Hey guys new problem. I built a new upper receiver to mate with a S&W M&P lower. But the takedown pins are so tight I have to use a punch and hammer to push the pins through. Any suggestions? Thanks!

Are they tight because the holes themselves are undersized? Or do you mean they're difficult to push in because the upper to lower fit is so tight? Most people dream of the latter.

Either way, oil and use should loosen them up.

Yep. Oil them. Use them. They'll wear in.

quote:

Originally posted by m499:
Are they tight because the holes themselves are undersized? Or do you mean they're difficult to push in because the upper to lower fit is so tight? Most people dream of the latter.

Either way, oil and use should loosen them up.

Good question, I'll have to take the calipers out and take some measurements.

Back in the ban era days, I had an Olympic Arms that was tight for every upper I tried ... I had to set it on the corner of the bench and press down and tap the rear pin in and tap it out with a punch when I needed/wanted to take it down. I think it was technically out of spec. It started to look like it was cracking at the detent pin hole, I decided to let it be someone else's problem.

To tighten up a receiver I like JP Enterprise tension pin, makes it rock solid by pulling the upper and lower together. The only drawback is that it takes an allan wrench to loosen/tighten it.

Can you easily push the pins in without an upper attached?

Maybe add a drop of oil to the detents if they do. Otherwise, its most likely the holes of the upper and lower that are slightly off, causing the pins to bind up as it tries to align the holes together.

It might be tolerance stacking. Larger lugs on the upper and a smaller lower. Or visa versa. You can try oil, you can play with the order you remove the pins, you can also set the rifle on it's stock and it might help.

I've had similar issues with an M16A2 in basic and my new Larue upper is very tight in a S&W lower.

What others have said. Drop of oil in the grove the detents run in. Then just shoot the gun.

How often are you taking the thing apart?

I've had to pull the pins out and roll up some fine grit sandpaper and clean the holes a tad. You can do that with the pins instead, they won't rust.

At a minimum I always run the correct drill bit through the holes to make sure they are OK.

I still have one upper/lower that requires a little tap with the delrin pin punch made just for this purpose. The detent slot is lubed with a light smear of grease when I assemble a lower.

Maybe clip a couple of coils off the detent springs...I've been to that rodeo including safety selector springs

Keep a little bit of oil or grease on the pins and they will loosen up a bit over time.


It isn't unusual. Some people pay extra or take extra steps to snug uppers and lowers.

I've owned like 40 of these rifles at this point. Tight pins, or super tight pins, I do not live with. Has to be hand tight pins that can be removed without tools for me.

Place the front pin in. Close the upper and lower together WITHOUT pushing the rear pin in. Use a flashlight and look in that hole. You will see tolerance stacking. You will see a raised area, where the upper and lower holes don't align properly.

Take a round file to the upper hole, and with only a few swipes, it will knock off the thousandth of so of tolerance stacking that was causing issues. Now your gun is perfect, with proper finger tight pins.

The reason tight pins are so common is that the holes in the takedown pins are NOT finished as they should be from an Engineering standpoint. What is missing is a small CHAMFER around the hole for the detent pin. As a result there is no "ramp" to ease disengaging the detent pin. Because when engaged what you have without any chamfer is a sharp edge digging into the 45 degree tip on the detent pin. What happens with usage is that over time the force applied by using a punch actually peens a small ramp into the takedown pin which allows it to release with less force. Downside of this is that you are using a hammer and punch to free the pins and chances of a missed strike causing damage are rather high.

Since I don't like using a hammer on any of my rifles I have used a small 2mm diamond ball shaped burr in a Dremel to chamfer the detent holes on any rifle I build. It only takes a minute or two and makes a huge difference in how the takedown pins release. BTW, by small chamfer I do mean small, using calipers measuring in the range of 0.020 to 0.030 inch for the complete width of the ramp, meaning a chamfer defined as 0.014-0.021 x 45 degrees. I also only apply the chamfer on the release side because there really isn't any need to do the full diameter. End result is that in only take a press with about 5 lbs. of force with my thumbnail to release either pin and for those concerned about pins popping loose think about where a force that would overcome this detent would have to come from in actual use. Hint, about the only force that would arise is in the event you dropped your rifle perfectly on a stick or rock shaped perfectly to pop the pin free. During live fire any forces produced by recoil are aligned with the direction of the barrel and are 90 degrees to the axis of the takedown pins.

Now, some bad news. Learned this lesson once when I forgot to chamfer the pins on a rifle I had just put together. The rear pin is easy peezy to get free simply by removing the handgrip. The front pivot pin isn't, it can be a bit of a PITA. However if you go in thru the slot in the pin with a straight angled blade dental pic with a with a bit of patience you can press that detent pin up into its recess and rotate the pivot pin 90 degrees. Then you can push the pin out with a 0.25 diameter punch to keep the detent pin in place. Note, leave the punch in place, if you allow that detent pin to escape you will have to remove the handguard and barrel nut. Then to re-assemble you push the punch out with the pivot pin and rotate the pin to release the detent into the slot.

quote:

Originally posted by Scooter123:
if you allow that detent pin to escape you will have to remove the handguard and barrel nut.

Why?

Just detach the upper from the lower. If that detent pin does happen to escape, you can easily reinstall the spring, detent pin, and takedown pin, just like you would when building a stripped lower.

It's certainly easier in the grand scheme to keep the detent pin captured as described, since getting the front takedown pin assembly put together can be a bit fiddly, but failing to do so does not by any means require disassembling your upper...

When I have a nasty fit I just use a small reamer to allow it all to go together with the amount of force I'm ok with. takes about 3 seconds. also has the effect of Scooter123's chamfer as the reamer is tapered.

You can also buy extended take down pins at Brownels or elsewhere. They give you something to grab.