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Discussion Starter · #1 · (Edited)
Recently started having gas problems in my Gen 2 MPX and got to the point of not being able to eject or feed anything. MPX is not my go-to rifle because I haven't taken the time to replace the flash hider with a 3-lug mount for suppressor use.

Only about 800-1000 rounds thru the MPX and most have either been under powered frangible or +P+ rounds. Big difference in the gas flow between the two types of ammo, but the MPX generally eat it up with an occasional stove pipe with the low gas frangible (happens with every firearm I've ever shoot with this stuff and just par for the course).

Took the Gen 2 out to test some Gen 1 mags I bought on sale and had nothing but problems. Just chalked it up to Gen 1 mags and didn't think much of it. Next outing with only Gen 2 mags and the MPX did even worse with nothing but stove pipes with the +P+ and the frangible barely moved the bolt with the spent case remaining in the chamber.

Decided to clean the gas system for the first time. Read the manual and watched a YouTube video to make sure I knew what I was doing. The gas system didn't look too dirty, but cleaned the carbon residue. The tappet and gas valve sub assembly didn't look like any issues so I cleaned and put back together.

Another range trip and again the MPX was way under gassed.

Back home and again clean the gas system and check before re-assembly. This time I double check the gas port wasn't clogged - should have done the first time. Again the tappet and gas valve sub assembly looked normal to my untrained eye - with untrained being key here.

Back to the range and still a no go. Still way under gassed.

Finally call Sig Customer Service. They agree there is some type of issue and agree to send a return shipping label.

I break the MPX down again to clean before sending it back. My biggest worry was that they were going to say I need to clean the gas system more and their test fire didn't have any issue. This time I immediately see that the screw-in value at the end of the gas valve assembly was backed out. Normally there had been about two threads exposed - there were four exposed threads now. The screw-in valve easily twisted by hand. This very much explains the under gassing.

A closer look at the gas value assembly, I see the retaining pin is missing. The pin held the screw-in valve in place with the pin going thru the screw threads. Without the pin the screw backed out causing gas to be released out valve instead of functioning the gas system.

What I don't know is whether there ever was a retaining pin or the pin backed out. I suspect that the pin backed out because there is a retaining pin mark on the threads. I questioned if that mark is machined in the threads and instead believe the pin was pushed thru the threads. For what ever reason, my pin didn't hold.

Does cause me concern and now will always think it will happen again.

Called Sig back to cancel shipping the MPX back and instead asked for the Gas Valve Sub Assembly (Part # 1810577-R). The Rep was honest and stated they are not shown in stock, but took the order and stated it would have to be filled from production with no shipping ETA. So now I wait to see how long before I get my new valve.

Tried to order a second one at my expense and he wouldn't let me. I figure all the production is going toward the exchange barrels Sig is finally putting out and eventually there will be more in stock. Do plan to get a spare in the future.

I know this post is long, but trying to add to the knowledge base for others having the issue in the future.
 

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Discussion Starter · #2 · (Edited)


Gas Valve Sub Assembly. Note the large amount of exposed threads. This is bad.






Inside of the Gen 2 Gas Valve Sub Assembly. Note the retaining pin hole toward the end of the assembly. Mine doesn't have the pin in place.







Screw-in valve at end of Gas Valve Sub Assembly. Note the pin mark thru the threads.







Empty retaining pin hole.
 

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Discussion Starter · #3 ·
Mutenaye, I did already order the ILWT Restricted Plugs before I found my exact problem. I posted in another thread asking if anyone was using these yet and got no responses. Guess I'll be one of the first to try them.
 

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Discussion Starter · #4 ·
Looks like we are on the same page. For now I plan on just leaving it the way it is on the off chance that Sig asks for the valve assembly back.

I figure that all I need is a new pin to rebuild the valve. Guess I'll also need a spanner wrench small enough to screw the valve back into the proper position. Figure if I line up with the old pin, insert new pin with adhesive and should be good to go.

Or may be worth to have a smith do it just so I don't have to buy a spanner wrench I'd likely never use again. Any ideas on who would be interested in rebuilding?
 

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I had the same issue...almost. Saw that the roll pin was almost all of the way out, so I tapped it back into place. Next outing and it started walking out almost immediately. I quit shooting it and this time, after tapping it back into place, I took some stainless wire and threaded it through the roll pin, and twisted the ends together below. That "temp" fix was about 2000 rounds ago, with no issues since.

Hope that helps
 

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Discussion Starter · #7 ·
Thanks for the info Mnolan660R. Seems like this is a potential issue that MPX users should keep an eye on.

On a side note I got a shipping notification from ILWT for new barrel and their gas valve set. Hope to try out both of them next week.
 

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After reading this thread I checked the roll pin in my MPX carbine each time I cleaned it.

Low and behold today during cleaning I noticed the roll pin walked out about an eighth of an inch.
I took the gas valve off the barrel and removed the roll pin. Cleaned the pin and hole with solvent and blew dry.
Installed pin using Loctite 620 retaining compound. This stuff is good to 450* F.

We'll see now if that little critter walks out. :)
 

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Discussion Starter · #9 ·
Good catch.

I've received my replacement from sig and the two "restricted" plugs from ILWT. Haven't had enough time to truly test the restricted plugs, so I haven't posted anything about them.

I just say at this time, I haven't noticed any difference between the factory and the restricted plugs. But remember that I haven't done any real testing yet. I can say that the ILWT plugs work. ILWT now has a standard plug and the best part is that none of their plugs use pins like the factory. So no worry about working loose.
 

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I'm glad to have found this forum and this discussion. The same incident just happened to my Gen2 MPX after only my 3rd round of extended firing (150 rounds or more). This time, upon removing the barrel and beginning to remove the tappet and gas valve for cleaning, I found that the tiny retaining pin holding the unnamed threaded end of the gas valve had backed out about 90% of the way, allowing this piece to loosen so that 3-4 threads were showing (should be none). This part is not addressed in the owner's manual so I was unsure whether or not it was meant to be cleaned at all. I removed the pin the rest of the way and cleaned it along with the end piece, but upon trying to reinsert the pin and get it lined up with the holes in the threads, I unfortunately bent the pin... A quick call to Sig and I've got a new one on the way, but if that doesn't work (or I mangle it again) then a replacement gas valve subassembly will likely be required. Thanks for providing the part number!

My advice to all MPX users is to check this part after every round of firing regardless of the extent to which you strip and clean the firearm... it seems there is a trend beginning to develop which should be addressed in a preventive manner. I plan to install the new pin with Loctite and keep a close eye on the gas valve, but now wonder if I'll have to worry about the gas valve failing at the range. The good news is, I had no issues during the firing session immediately prior to discovering the problem. The bad news, perhaps I was only a few more rounds away from being undergassed as aleve90 described. It makes me concerned for the long-term reliability of what has quickly become my favorite regular-use pistol.

Will be watching to see if the ILWT"restricted" plugs do the trick. Although the stainless wire securing method sounds intriguing too.
 

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Discussion Starter · #11 ·
Thank you for adding to the knowledge base JML36.

Sounds like we were in the same boat and I share your concerns about this happening again and until it is addressed I can't consider the MPX more than a range toy.

Also want to add that In Lead We Trust now has a regular "Gas Plug." https://www.inleadwetrust.com/collections/mpx-parts/products/mpx-style-gas-plug

I bought their two restricted gas plugs and have had success with them. Just haven't tested the "restricted" part too much. The good news about the ILWT plugs is that they do not rely on Sig's retaining pin. No pins to back out. Just insert and go. I think this is a winner just for getting rid of the Gen II flawed design.
 

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For some reason I cannot see the images posted. Instead of the image I get message to update my account to see third party hosting. I have had the same problem with my 300 barrel after a couple hundred rounds with no issue. Since I also have a 5.56 barrel I compared the two piston assemblies and I could find no difference. Could some kind soul enlighten me about enabling 3rd party images?
 

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For some reason I cannot see the images posted.
The Forum's having these issues with Attachments, which thus far remain unresolved...
 

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After reading this thread I checked the roll pin in my MPX carbine each time I cleaned it.

Low and behold today during cleaning I noticed the roll pin walked out about an eighth of an inch.
I took the gas valve off the barrel and removed the roll pin. Cleaned the pin and hole with solvent and blew dry.
Installed pin using Loctite 620 retaining compound. This stuff is good to 450* F.

We'll see now if that little critter walks out. :)
Just an update on my previous post.

I have a little over 1K rounds down range since I secured the little retaining pin. It has remained tight and hasn't moved. The Loctite 620 retaining compound I used is some fantastic stuff.
I believe what is happening is the gas valve expands at a different rate than the pin as the barrel heats up. Thus allowing the pin the move.
A tighter fitting pin would be the best fix if one could be found otherwise Loctite 620 or a retaining wire will work . . . . . . or a new gas valve.
 

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Thanks aleve90 for re-posting the photos (I couldn't see the previous ones either) and for the link to the ILWT regular "gas plug." I put one on order (free shipping is a nice touch!) pending receipt of the Sig replacement pin. Thanks too, Shagbark for your update. I will perhaps go that route first and retain the ILWT plug as a backup. Always have a backup! Does seem like a potential design flaw, in that a major malfunction due to such a tiny pin shouldn't be possible. I hope Sig will address this issue quickly. In the meantime, be wary!
 

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Update to my previous post. SiG customer service sent me the wrong part, the pin I received was much too large and I could not begin to imagine where on the firearm it is used. The next step was for SiG to send a shipping label and for me to box up the complete upper assembly and send it in. Inconvenient for sure and hard to be without a working MPX for a few weeks. But no charge to me so I can't complain.

I received the upper back a few days ago and removed the barrel to examine the gas valve. Guess what? The part they installed is a solid piece, no threaded end or tiny pin to worry about backing out. It looks very much like the ILWT piece I have now, except that the groove in the end that is inserted into the gas block goes all the way through the edge of the valve -- on the ILWT piece it's more like a small notch that does not penetrate the edge of the valve. I'll try to attach pictures to show the difference.

Bottom line, there is no more tiny pin to worry about... I think this constitutes a permanent fix. Good on SiG to come up with a replacement part that eliminates to possibility of failure. I'd advise anyone who has a gas valve with the pin to have it replaced. Unfortunately that means sending in the upper for a few weeks, but you can be confident that when it's returned to you your worries should be over.

First photo: New gas valve from SiG. Second photo: ILWT gas valve on the left, SiG valve on the right. Note that the groove in the SiG piece goes to the edge of the valve
 

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Discussion Starter · #18 ·
Thanks for the update.

Seems Sig has made an update and I think it is a step in the right direction.

Guess I'll have to wait for my replacement valve to fail again and request a replacement to the better design.

Right now I'm looking at the new ILWT adjustable valve. Just had a bunch of unexpected expenses and don't have the money for anything, so no new valve for me.
 

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Update to my previous post. SiG customer service sent me the wrong part, the pin I received was much too large and I could not begin to imagine where on the firearm it is used. The next step was for SiG to send a shipping label and for me to box up the complete upper assembly and send it in. Inconvenient for sure and hard to be without a working MPX for a few weeks. But no charge to me so I can't complain.

I received the upper back a few days ago and removed the barrel to examine the gas valve. Guess what? The part they installed is a solid piece, no threaded end or tiny pin to worry about backing out. It looks very much like the ILWT piece I have now, except that the groove in the end that is inserted into the gas block goes all the way through the edge of the valve -- on the ILWT piece it's more like a small notch that does not penetrate the edge of the valve. I'll try to attach pictures to show the difference.

Bottom line, there is no more tiny pin to worry about... I think this constitutes a permanent fix. Good on SiG to come up with a replacement part that eliminates to possibility of failure. I'd advise anyone who has a gas valve with the pin to have it replaced. Unfortunately that means sending in the upper for a few weeks, but you can be confident that when it's returned to you your worries should be over.

First photo: New gas valve from SiG. Second photo: ILWT gas valve on the left, SiG valve on the right. Note that the groove in the SiG piece goes to the edge of the valve
Am I reading you correctly that the new Sig gas valve is one piece without any guts? (bevel washers, threaded plug), just a straight hole thru and thru? If I took the old Sig valve and removed the guts, it would essentially perform the same as the new Sig valve? Thanks for your help!
 

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I am actually planning to pick up an MPX tomorrow and just happened to run across this thread during my lunch break. I'm so glad I learned about this before taking my new gun to the range and potentially having the same issue after a few times. I'm also wondering if Sig has addressed this in their production with the fix so I won't have to send the upper back for a proactive fix. Once I get my MPX I'll break it down and check and report back here.
 
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