Purpose of the hole on the right side of the pivot pin detent spring channel?

[SomeOtherGuy]

I ran a search and can't find anything. What is the purpose of the hole on the right side of the pivot pin detent spring channel? I'm attaching a photo with the hole circled to clarify what I'm asking about. I have a DSA stripped lower that doesn't have this hole but otherwise appears very well machined, and I want to know if I will ever miss having that pinhole.

The DSA is a $60 blem but I can't find anything else on it I would consider to be a blemish. As shipped the lower had some gunk in the spring channel, but per advice from a DSA tech I removed it with a straightened paper clip and the channel now seems to be normal in depth, etc. I'm wondering if the hole is just a machining artifact for removing drilled material.

9583

[Dionysusigma]

I always thought it was an oiling port.

[Heavy Metal]

Drain hole to allow grit and water to escape.

[Iraqgunz]

I believe it is a combination of what the last two posters stated.

[MarkG]

I'm wondering if the hole is just a machining artifact for removing drilled material.

This...

It is a chip clearance hole for the machining process.

[Animal_Mother556]

Pencil sharpener, perhaps?

I have placed drops of oil in it before...I don't really think I NEEDED to...just seemed like the thing to do....you know. Little bottle of CLP in your hand...your eye catches some random hole....

[GTifosi]

And the actual use from way back when:
It was used to help tear up a frozen pivot pin detent spring that was preventing dissasembly.

A suitable size drill bit run into the hole, a few turns and the spring stops being a spring allowing disassembly. After the pivot has been released, the hole can be used as an access to punch out bit of broken/stuck spring or the detent proper for that matter.

Initially there weren't holes there and the method only appeard in the higher level manuals, (I'm digging for the reference currently), but over the generations it became a default manufacturing process.


There is no need for a machining clearance hole there, the drill will have proper fluting to pull a proper strand of cut material out of the hole just like every other drill bit is designed to do.

EG: there's no 'relief hole' in the side of the takedown pin spring drillway, yet its the same length and uses the same spring in it.

[Robb Jensen]

I allows gunk/oil to be cleared out if you notice the pivot pin has a hole in it and stuff can find it way down into that channel.

[GTifosi]

TM 9-1005-249-24&P 6/1991
Page 3-65 describes putting the hole into an M16 receiver if it is not present.


Having a hell of a time finding the spring removal TM reference, but it is out there in one of the books for sure.
Along the lines of 'using handheld portable drill and a 1/16" twist drill...'

The hole in the pivot pin only lines up with the detent spring drillway when the pin is in the open/released position.
While a potential contamination hazard to be sure, it's unlikely to require a special port be made to relieve any debris infiltration.
Or if so, it wouldn't be that far back on the receiver from the pin proper.

That being said, it does offer a very good place to get a drop or three of oil in there or allow water infiltration to escape so it doesn't rust the spring all to hell.


EDIT:
To give an idea the very same manual, and on that very page above, describes filing or machining 2 flats into the end piece of the receiver extension tube so that a modern/regular extension wrench can be used to service same instead of the factory strap or collet type wrench it was originally installed with.
IOW, we're talking archival stuff here.

[markm]

Ignore the hole.

[GTifosi]

I tried that for a while once, but in retribution she stopped cooking and cleaning.

[polydeuces]

Just when I thought this would remain a dignified technical discourse amongst respected experts....:jester:

[SomeOtherGuy]

Thanks guys, M4C always impresses me with the expertise here.

[norinco982lover]

I was wondering this same thing yesterday...

Either way...I don't think the hole is that important haha. I guess they knew what they were doing when they made it or maybe they used to have a lot of springs freeze up?

[J_Hernandez]

I got a DSA lower too. it also doesnt have that same hole. had it for about 3 years now. its set up with a BCM upper. its my go-to AR. i havent had any problems with it at all. shot over 1000's of rounds through it too.

[Magic_Salad0892]

I tried that for a while once, but in retribution she stopped cooking and cleaning.

Cleaning the hole?

Is that why you ignored it?...

[ThirdWatcher]

Wow, it's been 38+ years since I first picked up my first M16A1 and this is the first time I ever noticed that little hole.

[Axcelea]

Heh, I always thought it was some lubing/cleaning/drainage port but its really left over from being able to remove seized up springs?

Next up the human appendix.

[GTifosi]

In the wayback long ago there was neither hole in the spring tube nor in the pivot pin proper.
This is why there is/was a 'pivot pin removal tool' in the manuals under the 'locally fabricated tools' section.

The method of the time was that you used the pivot pin removal tool, made from a 1/16" allen wrench, to lever back the detent so the pivot pin could be removed from the receiver.

However, if the spring tube got full of crap, the spring overly corroded, the detent mushroomed, or other such thing that wouldn't let it move properly and fully, the tool couldn't push the detent back far enough, if at all, so that the pin could come out to fix it.

Net result was drilling the tube in that location and into the spring with the deliberate intent of breaking the spring.
Once the spring is broken, it makes just enough clearance room to get the tool into the pivot pin groove, lever the detent back and remove the pivot pin.
Picking at the pieces of spring to remove them after was accomplished through the now two holes in the tube.
Its also possible to fish a bit of safety wire into the side hole and push out a stuck in the tube detent as there's no other way you could actually get on it or at it from the pivot pin end if its stuck in the spring tube with little to nothing protruding to get pliers or whatever on it and slamming the receiver front first against the pivot pin receiver holes/loops on a hard surface to try and dislodge one.


Later down the line the hole was incorporated by default to allow the *drilling of the spring, and/or allow lube and drainage of the spring tube.

*dependant upon when the pivot pin itself grew that hole, which I'm pretty sure was much later as the earlier pivot pins were drilled through thier length but not drilled crosswise so a simple pin punch through the pivot pin could be used to push the detent back as we do today.
Doing such would have caused the detent to bind like mad as it went stupid deep into the linear drillway through the pin where the two crossed paths.

I don't think 'lube/drain port' was the deliberate intent, but it does work very well for that.
Certainly more useful today for that purpose than as a point to get a tool in to help a persuade particularly difficult teardown to be accomplished.

[g5m]

It was also stated on some post that that hole allowed for anodizing solution to drain out.

[scottryan]

It was also stated on some post that that hole allowed for anodizing solution to drain out.


Yes it is that and the vent hole that MK18 Pilot posted about.

It is to allow milling particles, machining oil/coolant, and and anodizing chemicals to drain/vent so they don't blead out into the anodizing.