barrel pin

[uxo2]

have a question.

bought a off brand barrel M4 style.




The barrel pin does not drop in but is clearly oversize.

Is this normal or acceptable.
Or send it back.

3 different recivers same thing

[tujungatoes]

Shouldn't be a problem. Just massage the pin with a file till it fits in the notch, and rock on.

[Gunsmith Dan]

well a much better technique than filing (which unless you are a expert because it will cause flat spots on the pin) is using a variable speed power drill and fine sand paper ( around 500 to 600 grit ).

Take the pin and put it in the drill chuck (just enough for the chuck to grab it) and tighten it. Then take the sandpaper and cut it down to size so it is slightly longer than the pin sticking out, but enough so it can be foled at least once.

Spin the drill at a slow speed and press the sandpaper lightly onto the pin. Stop every so often (maybe every 10 seconds or so) take out the pin and try it in the hole. This will give you a pin that should fit nice and snug plus not wobble because it has a flat spot. Just do not be to aggressive as you can not put back the metal you took off.

Hope this helps

[tujungatoes]

As I recall those pins are pressed into the extension. Removing it to do a ghetto lathe operation may not be an option, and trying to remove it may mar the pin and cause further headaches. Hence my recommendation for using a file.

[BSlacker]

The pin is backed up by a fence(barrel shoulder) good luck running a file passed that. Removing a pressed in pin that close to a fence without damage to some part or the other is next to impossible. You will need a new pin after you scar up your new barrel.
Call the maker. It is your best option. If you can, measure the slot and pin just to be sure which one is out of spec before you call. One or both are out of spec why mess with a lousy fix on your build.

[BSlacker]

One or both are out of spec

It maybe the case that each is just slightly out on the opposite ends of the specs. Barrel makers want a tight lock up with the pin and may tend to be on the large end. Upper makers may want to have a tight pin fit and are nearer the small end and bang they won't fit.
It seems simple but taking a pressed pin out near another object is a pain. This pin locates the front of the rifle why not just start right and let the rest follow.

[kcstott]

I say measure the pin and the slot and figure out how much needs to be removed to make it right.
The technique applied is set by the amount of material needed to be removed.
If they are less the .002" difference just use the barrel nut. More then that and filing or a light dermal touch would be needed.
Sanding a pin down .001" would take some time. More then that and you could be there for a while sanding a pin.

[Dreaded Claymore]

Eh?

I say measure the pin and the slot and figure out how much needs to be removed to make it right.
The technique applied is set by the amount of material needed to be removed.
If they are less the .002" difference just use the barrel nut. More then that and filing or a light dermal touch would be needed.
Sanding a pin down .001" would take some time. More then that and you could be there for a while sanding a pin.

Whadda ya mean, use the barrel nut? Use it for what?

[fourtraxmc]

Use the barrel nut to pull the barrel into the receiver. Lube up threads on receiver, install nut, snug nut, then loosen- repeat. Barrel should seat, unless interference is too much.

[BSlacker]

Barrel should seat, unless interference is too much.

Then what?
Measure the pin and slot to determine which is what dimension. Call the barrel or upper maker or both. Then you can decide to use the nut to pull it in or not. The pin .002" more than slot and it might work .003" and it might not. Just what you need is a nut stuck or nut thread buggered or short seated barrel due to material pushed up by the interference fit of the pin and slot inside the threads of the barrel nut.
Get some actual info before filing or sanding or taking action you may not like later. It seems like a simple part but a whole chain is depending on this fit. People will assume the pin is bad because they see with their own eyes it doesn't fit into the slot but the slot could be small or off center as well, I know the OP said several uppers didn't fit but I worked in QC and tested thousands of things that didn't fit three would not be a surprise. All that is known is that it don't fit! There is no useful info to base an action decision on without the measurement.

[BSlacker]

These are general measurements. Pin .125" slot .126". If you can slip a 1/8" drill shank into the slot it is ok.

[Gunsmith Dan]

tujungatoes:

tujungatoes said:

WTF?!? Dude...did you even read the thread?

I guess I will just recycle your comments as it seem I will be using it alot when you post.

OP posted he could not even get the pin in that is was oversized for the hole and I never said he needed to set the pin all the way in. The pin is not tapered so once it fits into the hole entrance snug it should go be able to be tapped in all the way, so no extraction would be needed.

The technique I posted is used by many gunsmiths for small pins that are either to small or take to much time to setup in a lathe to size down. This of course is normally done in a drill press but I can not assume OP has access to one and hand drill is just as good. Using a flat object around a stationary round object will never produce a rounded object. That is why you rotate the round object and keep the flat object stationary on it. I suggested the use of fine sandpaper because unless you are experienced a file will take off material at a faster rate.

The only ghetto technique here is hand filing around a pin and expecting it to stay round.

You can continue to flame me and try to make me look clueless like you have in some other posts. The facts are the facts and you maybe should read all the posts start to finish before posting something to try to make someone else look bad.

I know you want to bring up the whole Saiga rifle post but go here if you actually want answers and not just flame others.

If you don't that is fine but I am no longer gonna let personal attacks against me to try to discredit me get a pass any longer.

To the rest sorry about this temp hijack but I had to take a stand on this. This is getting out of hand with the arm chair gunsmiths who think they can make a professional look stupid, then go around yelling "WHO DA MAN".

[fourtraxmc]

Then what?
Measure the pin and slot to determine which is what dimension. Call the barrel or upper maker or both. Then you can decide to use the nut to pull it in or not. The pin .002" more than slot and it might work .003" and it might not. Just what you need is a nut stuck or nut thread buggered or short seated barrel due to material pushed up by the interference fit of the pin and slot inside the threads of the barrel nut.
Get some actual info before filing or sanding or taking action you may not like later. It seems like a simple part but a whole chain is depending on this fit. People will assume the pin is bad because they see with their own eyes it doesn't fit into the slot but the slot could be small or off center as well, I know the OP said several uppers didn't fit but I worked in QC and tested thousands of things that didn't fit three would not be a surprise. All that is known is that it don't fit! There is no useful info to base an action decision on without the measurement.

If the barrel would not fit and was just too tight I would open up the notch on the upper reciever. If it came to that a small file and one or two passes should do the trick. You would also need to make sure the feed ramps will line up properly, if the notch was off center you may only need to open one side. Disclaimer - I am not a gunsmith and do not pretend to be. When it comes down to it it is your gun you need to feel comfortable with the action taken.

[kcstott]

Whadda ya mean, use the barrel nut? Use it for what?

As in just crank it down the aluminum upper will get out of the way.

.002" press on a tiny pin like that on two sides only is going to equal a .0005" press on the same size pin at full diameter. Very little tension very little deflection of the upper. No big deal shove it on.

[BSlacker]

OP posted he could not even get the pin in that is was oversized for the hole and I never said he needed to set the pin all the way in. The pin is not tapered so once it fits into the hole entrance snug it should go be able to be tapped in all the way, so no extraction would be needed.

The pin is in the extension already it won't fit in the slot in the upper that is the problem. Are you thinking the pin does not fit the extension hole? It should be obvious from the pic that the pin is already installed in the extension. You and the other fellow are talking about two different things.

Can anyone explain how you pass a file passed the pin while it is installed and effect the width? There is the barrel shoulder to stop any file action.
Did the OP leave the building?