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View Full Version : Anyone interested in tighter-fitting AR takedown pins?


gargodude
April 1, 2012, 09:22 PM
I am building an AR rifle with a BCM upper and Aero Precision lower. I bought a Spikes Tactical lower parts kit and decided to test fit the upper and lower before I built the lower. I notice a little slop in the fit between my upper and lower if I use the takedown pins in the kit. Nothing abnormal, like some cheapo rifles I've played with, but noticeable. I measured the diameters of these pins (the right way, with a micrometer) and they both measure .2482". The holes in my lower and upper are between .2505" and .2510" (measured with gage pins). I imagine the pins in most kits are intentionally made on the small side so that they will fit any rifle out there, no matter what. I'm a bit fussy about things that fit together, being a machinist for 34 years, but that's just the way my brain works. I haven't put the detents in the lower yet, so I substituted a pair of 1/4" steel dowel pins (.2502" dia.) for the takedown pins and the slop is GONE. They slide in and out like a dream. This is how I want my rifle to feel. I scoured the internet for someone who makes precision takedown pins of a slightly larger size and all I can find are removable ones with push buttons or huge heads, but neither manufacturer states the actual diameter of the pins. Also, I want to retain the detents so the pins stay with the rifle and neither of these pins do this. My intent is to manufacture some custom pins with the same design as the stock pins, but with a larger diameter, say .2490-.2498". I own a small machine shop, so this is do-able, but I'm wondering if this would be worth making several sets and offering them for sale, since it appears that no one else makes them. I would probably make them out of 17-4PH stainless steel that is pre-hardened to Rockwell C45 and bead-blast the exposed areas so they aren’t reflective. I have a couple of friends with ARs who would like a set, but I was curious if anyone else out there would be interested in something like this.

HiBC
April 2, 2012, 03:17 AM
I made my own already.The ones I was finding ran about .245.Minimum material condition!

I used mold core pins.They are H-13 and come in two different hardnesses,one is in the low to mid 40's on the "C" scale.They are through hard and a few 10ths over nominal.

I would not try mold ejector pins,they are usuallt .0004 to .0007 under nominal,but they are nitrided harder than a woodpecker's lips!!They do not machine well

CowTowner
April 2, 2012, 06:42 AM
Here's the solution I used on my last build:
http://www.brownells.com/.aspx/pid=7591/Product/AR-15-AR-10-SR-25-REAR-TENSIONING-PIN

Granted, the pin doesn't just pop out any longer, but the upper sure is kept tight to the lower.

madcratebuilder
April 2, 2012, 08:43 AM
My intent is to manufacture some custom pins with the same design as the stock pins, but with a larger diameter, say .2490-.2498". I own a small machine shop, so this is do-able, but I'm wondering if this would be worth making several sets and offering them for sale, since it appears that no one else makes them. I would probably make them out of 17-4PH stainless steel that is pre-hardened to Rockwell C45 and bead-blast the exposed areas so they aren’t reflective. I have a couple of friends with ARs who would like a set, but I was curious if anyone else out there would be interested in something like this.

I'm sure you would have a niche market for these. It would grow as the word circulated on the gun boards.

Would you offer custom sizing, or just a standard .2502?

The problem would be dealing with people that have no clue on how to do a precision measurement. Asking people to measure to a ten-thousandth when most have a problem with a hundredth or even a tenth.

On my uppers that had a sloppy fit it is normally from a oversize takedown pin hole. I have made inserts for these to tighten them back up.

gargodude
April 2, 2012, 11:47 AM
Madcratebuilder, you have a point there. Without any reference size gage, the customer would have no way to determine what size he needs. The only thing out there that is commonly available would be steel dowel pins which are .2502". Most good hardware stores have these. If that does the trick, as in my case, then there’s your size. I have a machining drawing for an AR lower and the takedown pin holes are dimensioned .251±.001, so they can be as large as .252, perhaps larger if they are already worn out. In order to offer custom sizing, I would have to keep several sizes in stock and offer an opportunity to exchange them for a different size if they didn’t fit the first time. Sounds like more of a pain than it’s worth. Definitely not a money-maker. Maybe I’ll just offer a .2495-.2500” size for those with new or near new rifles. Anything’s better than the pins that come in the kits. Thanks for the insight.

DnPRK
April 4, 2012, 03:03 AM
Already done.
Oversized take down pin (http://www.armalite.com/ItemForm.aspx?item=ENM002&Category=5e96660f-cc85-4db9-ab9d-8a3c238a7007)
Oversized pivot pin (http://www.armalite.com/ItemForm.aspx?item=ENM001&Category=5e96660f-cc85-4db9-ab9d-8a3c238a7007)

madcratebuilder
April 4, 2012, 10:24 AM
Yup, those are +.001 and should be .2495.

I would love to get my hands on a box of 50 and mic them to see what the tolerance is. Can they mass produce at +/-.00025?

gargodude
April 5, 2012, 08:14 PM
Thanks DnPRK! $20 sounds worth it to me.:)

ScottRiqui
April 5, 2012, 08:21 PM
I would love to get my hands on a box of 50 and mic them to see what the tolerance is. Can they mass produce at +/-.00025?

A tolerance of +/- 0.0001" isn't uncommon for machined parts. I'm sure the necessary equipment to do high-volume production would be expensive, but they're probably not having to manufacture 5,000 AR pins per day.