run great ever since.

[Ryo]

 

...Another idea is add material in the receiver which would push the bolt down a bit... but it's such a small amount not sure how to do it reliably yet. Probably has to be something like thin steel but thin steel can flex more easier..

 

What do you guys think? Yes maybe they are stupid ideas.. but I'm trying to come up with a solution.

 

Maybe braze a couple of pieces of brass shim stock to the underside of the top of the receiver, one on either side of the actuator slot?

That might discolor the blueing on the receiver but it should solve the problem.

Yeah.. I probably would avoid that. I'm one of those people who would get super bothered by that, and I don't trust the WH metallurgy. I'd rather have a expert like PK or Alex to do work since they will refinish it back to new. Though for PK I'd wait 5 years to have to get on to his 5 year list.

 

My WWII M1 did the same a couple of years ago. I took the lower, submerged it upside down in my ultrasonic cleaner with simple green. I let it sit for a couple of minutes and then let the ultrasonic run for a couple minutes. While the lower "looked" clean, there was enough grit coming out of it to fill a sand box. I was surprised. It has run great ever since.

I can tell my lower has a lot of grit in it. Maybe I'll try that first but based on my measurements it doesn't seem like this would work.. but the lower does need a cleaning!

I'll complete my disassembly and dump the parts in the ultrasonic cleaner.

[TSMGguy]

I'd avoid guessing, and engage the services of any one of about four or five excellent Thompson smiths that frequent these boards. I especially would want to avoid the modification of an irreplaceable full auto receiver, unless it's done by an experienced expert.

[TD.]

I did not know the bolt cavities on some West Hurley guns are milled too deep - but it makes perfect sense. And I am not surprised. Runaway fire seems to be a common problem for some West Hurley Thompson guns. I agree with TSMGguy that working on the receiver without knowing exactly what you are going to end up with could be disastrous.

 

Question: How much trouble would it be to manufacture a slightly taller rocker? If you have the skills to make one, then make several of slightly different heights. This may be a simple fix that solves the problem for your WH and creates a market for others with the same problem.

[timkel]

Don't over think this. Start with the basics. Are you using the proper trigger pull? It's a sharp quick pull then release. Do not squeeze slowly like for target shooting.

Still no semi?

Then pull lower, disassemble, soak/clean, replace springs, replace any worn parts.

[gijive]

I really believe everyone is overthinking this. As Mike pointed out, never cleaning the internal parts of the grip frame and letting crud and oil build up can also affect the function of the sear, sear lever and disconnector spring. It's called maintenance: shoot gun, disassemble, clean parts, lightly oil, check spring lengths, and reassemble. Not a difficult practice to follow if you want your firearms to function properly. That is why the military made manuals to instruct users on the proper care of firearms.

 

You really may not have to rebuild the entire gun. Maintenance and cleaning may solve your problem.

[Ryo]

I wouldn't do any permanent modification on a expensive receiver.. at least not without a expert doing it.

I have a engineering background so I'm very analytical. I'm constantly troubleshooting for work. I'm applying my skill trying to figure out this.

 

I didn't know some WH runaway issues nor the depth issue.. though the latter is not a surprise for me as well considering the history of how they cut corners to build them. You can tell by the workmanship of the milling of the internal.

 

Any case looks like I need to shoot this in full auto to see if I have a runaway in this mode. This should by pass the rocker issue since the rocker is lowered in F/A. If it does then I may have a secondary problem dealing with the sear. Only reason i mention that is that my friend said he could have sworn he had issues when in F/A but he wasn't sure. Since I didn't see this I can't rule it out, but it seemed to lock back when doing it manually.. but we all know manual cycling isn't a substitute for live ammo.

 

Well this has been interesting.. learning new stuff. What better way to learn about your firearm than to needing to debug a problem. This month has been really strange.. discovered my runaway MG34 occurred due to a dust cover spring interfering with the trigger and anther part getting worn down.

 

Thanks again for the inputs. I greatly appreciate it.

[Ryo]

I really believe everyone is overthinking this. As Mike pointed out, never cleaning the internal parts of the grip frame and letting crud and oil build up can also affect the function of the sear, sear lever and disconnector spring. It's called maintenance: shoot gun, disassemble, clean parts, lightly oil, check spring lengths, and reassemble. Not a difficult practice to follow if you want your firearms to function properly. That is why the military made manuals to instruct users on the proper care of firearms.

 

You really may not have to rebuild the entire gun. Maintenance and cleaning may solve your problem.

 

Sorry I missed your post before responding. I do plan to clean it first. I typically like to keep my MG (and my other firearms) cleaned and oiled. Only my long range bolts I don't clean the barrel aggressively unless I have a lot of rounds through it.

 

Tonight I plan to pull my ultrasonic cleaner out. Got some Lyman cleaner and lubes to protect the metal after taking it out.

[zavrit]

If you have a runaway using the full auto setting, the rocker is not the issue. First, I would look at the sear. Is it worn/rounded? Second, look at the sear notch (or notches) in the bolt. Is it worn/rounded? When I've had runaway guns, it was worn/rounded sears (MG34 and MK760). Take pics and post if possible of these two items. Next I would look into the sear spring and sear lever spring. But, without spare springs you won't a way to compare length and/or tension.

 

The Thompson has two sear notches in an original surplus bolt. It's common to see the bolt ride over the first notch and catch on the second notch when a Thompson is being shot. Click on the link to see this occur (watch the charging handle):

Does your West Hurley conversion use an unmodified surplus bolt or a modified bolt? When I converted my West Hurley to a machine gun (post sample), I used the West Hurley semi auto bolt and can only have one sear notch in the bolt when I converted it to a fixed firing pin full auto bolt. Seeing how the sear fails to catch the first sear notch in the surplus bolt at times, that was an indication to me to make my sear cut in the bolt at a slightly sharper angle and modify the sear at the corresponding angle. The trigger pull is a little heavier than an original WWII Thompson, but I have zero issues of the bolt not being caught by the sear. And, my bolt moves up and down slightly compared to a friend's Thompson. I didn't take a measurement, but the bolt has sloppy up and down movement in my West Hurley.

 

To clean the lower, I just use brake cleaner and compressed air. Spray and blast with air a few times and that really cleans it out. I don't see a need to take it apart unless you want to inspect parts and springs further.

 

Dan

[Ryo]

Dan, you know what.. your right. Even if I'm on Single mode, if I let go of the trigger, the sear should pop up no matter what, hence stop firing. I do still think the rocker will be an issue but not the main issue I'm trying to solve. So likely I have 2 issues occurring at the same time. I've just been too focused with the first problem and didn't think it through.

 

Time to give it a bit of TLC to determine the issue. When I took it apart for this problem I did look at the sear and bolt. They believe both are surplus, the guts were changed by an previous owner. I remember wondering if the bolt would catch with the sear at it's current angle but I stopped looking at it since I found the rocker issue. I'll have to relook at the interfacing angles.

 

I serious really appreciate the information I'm getting here on this. I was planning already to donate to this site because of the information here.. but the help I've been receiving here has been a bonus.

Edited June 4, 2021 by Ryo

[deerslayer]

Some west Hurley Sears are cut in the front bottom edge to allow the back of the sear to rise higher.. And engage the sear notch better. Building up a rocker is the simple solution.

[reconbob]

Well, this has already been beat to death but here goes...

 

You can check the function of the rocker/semi auto by dry firing

The gun on semi. When the bolt slams shut the rocker should

push the disconnector out from under the sear lever and the

sear should snap up ready to hold the bolt back when it recoils.

 

1.327" is way too deep. The specification for this dimension is

1.295 + 0.005. With this depth if the sear is worn and/or the sear

notches in the bolt are worn the bolt can "jump" or over-ride the

sear instead of being held back by the sear.

 

The ammo should be checked. If not hot enough the bolt could recoil

far enough to eject and feed, but not far enough to be held by the

sear.

 

Bob

[cbmott]

If you do have to have some work done because the other troubleshooting doesn’t work, I know John Andrewski does the heavier machining if PK doesn’t have availability. He’s pretty responsive by email or phone and his turnaround is pretty good.

[Ryo]

Well I confirmed the rocker is one of the issues. With my 3 hands I manage to test that the bolt is gliding over the rocker, not triggering it by putting downward pressure on the back edge of the actuator while it is sliding closed. With no pressure the trigger is not reset and the bolt is allowed to slide forward after cycling. When pressure is applied from top of receiver's slot, the trigger has a very audible click, the trigger resets, and the bolt will lock back.

 

However the issue of the runaway is not caused by the rocker since it has something to do with the sear. I don't think it has anything to do with the ammo since I did shoot different ammo through it, and so did my friends. From Wolf, PMC, American Eagle, and Magtech. It feels like a solid lock. Manually cycling it always locks back. Of course hand cycling is way different from the cycle of shooting since the angle of torque applied would push the bolt downwards. I suspect reconbob is correct about it jumping over the sear. That would make the most sense since it appears the sear moves back in place. I suppose it could be a weak spring (or filthy lower) where it bounces, but I haven't gotten a chance to test either of those theories.

 

I did clean the lower.. and it was dirty with unburnt powder. Looked like it needed some TLC since I saw brown surface residue.. which I'm maybe guessing rust? No pitting though.

I'm going to go over the measurement of the sear and compare it to the height of the bolt and the receiver height to see if there's enough gap to cause it to go over the sear. Also I need to inspect the sear to see how worn it is. It looks like it has been used a bit, but I am far from a expert on Thompsons to know what a worn out sear looks like versus a well used one. Only reason I am leaning more over it jumping over the sear is that both issue I am having can be explained by the depth the receiver has been milled.. 0.03" doesn't sound like a lot to me.. but with tight tolerance, it's a lot.

 

Once again thanks for the suggestions. I plan to see if I can get it going without doing major rework, I enjoy this type of work. It lets my engineering brain get a bit of a workout.. and helps me understand my firearms. I'll leave any major work to the experts if I can't get it going. I know PK won't do work since I had contacted him a couple months ago and he's not even taking names down. I'm not sure what's John Andrewski handle on this site but I've definitely heard his name thrown around.. The other person I hear good work is done is by Dan Block (deerslayer).

[cbmott]

John Andrewski isn’t on the forum, he’s another well known NFA gunsmith though.

http://johnandrewski.com/

[gijive]

Well I confirmed the rocker is one of the issues. With my 3 hands I manage to test that the bolt is gliding over the rocker, not triggering it by putting downward pressure on the back edge of the actuator while it is sliding closed. With no pressure the trigger is not reset and the bolt is allowed to slide forward after cycling. When pressure is applied from top of receiver's slot, the trigger has a very audible click, the trigger resets, and the bolt will lock back.

 

However the issue of the runaway is not caused by the rocker since it has something to do with the sear. I don't think it has anything to do with the ammo since I did shoot different ammo through it, and so did my friends. From Wolf, PMC, American Eagle, and Magtech. It feels like a solid lock. Manually cycling it always locks back. Of course hand cycling is way different from the cycle of shooting since the angle of torque applied would push the bolt downwards. I suspect reconbob is correct about it jumping over the sear. That would make the most sense since it appears the sear moves back in place. I suppose it could be a weak spring (or filthy lower) where it bounces, but I haven't gotten a chance to test either of those theories.

 

I did clean the lower.. and it was dirty with unburnt powder. Looked like it needed some TLC since I saw brown surface residue.. which I'm maybe guessing rust? No pitting though.

I'm going to go over the measurement of the sear and compare it to the height of the bolt and the receiver height to see if there's enough gap to cause it to go over the sear. Also I need to inspect the sear to see how worn it is. It looks like it has been used a bit, but I am far from a expert on Thompsons to know what a worn out sear looks like versus a well used one. Only reason I am leaning more over it jumping over the sear is that both issue I am having can be explained by the depth the receiver has been milled.. 0.03" doesn't sound like a lot to me.. but with tight tolerance, it's a lot.

 

Once again thanks for the suggestions. I plan to see if I can get it going without doing major rework, I enjoy this type of work. It lets my engineering brain get a bit of a workout.. and helps me understand my firearms. I'll leave any major work to the experts if I can't get it going. I know PK won't do work since I had contacted him a couple months ago and he's not even taking names down. I'm not sure what's John Andrewski handle on this site but I've definitely heard his name thrown around.. The other person I hear good work is done is by Dan Block (deerslayer).

By the way, did you ever replace the sear and sear lever springs while you were in there cleaning the internal trigger parts? If you had rust in there, that can certainly damage the springs and cause them to break.