View Full Version : Rear takedown pin detent pin locking takedown pin in place
Got a weird issue that I cant figure out.
My buddy got a CMMG bargain bin rifle, even though I told him not to and offered to help him build a BCM or something else. Well he didnt follow my advice because he thought he knew better. Well he also didnt listen to me when I said to make sure the firing pin is installed in the BCG before re-installing it. He locked the gun up good and we had to take off the receiver extension to get it all un-stuck. Here in lies the issue.
Upon re-assembly the detent pin for the rear takedown pin keeps locking up the rear takedown pin. If its allowed to drop down into the detent in the pin(on either side) it locks into place and not even a hammer would break it loose. I tried flipping the detent pin over with no success. I am thinking it needs a new takedown pin plus detent pin and spring because I am thinking the parts kit in the lower is not good(well I know its not good, but I digress). Anybody seen this issue before and was the fix simply to replace the 3 offending parts? Anybody have a source for individual parts so he doesnt have to buy a whole LPK?
There are several places that have the parts you need.
G&R
Brownells
Midway
PK Arms
There are several places that have the parts you need.
G&R
Brownells
Midway
PK Arms
I didnt think about brownells selling just the parts. I dont think Grant sells anything besides full LPK's, at least not that I can find on his website.
I didnt think about brownells selling just the parts. I dont think Grant sells anything besides full LPK's, at least not that I can find on his website.
Look starting here: http://www.gandrtactical.com/cgi-bin/commerce.cgi?search=action&category=GRTA
Although you might as well order some other stuff too to make shipping worth your while.
It's possible that the hole in the takedown pin itself is too deep and therefore when the detent goes in, it locks up. Try putting a drop of some lube in the area for shits and giggles.
Got a weird issue that I cant figure out.
My buddy got a CMMG bargain bin rifle, even though I told him not to and offered to help him build a BCM or something else. Well he didnt follow my advice because he thought he knew better. Well he also didnt listen to me when I said to make sure the firing pin is installed in the BCG before re-installing it. He locked the gun up good and we had to take off the receiver extension to get it all un-stuck. Here in lies the issue.
Upon re-assembly the detent pin for the rear takedown pin keeps locking up the rear takedown pin. If its allowed to drop down into the detent in the pin(on either side) it locks into place and not even a hammer would break it loose. I tried flipping the detent pin over with no success. I am thinking it needs a new takedown pin plus detent pin and spring because I am thinking the parts kit in the lower is not good(well I know its not good, but I digress). Anybody seen this issue before and was the fix simply to replace the 3 offending parts? Anybody have a source for individual parts so he doesnt have to buy a whole LPK?
I had a similar problem on a new build last night except it was with safety/selector. When I installed detent pin and spring it would not work, took pin and spring out and it works fine. Upon examining detent it had a very sharp point and was burred over. My solution was to hold pin in a drill chuck and took it over to my grinder buffer, I used a leather and then hard felt polishin wheel, and buffed the burr off, then slightly rounded the sharp tip and finished up with a high polish. The end result was a smooth positive operating selector. I have done similar things on take down/ pivot pins and my do the take down on this one before everything is finished as it is a bit stiffer than I like but will try and allow it to wear in a bit first.
It's possible that the hole in the takedown pin itself is too deep and therefore when the detent goes in, it locks up. Try putting a drop of some lube in the area for shits and giggles.
Well either the takedown pin is machined incorrectly or the detent pin is not good, I am leaning towards it being the detent pin being out of spec.
I had a similar problem on a new build last night except it was with safety/selector. When I installed detent pin and spring it would not work, took pin and spring out and it works fine. Upon examining detent it had a very sharp point and was burred over. My solution was to hold pin in a drill chuck and took it over to my grinder buffer, I used a leather and then hard felt polishin wheel, and buffed the burr off, then slightly rounded the sharp tip and finished up with a high polish. The end result was a smooth positive operating selector. I have done similar things on take down/ pivot pins and my do the take down on this one before everything is finished as it is a bit stiffer than I like but will try and allow it to wear in a bit first.
Well the detent pin is not brass like it is on my DDM4, it appears to be aluminum or steel and is not very rounded on the ends. I think that is the biggest problem. Instead of trying to troubleshoot one part at a time, I just recommended that he buy a new takedown pin, detent pin, and spring and we will replace it all and be done with it.
I must have been working prior to the "work" he did on it? Correct or not?
I must have been working prior to the "work" he did on it? Correct or not?
It was working prior to pulling it apart, it was really stiff and usually required a dowel rod and light hammer hit to get it apart.
I pulled it apart, had to take the receiver extension off to get the BCG freed up.
I wonder if it wasn't installed correctly the first time. I had him order new, good pieces and I will swap them in next week.
Actually some detents are gold-ish colored and some are silver looking. I have also seen many new ones that had more of point to them than rounded.
It was working prior to pulling it apart, it was really stiff and usually required a dowel rod and light hammer hit to get it apart.
I pulled it apart, had to take the receiver extension off to get the BCG freed up.
I wonder if it wasn't installed correctly the first time. I had him order new, good pieces and I will swap them in next week.
I'll bet the pins have a sharp point on them. I got a bunch of pins from Brownells & found them to be VERY sharp. I believe they ARE steel. highly polished, silver in color, and somewhat hard. All that needs to be done is rub the sharp point on some 320-400 grit wet/dry sandpaper a little bit at a time until you get the proper "feel". I usually will take off material until there is about a .030" ( 1/32" ) flat spot on the tip. then you still have the positive stop of the detent, without having to take a hammer to the pin to get it past the detent.
Ty Hoeffer
Palmyra, VA
I worked mine back and fourth a couple of dozen times and it smoothed out so I can start it out by hand now, no tapping with a punch. These parts were in a DPMS kit from Brownells. All the detent pins had sharp points and are very hard, and like I said earlier the selector pin actually had a sharp burr on the end.
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