Rear Take down Pin Won't Go Through Lower and Upper

[johnthomas]

Make sure your springs for the trigger are in correctly as not to impede the pin going in.

[TurboChrisB]

I had a upper /lower set once that it was incredibly tight....like I had to tap on the pin with a mallet to get it in and tap on it with a dowel to get it out...and I had a ring on it. So it wasn't like I couldn't pull hard. I just took out my drill index and found the first bit that fit it exactly and then went up one and drilled it out. Basically just took the finish out inside the holes.

I figured I just sped up the mating process. Problem solved.Wasn't loose after or anything. It's been a couple of years and still works great.

[protohyp]

[jimbo74]

sounds like it could be an issue of tolerance stacking

[4GLOCK30]

What would you recommend sanding, the lower or the upper?

I took a hammer to the pin again and it went in all the way. Hopefully I won't need to break the action anytime soon.

Thanks for the help!

I would not sand or hone anything until you confirm what is "causing" the misalignment. As noted the trigger assembly could be affecting it. Mis-matched parts can have variances which then can be lightly sanded (emery cloth) or honed.

You should never have to use a mallet to get a take down pin in.

I assume you pushed back the detent pin to get the take down pin all the way in.

Put a few hundred rounds through it and see if it "settles" and the take down pin does not require any force to install or remove

and yes if you have another upper try it on the lower to see if you have the same issue.

[rmnc3r]

Open and close the upper and lower a few times - any rub marks on the back corners of the lower's lug recess? Yes?? file a slight chamfer the back edges /corners of the lug and try for fitment again.

Still no fit?

Measure the lug on the upper, Measure the lug recess in the lower. Compare
the recess needs to be deeper, which can't be done unless you have a mill set up, so file off a little bit of the bottom of the lug. check fit.

Of the two things I mentioned, the first usually fixes the problem.

[W.R.Buchanan]

Some good info here and alot of BS.

First: do the takedown pins have chamfers on the lead edge? If they do it will facilitate assembly even if the holes are slightly misaligned. IF they aren't and have square ends even if the holes are in perfect alignment when you put them together you will scrape off metal.

Second: it is not unusual for the holes to be misaligned and the parts still be in tolerance. They are just on either end of the tolerance. IE the distance between the uppers holes are on the plus side and the holes in the lower are on the low side of the nominal dimension. With parts from two different manufacturers this is not unusual at all. Next time buy both from the same outfit.

If necessary you will have to chase the holes with a .251 Chucking Reamer like Randall Rauch suggested in post #14 This will line bore the holes into a common centerline.

Third: You can fix the anodizing with a Sharpie! This is standard machine shop and plater practice for dealing with small imperfections. Please note: His lower receiver is NOT hard anodized or else it wouldn't have scratched that easily. Also regular anodizing adds virtually nothing to the size of anything.(Less than .0001!) Hard anodizing usually will have a call out as to thickness. That thickness will be divided between the outside and interior of the surface. In other words it penetrates the same amount into the metal as is added to the outside. Seldom is hard anodizing deposited more than .001 thick. In a hole this would reduce the diameter by .002 ( .001 on each side of the hole, and in Threads it is +.004 due to plating of both sides of the thread profile.)

Since you already beat the snot out of this gun I would suggest having someone who actually knows what they are doing ream your holes for you. It will be the difference between ruining your parts or making them right.

Hope this helps.

Randy

[67Cuda]

My DPMS .308 upper and Colfax lower would mate up great, until you tried to put the pins in, the holes were out of alignment.
What I did was use pins, I have pin sets, and find out how close I could get each pin to each other and have it fit together.
I then turn down the lower pins to match. It was like -.002 on one and -.003 on the other.
Fits nice and tight, solid when together. Push the rear pin out to open and it's, loosy goosy.
Doesn't hurt its accuracy.

[bauern]

Binding at the front pivot pin will also cause the misalignment at the rear that you're experiencing. Check for any witness marks in the area between the lower's front ears and the back of the upper's front lug. Can you insert the rear pin through both upper and lower if you don't insert the front pin?

[sbo80]

He already said the pin went through the lower fine when alone. I wouldn't ream the holes through the lower just yet. I had one just like that, you could clearly see the upper didn't go far enough down, it was a forged upper on a billet lower. Some light sanding and filing on the rear lug on the upper did the trick. It wasn't the through hole that was the problem, it was the edges of the lug rubbing and keeping it from seating far enough down.

[FlyingShooter]

Take the upper & lower to a gunsmith and have them drill through the rear takedown pin holes with both attached. They’ll then hit it with some Alum-black and you’ll never tell anything was done, yet the pin will slide through with ease. Usually $20-$30 at most places...

[17+1]

[BarrettM99]

Try different uppers on that lower and vice versa. That should be step 1.

[bauern]

He already said the pin went through the lower fine when alone.

That's why I asked if it goes through both obviously when assemble without using front pin first.

[ZombieLivesMatter]

If you were by LA I was going to offer you test on the few stripped lowers and uppers I have, but it sounds like you got it it in already.