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Centerfire Rifles - Semiautomatic or Gas Operated Centerfire rifles, carbines and other gas operated rifles.

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Old 05-04-2013, 8:33 PM
BigDave101 BigDave101 is offline
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Default Adams Arms kit fitting for your MACHINIST

This old thread upset me because Adams Arms kit owners may be referencing it for help with their issues. While there was some good advice, everyone totally missed a couple of KEY areas that MUST be checked in case they are out of spec. Out of spec does happen....especially when mixing manufactures. This thread:

I am going to resurrect/supplant this old thread because many AA owners may be reading that thread looking for answers. It did NOT have all of them. I have them. Well, most answers. Read on.

I had the same symptoms as the OP.,,short stroking, FTE, gas block that moves no matter how tight I cranked on it. I may or may not have had more out of spec stuff than the OP. IDK. I DO know I got everything PERFECT and working flawlessly in 1.5 hours instead of 4 to 6 weeks.

No where in any of the posts did I read( I read ALL four pages) about checking the gap between the piston cup and the gas block. THAT is the gap that should be about .020 or 2 business cards. Once you have the gas block in the correct position(more on that later), then and only then should you be measuring for the piston head gap. IF something is out of spec like the rod being too long or the gas hole and or shoulder being slightly off, this may make the piston rod a VERY tight fit with NO play. My DD barrel was so out of spec that my bolt would not even go into battery becasue the rod length prevented it.

Solution: Mill, not sand...mill the rod shorter to prevent heat up and change of temper. Use a carbide bit. Mill until you have a .020 gap in that piston cup.
To test, make sure the BOLT is in full battery and remove the spring from the op rod. Re insert the op rod and the gas adjustment pin. Lock the pin in place. you should be able to move the op rod back and forth 0.020 inches. It is just a little click click back and forth. Now TILT the barrel back and forth. The op rod should MOVE back and forth just from gravity and its own weight.
Re install the spring. You are good to go. Almost.

These are the things you need to do BEFORE messing with the op rod. Yes, I know I an not addressing things in order. Bear with me. Forget about business cards and half cut guards to measure where your gas BLOCK needs to be positioned. To REALLY get it right, you need to MEASURE the distance where the GAS HOLE in the barrel was drilled in relation to the SHOULDER on the barrel. That distance MUST meet up with the hole in the gas pin which lines up with another receiving hole in the gas block. Note you DO have a bit of leeway since the AA receiving holes are several times larger than any barrel gas hole. None the less getting all this PERFECTLY lined up will maximize your gas FLOW.

Make sure to measure the SIZE of the gas mid hole too. It is supposed to be .078". Find a shank of a milling or a drill bit that JUST fits in the hole and measure the shank. Anywhere over .070 is probably OK. Mine is .072 and works fine. Industry standard is .078. IDK what AA uses as their size.

The set screws are 10-28x1/8" I believe. They are almost impossible to find. You can find them in 1/4 inch long which will work if you loose one. Note the AA screws ARE knurled on the end which will bite into your steel. But DIMPLING the barrel... again using a MILLING bit is necessary IMO. Even with the dimpling use RED locktite. IDK what the max torque is but you will round out the holes before you will snap one. MAKE SURE your allen key is TIGHT in the set screw hole and torque the crap out of it soaked in red loctite. Even with the heat of the barre you MAY need a powerful heat gun to get them apart if you ever need to. I have had smaller screws strip out inside with just blue loctite with no heat gun. Buy a heat gun for 35 bux.

What you have now is a gas block that is PERFECTLY aligned with a PROPER sized gas hole. Why? Because you have carefully measured and checked all these things. My gas block ended up TIGHT against the shoulder as the BEST location for gas flow. The ONLY way you can tell for sure is by measuring and matching up barrel gas hole to AA gas hole receiving position. Never ASSUME that gas holes are the correct size and are in the right position. Chances are good that all is in spec....but it very well may NOT be. This is WHY you must measure to be absolutely certain.

Now it is a pure JOY to remove or move the gas block should the rail need to come off. Re alignment is a SNAP. Make sure your dimples are PERFECTLY placed and JUST large enough to capture the diameter of the set screw. Depth is up to your machinist. The dimples need not be overly deep but my set screws are now FLUSH with my LITE gas block when they are tightened. All original AA stuff. No need to go oversize or drill addl set screws. DIMPLE and do it WELL.

IF you DONT fit the piston rod correctly and the rod happens to be TIGHT against the gas block, it will hammer away at the gas block. Heat will make the fit even tighter and the hammering greater on the gas block. Something WILL break in this situation...dimples, loctited set screws and an op rod that is too long/tight. Just get that nice .020 gap between the op rod and the gas block and nothing will move or be overly stressed. As I said, MILL the rod shorter if you MUST, but get that .020 gap that way rather than move the gas block out of optimal positioning with the barrel gas hole. This is the way I think it should be done. You CAN move the gas block forward and rearward TEENY TINY amounts, because, yes, the receiver hole IS larger than the barrel gas hole. But not THAT much larger. I really prefer to CENTER and ALIGN the gas hole front to back using careful measurements and calculations to center the gas block receiving hole over the barrel gas hole. Now you can move the gas block side to side ever so slightly so the piston does not bind. You still will be over that barrel gas hole and KNOW you will get maximum gas flow.
Realize AA does give you a RANGE for clearance of the piston head to the gas block gap. It is .015"-.025". I pick .020 since it is right in the middle.

This information and SO MUCH MORE IS IN AA'S FAQ's. READ ALL OF THEM!
The gap question/answer is here:


Dimpling your barrel is up to you. If your gas block does not move in 2000 rounds then you are fine. But IF you take your system apart a lot or remove your rail a lot then consider dimpling only for the reason that re assembly takes seconds. Perfect alignment is automatic if your dimples are perfectly sized.

Sorry to write SO much and so much detail. But this post can be handed to ANY machinist willing to work on your gun. With NO fire arm knowledge, the machinist now has exact directions of what to do.
My machinist has NEVER worked on a fire arm and he accomplished every single task(under my direct supervision) in this post in less time than it took me to write it. It cost me almost nothing....maybe twice the price of SHIPPING my upper to AA. I could have sent all my stuff up to AA and waited 4? to 6? weeks or accomplished ALL this in about 1.5 hours. It is EASY when you really sit and LOOK at the system. The things that need to happen are so simple. Getting the pieces there may take careful machining but nothing extensive. ANY good machine shop with a milling machine and GOOD carbide bits should be able to do this. When in doubt take a LITTLE off at a time and re insert the parts to check. You cant ADD BACK material!

Notes: Tell you machinist to clamp your barrel on using the thicker gas block portion of the barrel so the bore does not get crushed if you have a pencil barrel. No leather or soft jaws are needed in a smooth jawed milling vise. Steel on steel is just fine. But use the gas block shoulder.

Before dimpling check the run out to make sure the barrel is perfectly LEVEL.
This will take minutes with a good machinist.

Insist on using fine carbide bits and tooling. A slow to medium speed does not even make the parts warm when they are machined.

Last edited by BigDave101; 05-04-2013 at 8:47 PM..
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Old 05-04-2013, 10:30 PM
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BrassCase BrassCase is offline
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Nice article in Shotgun News (available at Barnes & Noble) on AR15 gunsmithing this month. Some of this is covered in the article. Check it out.
I'd agree with you but then we'd both be wrong...
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