tight trigger pins

[BoilerUp]

I was helping a friend build an AR last night and the trigger pins from his CMC trigger were just a touch too large to fit in the lower. I had some extra pins from a mil-spec trigger group and a Geissele slave pin which all fit fine. They measure 0.153" whereas the CMC pins measured 0.1545". So, it appears the receiver (Aero) is in spec. Any suggestions?

[SINNER]

Receiver spec is .156 so my money is on the receiver not being in spec. Is it coated or anodized?

[Stone]

If you have to use the CMC pins just roll up a piece of 1000 grit wet sand paper and lightly work the trigger pin holes.

[BoilerUp]

Anodized receiver. Seems like 0.154 is what most spec out their pins. It's not my receiver but I'd be reluctant to enlarge the holes, but since we're probably talking about less than a .001" maybe my friend will be OK with that.

[alamo5000]

Have you measured the holes on both sides and both ends of the pins as well?

If I recall correctly when I was assembling both of my AR's my CMC pins would not go in one way but they would go in most of the way the other direction. The torque used to tighten them up made a very snug fit.

[UWone77]

Receiver spec is .156 so my money is on the receiver not being in spec. Is it coated or anodized?

I thought Mil-Spec is .155?

Either way, that lower is out of spec if .1545 pins won't fit.... well it's out of "mil-spec" at least.

I love Aero Products, but over the years I've had to do a lot of fitting to some of their parts to get them to install.

[SINNER]

I thought Mil-Spec is .155?

Either way, that lower is out of spec if .1545 pins won't fit.... well it's out of "mil-spec" at least.

I love Aero Products, but over the years I've had to do a lot of fitting to some of their parts to get them to install.

It's actually .156 +- .0015 so technically we are both correct.

I agree with Aero having some spec issues even to this day. Usually Jon issue items but I've had some serious flaws also.

I use a .156 reamer to dress just about every receiver I build. CMT and Mega are the few companies that have always been in spec. Cerakote is enough to throw almost all out of spec.

Also I wouldn't use sandpaper. Trigger pin holes are one of the most critical dimensions on a lower IMO. The $20 to have the correct sized reamer is a no brainer.

[SINNER]

[UWone77]

Nice Find on the specs. I was looking for it, but couldn't locate it.

[BoilerUp]

If you have to use the CMC pins just roll up a piece of 1000 grit wet sand paper and lightly work the trigger pin holes.

That worked. We wrapped some fine emery cloth around some rod and worked the pin holes. Didn't take much at all, probably not even a thou. I think the anodizing was just a touch thick.

[Stone]

Glad that worked out for you, I figured it was just a small build up of anodizing.

[FortTom]

I know it's a little late, but if the holes were in spec. I would have taken a little off of the pins and left the holes just like they were. Probably get bludgeoned for my line of thinking, but just saying.....

[KW900A]

I'm chiming in late too, but the build I'm currently working on- aero, the forward trigger pin gave me a fit trying to get in. But it was almost more like the holes were not aligned properly. I assume when it's milled the holes are drilled simultaneously with the same bit. I checked my pins (anti walk) to see if they were true, but no matter which side I started from, the opposite side was difficult. I got it, it just took more wiggling than I anticipated.

It was also my first round with a drop in trigger, so perhaps that was the issue.