Takedown Pin Won’t Fully Insert with New Upper

[cl2]

Searched, but didn’t see this topic listed…

I received a name-brand upper (I’m reaching out to the upper manufacturer to see if they have any thoughts as well) and went to put it on an existing lower I have, but the takedown pin won’t fully insert.

The takedown pin will encounter severe resistance once it gets about 3/4 of the way to being fully seated. If I wish to get it in all the way I have to pound it in with a rubber hammer. Taking it out is equally difficult. The pivot pin has no issues.

The lower in question has had two different uppers on it without encountering this issue and was sitting in the safe alone, hence the new upper.

The new upper fits perfectly fine on the four other lowers I’ve tried it on (each has an upper associated with it already which were tested with the problem lower).

The old lower fits 2 of 4 uppers fine, a third is a bit of a tight fit, but no real problems, and the fourth goes about 4/5 of the way before extreme force is required to finish the job.

I initially thought the takedown pin might be on the large side so I tried sanding it, but that didn’t really help. I pulled the takedown pin from the lower and it drops right into the retaining hole in the upper without any resistance. A new takedown pin similarly drops right into the upper.

Looking through the takedown pin hole with the receivers together there is a bit of a high spot in the lower when compared to the upper.

Seeing as how this new upper fits other uppers with now trouble and the manufacturer clearly test fired it (brass marks on the deflector) I don’t think it it the problem.

Does anyone have any suggestions for a home-fix for this?

Thanks for any suggestions


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[georgeib]

Try sanding off a couple thousandths off the bottom of the upper's takedown boss. It sounds like it's not allowing the upper to fully seat in the lower. You could try using some layout fluid first to verify that, but it's probably easier to just do it.

[cl2]

Thanks for the suggestion! I’ll try that next.


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[bamashooter]

The bullet style / shaped Cratex abrasives for rotary tools are extremely useful for applications such as this. Wide-range of coarsenesses and are very forgiving. Your anomaly is not all that uncommon. I've cleaned up 3-4-5-6, etc pin openings using these. In my experience it's typically a tolerance-stacking issue as compared to a nano-burr.

[cl2]

I put a dab of lithium grease in lieu of the fluid George suggested on the bottom of the takedown lug and put the upper and lower back together, but there was no trace of grease on the lower or signs of the grease on the lug being disturbed.

I tried filing the upper to see if the would open up the clearance enough to get the pin to work.

When I put the pin into the receivers it encountered no resistance and dropped tight in. Except the pin was inadvertently rotated 90 degrees (cut for the retraining pin was facing down, not towards the back of the receiver). The pin would not insert rotated 180 or 270 degrees. The spare takedown pin I have behaved the same.

I think it’s a tolerance stacking issue as well as separately both lower will work with other components.


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[cl2]

The bullet style / shaped Cratex abrasives for rotary tools are extremely useful for applications such as this. Wide-range of coarsenesses and are very forgiving. Your anomaly is not all that uncommon. I've cleaned up 3-4-5-6, etc pin openings using these. In my experience it's typically a tolerance-stacking issue as compared to a nano-burr.

Based on what I found with the takedown pin experiment, this seems like a viable path.

Any suggestions on the correct coarseness for this application?


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[cl2]

I got out the Dremel and ground one section of the takedown pin flat.

Problem solved

Thanks for the suggestions George and bamashooter.


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[georgeib]

I got out the Dremel and ground one section of the takedown pin flat.

Problem solved

Thanks for the suggestions George and bamashooter.


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Glad you got it solved. Thank you for the update.

[Todd.K]

Is the upper or lower painted?

[cl2]

Neither were painted. Just factory anodizing.

I was able to try the new upper in several other lowers and it plugged right in to all of them. It was just this one lower that requested customizing.

I didn’t try the lower in any more uppers before I went to work on the takedown pin.

I guess the tolerances stacked the wrong way for those two.


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