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MGySgt USMC (ret)
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Discussion Starter · #1 · (Edited)
Part I

We get a LOT of questions on the forum about gas pistons and I realized that I don’t have a thread that really discusses them in detail. I have written posts in other threads, but figured it would be a good idea to have one thread here where the information is collected all together. No doubt this thread will have to include more than one post, so if you don’t find an answer to a question you have, please read all the posts. I also realize this information may bring up new questions, so if you have additional questions, please put them up here. Chances are if you have a question, another person may have a similar question.

I think it would be best to first begin by talking about the different types of Gas Pistons you may run across.

G.I. Gas Pistons – These “are the standard” that all other gas pistons must be measured against. They were made from a Stainless Steel Alloy and surface hardened. The piston heads were PRECISION GROUND after hardening to very exacting tolerances of only about FIVE Ten Thousandths of an inch in the entire range of tolerance. Folks, that is a manufacturing range of .0005” and that’s only ONE HALF a thousandth of an inch. You don’t get that kind of accuracy over the surface of the head unless you precision grind them.

There are grooves around the surface of the head of the piston that were put there deliberately. They are meant as a “self cleaning” feature over a smooth surface that was found on the pistons of the M1 Carbine. After the surface of the head was hardened and ground, the edges of these grooves were left sharp so they would scrape carbon fouling loose as the piston operated. THAT’S part of the reason why you should not polish the piston heads because you would dull those sharps surfaces and cause them not to scrape carbon as well. This also does not mean they were meant to keep you from having to clean the gas piston surface. It does mean it was intended to allow the rifle to be fired in prolonged battle conditions without having the piston seize up and stopping the rifle from operating.

Sadlak Gas Pistons -- These are probably the very best commercially made gas pistons. They are hardened and precision ground. They also make ones that are Tn coated for more lubricity. Personally, I have never used the Tn coated ones, but there are times that coating could be useful. If I could not get a G.I. gas piston to give the best accuracy in a rifle, the uncoated ones they make would be my first choice of a commercial gas piston.

Other U.S. Commercial Gas Pistons - Other commercially made pistons range from fair to poor to downright junk. Some of them are not precision ground and some are so poorly machined that you can feel the machining rings on the heads from extremely poor lathe work. If your piston is one of these and functions the rifle correctly as a standard Infantry Grade arm, you may not have to replace them. However, I have found it is always a good idea to replace the poorly made ones for accuracy purposes.

Chinese Gas Pistons -- I have to admit I am not extremely familiar with these pistons as I have only worked about a dozen Chinese rifles and none of them for accuracy work. I do not know the tolerance range of them and how much different they are from G.I. spec. Chinese gas cylinders can also be a little different spec. than G.I. The best I can offer is that some people have found G.I. or Sadlak pistons have improved accuracy when put in some Chinese rifles. If anyone knows the tolerance range for these pistons, I would appreciate them adding that information to this thread.


Cleaning and Maintenance of Gas Pistons and Gas Cylinders

1. How often should you clean the Gas Cylinder and Piston? For use as a battle rifle, the answer is as soon as you can after every time you fire it. However, there is a down side to that. After you clean and reinstall the piston, it takes a couple three rounds for everything to settle back into place. No problem in a battle situation, but not good for the first few rounds fired in competition as they will go a little outside your normal group placement.

However, even in the military there were times we did not clean the Gas Cylinder/Piston after every time we fired it and that was on the Qualification or Re-Qualification .Range Course. We fired 50 rounds per day for five days for a total of 250 rounds without cleaning the gas system. That was a total of 250 rounds fired when accuracy on the last 50 were SUPER important. On THE Marine Corps Rifle Team, we found many rifles would go 300 to as many as 400 rounds before they started negatively affecting the accuracy by being too dirty. However, there is a down side to that as well. We fired that many rounds in three or four consecutive days of firing and THEN cleaned the gas systems. So we did not get RUST developing in the Gas system from them sitting for long periods after firing and yes, even though the GC, Piston and Gas Plug are stainless steel, they WILL STILL RUST. We used to clean all Gas Pistons as soon as the shooters came off the line on a Match day or on the very first practice day after they shot a match. We always wanted at least one day of firing after we cleaned them before a match so their first shots would not be off call. The problem with letting them sit dirty for long periods of time is the rust will eventually cause pitting and that will shorten the life of a Piston before anything else in normal use.

My own personal take on how often to clean the Gas Cylinder and Piston is you should clean it before you are going to let it sit for a couple weeks or more. That should ensure you will never get more than light surface rust that should not pit the piston. However, if the rifle was used in the rain or near Salt Water, then you really should clean the system every time after you fire it. Edited to add: I forgot to mention that if you were in a sand or heavy dust storm, I would also clean the entire gas system as soon as possible. Even dust will cause a lapping situation in the cylinder that will get worse and worse until you clean it out.

2. How do you clean the Gas Cylinder and Piston?

Let’s begin with inside the piston. As carbon builds up in inside of the piston, the piston will become sluggish and wind up negatively affecting accuracy. You need BOTH a standard “jobber length” LETTER size “P” drill bit (.3230” diameter) AND a 6” or 8” “aircraft length” NUMBER size 15 drill bit (.1800” diameter) to thoroughly clean the carbon out of the inside of the Piston. The smaller hole in the piston is deeper, so you need the longer aircraft length drill bit to clean it out thoroughly. USUALLY you can also clean out the interior of the Gas plug with the Letter size “P” drill bit, but on some G.I. gas plugs, you need a slightly smaller Letter Size “O” bit to clean them out.

To clean the exterior of the piston, it is best to use standard bore cleaner and a bronze/brass bristle brush to clean them. If you use one of the newer “Super Copper Dissolving” bore cleaners then DO NOT let the piston soak for more than about 20 minutes or it will finely pit the precision ground surface and pretty much ruin the piston. If it is rusted, you can use OOO or OOOO steel wool soaked in bore cleaner to remove the rust. DO NOT use a stainless steel brush or any kind of sandpaper or buffing to clean the head of the piston as you will damage it even though it looks like you didn’t. After it is clean, wipe it dry, then wipe some good gun oil on it and wipe it dry again. That will leave a TINY amount of oil in the pores of the metal and will help keep it from rusting.

You clean the interior of the cylinder the same way, though usually with a bronze bristle brush meant for a .45 cal. pistol. You also want to wipe it dry, oil it and wipe it dry again for the same reason we did it on the piston. The inside of the piston and gas plug should also have a light coat of oil that is wiped dry after oiling.

3. NEVER polish the piston. When we first started building NM M14 rifles in the 70’s someone came up with the idea that if we polished the pistons, carbon would not stick to it as soon. We buffed them on a buffing wheel when we had one available or used 600 grit Crocus Cloth when it wasn’t available. WE WERE WRONG. Not only did it not stop carbon from sticking as soon, it caused the sharp edges of the piston head to be dulled and we actually had to clean the pistons more often. Further, it totally messed up the fine precision ground surface and actually caused the rifles to be less accurate. So don’t do it.

4. What about pitted or chipped pistons? It is difficult to describe how much pitting is too much pitting as all good pistons will normally develop some pitting in their service life. I would suggest that if you begin getting quirky groups that you can not identify from shooter, ammo or other mechanical reasons, it is probably time to replace a pitted piston.

Chipping is normally seen on the very front edge of the head surface of the piston. When you see that, then I advise you to replace the piston as that demonstrates the piston has already been shot enough that metal fatigue has set in.

OK, this is the End of Part I. More to come in Part II.
 

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Outstanding information from you, as usual. Thanks a million from all of us M14 nuts...GI1
 

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Thank you so much Gus for all the work you do. It really helps new guys like me who have a steep learning curve already to make things easier.
 

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System cleaning

Hello Gus, This is a timely post for all of us, much appreciated.

There is a question that comes to mind, perhaps I am jumping the gun but an answer would be valuable to others as well..

This has to do with the three impact areas of the system. The front of the piston, the face of the plug and the end of the piston stem. In my rifles I notice that over time these impact areas have a tendency to flair. this is a small amount of mushrooming so to speak. Using high quality tools I can detect this on an otherwise normal [new] condition.. The open end of the piston, which I refer to as the head, and to a lesser extent the open end of the plug change diameters, very little, but they get bigger in time., just like many of us.

I address this situation in my own way but have wondered if or how you would address it?

Best regards, Art
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Good post, Gus. Regarding Chicom pistons, they are carbon steel, and the head exterior only is chrome plated, not the interior nor the stem. These pistons will rust when exposed to moisture. I have only measured a few, the pistons tend to be over USGI spec. About 0.499" , where US spec is 0.4970" to 0.4975" . But the Chicoms have been very uniform, and have clean sharp corners on the grooves.

So if used in a new USGI gas cylinder, the Chicom piston will be "too tight" and not allow the proper amount of "blow-by" when the piston is moving, since the min. spec on the cylinder is 0.500".

OTOH, the Chicom gas cylinder, which is also carbon steel and will rust, tends to be a bit oversize. If you keep the chicom gas piston and cylinder together, things will be OK. BTW, the gas cylinder spindle valve and spring also will rust.

I wonder how the hardness of the chrome plating on the chicom cylinder compares to the SADLAK TiN plating? It's not clear to me that any plating is necessarily a good idea.

JWB
 

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Discussion Starter · #7 ·
Part II

Folks, you may not believe this one, but I assure you it's true. You can have an excellent quality NM barrel, sights, glass job, et all, and your pet rifle may not shoot that well. I have built quite a few "improved" rifles with standard G.I. barrels that shoot almost as well or better than other people's match rifles. Since I first wrote some of this back in 2007 or so, some forum members here have tried it and found it does indeed work.

I don’t know or remember how The Armorers/Shooters stumbled across trying different gas pistons in our NM M14’s back in the early to mid 70's. I do know it was a Quantum Leap in accuracy when we did it. We found you have to try different gas pistons in your rifle to find the best piston that makes your rifle shoot the best in YOUR rifle WITH the most accurate ammunition you have, or even the standard ammo you use. (More on this later.)

The tolerance for the gas piston is a range of .0005”. We used a precision set of micrometers to separate the pistons into groups by .0001” “sizes,” as even the best Dial Calipers are not precise enough for this kind of measurement. Then we tried pistons from the different “size” groups until we found the size piston with which the rifle shot best. You actually see groups shrink or swell, depending on which size piston is used. Generally, but not always, the pistons in the middle range of the manufacturing tolerances would make the rifles shoot the best. Only every now and then would a piston from the smallest or largest size shoot the best in a rifle.

At this point it is a good idea to describe how we tested the rifles because it can give you even more help should you wish to test different pistons in your rifle. There are some “must do” things or you will not get the best test results.

1. You must have “control” ammunition that is the most accurate ammo you can find or normally use. If you use junk or poor quality ammo, you really won’t get anything out of the test. We used a Proprietary Load from Federal that became Gold Medal Match, NM Lake City Ammo and a special hand load we made up in quantities. We could not and did not hand load for every individual rifle as we did not have enough people or equipment to do that and we found it was not necessary. We never used Winchester/Olin Matheison ammo as they would not provide super accurate ammo like Federal did and we had an agreement with Federal to only use their commercial ammo. In more recent years, I tried Winchester Police Sniper 7.62mm Ammo and found it was also excellent as far as accuracy. I have not tested TAP rounds for accuracy, though there are many good reports with that ammo. If I were going to use TAP ammo for an Issue Law Enforcement rifle, then I would use the TAP ammo in the piston tests.

With the three kinds of ammo we used on THE Marine Corps Rifle Team and when we built double lugged/double torque screw rifles with Krieger barrels and McMillan fiberglass stocks, our testing requirement was a 10 shot group fired at 300 yards from our super expensive machine rest had to be less than 2 ½” in diameter. That’s less than a Minute of Angle as actually fired at 300 yards. Now, it was not at all common that a rifle would fire that well with ALL THREE types of ammo we used. Only a small percentage of the rifles shot that well with all three types of ammo. Most would fire two of the three types of ammo within that testing requirement. Some rifles would only shoot one type of ammo to that testing requirement and when it was only one type of ammo, it was almost always the Federal Ammo the rifles liked best. BOTTOM LINE is you must test the piston in the rifle WITH the ammo you are going to use to get the best accuracy from the piston changing test.

BTW, one of the things that makes older Armorers or Gunsmiths the most crotchety and “out of sorts” is when we build a really nice and accurate rifle and then someone puts really cheap/trashy ammo in the rifle and complains the rifle won’t shoot well. You are only going to get the best accuracy out of the best ammo. Cheap/trashy/poor quality ammo is going to make the best and most accurate rifle shoot poorly. There is nothing wrong with using cheap/poor quality ammo for plinking, but don’t complain when the rifle won’t shoot it accurately.
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2. You really MUST use a Foot Pound Torque Wrench to tighten the Gas Plug when you change the pistons. This is the only way to ensure uniformity for test purposes. The Army used 15 ft lbs. as their torque setting, but we Marines used 23 ft. lbs. as we found in a very few rifles 15 ft. lbs would not keep the gas plugs tight. 23 ft. lbs of torque ALWAYS kept all the gas plugs tight between cleanings. It is kind of funny how we came up with 23 ft. lbs. as the standard. We averaged what four Armorers on THE Marine Corps Rifle Team used individually “by feel” when they tightened up the gas plugs and found it was enough to keep the plugs tight. So it wasn’t from rare mechanical engineering or even scientific analysis, but rather what WORKED.

Some people have tried the test without a torque wrench and by painting or marking a line across the GC lock and Gas Plug to align them each time. Some people actually filed a thin/shallow line across both parts to align them. Either of these things is better than using nothing to uniformly align the Gas Plug each time, but the results will vary much more than when using the Torque Wrench. BTW, if you are a NM shooter or a Long Range shooter, you are only kidding yourself and setting yourself up for A LOT of frustration, if you do not get and use a Torque Wrench.

3. After you change the piston, you should fire one or two rounds away from the test group to allow the piston to settle in and properly seat in the Gas Cylinder. Then begin the accuracy test with that piston. Remember in Part I when I brought this up? If you don’t do this, you may and probably won’t choose the best size piston for your rifle.

Oh, and I guess I should mention again a little on the G.I. cylinders we used in NM M14 rifles in the USMC competition in arms program. Most of the rifles we used to initially build NM rifles had been arsenal reworked years earlier. The gas systems ranged from used to brand new, depending on if they passed the Arsenal specs or not in those Arsenal rebuilt rifles. When we made additional unitized cylinders, we would get NOS cylinders "two in a black paper container with metal ends," through the supply system OR use good condition used cylinders we had on hand. As such, we did not use "JUST" NOS cylinders. Used cylinders, as long as they were still in spec., were probably as common, if not more common on our NM rifles.

Now for the bad news, we never came up with what size Gas Piston will automatically work in a gas cylinder. We purchased a very expensive and accurate set of inside dial calipers and telescoping gages to measure the internal diameter of the gas cylinders and used micrometers to measure the outside dimension of the piston that fit best. We hoped we would find some kind of rule of thumb that for example if the gas cylinder diameter was "this size," then the correct piston diameter would be "this other size." Unfortunately, we couldn't find such a rule of thumb even though we inspected a couple hundred rifles with the gas pistons so fitted. Then we tried to come up with an explanation of why we couldn't get some kind of rule and asked engineers about it as well. The answer probably lies with there are so many other differences affecting accuracy, that we couldn't come up with a rule. So, we just kept right on testing each individual rifle to find the piston that worked best in it.

Now is a good time to address a phenomenon some folks have found after changing pistons in their rifle to either a different NOS G.I. piston or a coated or uncoated Brookfield or Sadlak Piston. All of a sudden, their rifle shot less accurately or accuracy really went in the dumps. Because these shooters do not understand how variations as small as .0001” in the diameter of a gas piston may negatively affect accuracy, they get confused, frustrated or sometimes really upset. It is not the fault of the piston or maker, per se, but rather that is not the optimal size piston for the rifle. I have seen customers' faces light up after I told them about this. "That's the reason my rifle won't shoot any more!” is the normal reply. They put the original piston in their rifle and it shoots well again.

The problem for most shooters is they can not afford to have a half dozen or more pistons to test in their rifles and find the most accurate one for their rifle. Normally, the only people who will have a bunch of pistons on hand to test are the Armorers/Gunsmiths who specialize in the M14. My hope is that at least with the information provided here, folks will better understand how accuracy is controlled by choosing the correct size gas piston.

If you have read much of what I have written before, you know I like to add a Sea Story to illustrate a point and I have a great one for this thread:

This takes me back to the days when I first went to Edson Range as the NM Armorer for the MCRD Rifle and Pistol Team. They were one of the few teams who had "found" the money to have double lugged torque screw rifles built and had regular single lug, no torque screw rifles as well. They told me they weren't happy with the accuracy of the rifles when I reported aboard. OK, so I went through them and they all needed skim bedding and other work and touch ups done on them.

THEN I tried to get them to do a gas piston test on all the rifles. I hit a brick wall and what was surprising was our best shooter was Double Distinguished and he had a personal rifle that was built at the RTE Shop at Quantico and he had WATCHED them do the piston test to choose the right size for his rifle. (That rifle shot as well as any we built for The Marine Corps Rifle Team.) I could not get him (a CWO) to push the Team Captain (who actually was a Marine Captain) to do the piston test. I brought this up to him time and again over the next 16 months, but the piston test was not done.

OK, so the Team Captain was transferred and we got a different Captain as Team Captain. He wisely asked me if there was anything he could do to help me to improve the firearms or anything else I needed. I JUMPED at the chance and told him we needed to do a piston test on the rifles. He said, "OK, Gunny, if you say we need to do it, then we will do it."

So I got ALL our rifles ready to go and then I covered the serial numbers on the rifles with strong tape. I just wrote 1, 2, 3, etc. on the tape so we could record the groups for each rifle. I told them it had to be a "blind" test of the rifles as well as we could make it, or personal feelings might make the tests go awry. I had gotten with the Team Captain prior to this that the rifles would all be "re-issued" after the tests.

I wound up using that Double Distinguished CWO as my "living test rack" as he could hold a rifle almost as good as the super expensive test rack at Quantico. We DRAMATICALLY decreased the group sizes on every rifle at 300 yards when we found the piston the rifles liked the best - though the CWO about got beat to death firing so many rounds for accuracy out of about 16 rifles all told with different piston sizes. What was GREAT was the entire MCRD Rifle Team was there and they SAW the results of the test and how accurate the rifles now were. One single lugged, wood stocked rifle made the most dramatic change of all; it went from an 11" group size at 300 down to a 3 7/8" group with the piston it liked the best. Got to admit that even surprised me and I expected dramatic group size changes.

After we cleaned the rifles and got back to the Armory, I told the shooters the Team Captain would choose which rifles to issue to what shooters and supplied him with the group sizes. My second best shooter had been complaining his rifle didn't shoot that well and he got permission from the Team Captain to pick out any double lugged rifle he wanted. I knew which rifles they were, but the shooters didn't. So I showed him the test results and of course, he picked the rifle that shot the best groups. It turned out to be the rifle that had been issued to him prior to the test. His heart sank when he found out it was the best rifle. I told him it would shoot better now and he should try it again. Well, he had a psychological block against the rifle and came back a few weeks later saying he couldn't get it to shoot. Now, this is where a NM Team Armorer has to be a pseudo psychologist on top of a gun plumber. I told him no sweat, he had a psychological block and I had saved the second best rifle for him because I sort of expected this to happen. The difference in group size on the second rifle was only about 3/16" at 300 yards. Well, with THAT rifle - he shot the kind of scores he was capable of doing. He came back a few weeks later and asked what the difference was. I told him he had given the second rifle a chance that he had not given the first rifle. The first rifle went to the CWO and he shot it like a house afire.

OK, so now my team had really accurate rifles that they had seen how well they would shoot. So the confidence level of my Team went up dramatically. (ANOTHER reason I wanted to do the test with the whole Team there during the test. Like I said, you have to be a pseudo psychologist to be a good Team Armorer. Grin.)

A couple months later, the Team was shooting at the monthly NRA National Match Course match. I was walking the line as the Armorer, waiting to see if my Team needed me for anything. The CWO, who had not given me enough support to do the piston test prior to the new Team Captain coming on board, was talking to a civilian shooter. As I approached, I overheard him telling how important trying different pistons was in a NM M14 or M1A, as if it was "The Word" from the Burning Bush on top of God's Sacred Mountain. I didn't say anything as I walked by, but when I was close and the CWO was looking at me, I just winked and smiled. He was embarrassed and even blushed a bit, but the civilian really didn't notice it.

Later on when the CWO was alone, I deliberately came up to him and said, "Well, Sir, I see you are now a "True Believer" in doing piston testing." This with a gentle grin on my face. He blushed and said, "Gunny, I knew your reputation as a NM Armorer before you came and I really should have listened to you. It was just that I didn't really think changing pistons would do THAT much difference in accuracy." I then reminded him that he had SEEN the piston test done on his rifle at Quantico and should have thought about that. He blushed again. Then I told him, "Sir, no problem, but from now on when I really strongly suggest something..........." Then he stopped me and said, "I got it Gunny, LISTEN TO YOUR ARMORER !" Then we both had a good chuckle.
 

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Discussion Starter · #8 ·
Folks, thanks for the kind words and your replies. I am too tired to answer them right now after finishing Part II, but please understand I appreciate them and will answer them at a future time after my eyeballs readjust. Grin.
 

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Another great post from Gus ...
and I definitely learned some new things that I'll be testing out at the range next week.

As for the Chinese gas assemblies ...
no surprise here ... they vary in quality from decent to NFG.
These are chrome lined, with [ as Jbrooks mentioned in his excellent post above ] slightly larger than GI IDs.

I've seen several of these with the hole in the back drilled WAY off center, which makes it awkward to try to align the tip of the op rod with the tip of the piston. I've also seen a couple where the hole was drilled at an angle. Neither of these is great for accuracy, but at least these functioned.

One brand new Chinese rifle would not function at all. Simply swapping in the Chinese gas assembly from my personal rifle got that one shooting fine. And the funny thing was, his gas assembly worked just fione in my rifle.
Go Figger??

Also a reminder that the threads for the gas plug and the gas cylinder are METRIC,and just close enough that you could force a GI plug into a Chinese gas assembly and Vice versa ... but you don't really want to go there. Same for the barrel and the 8 ring.
[;(

And one of my own Sea Stories,
just to illustrate how insane the world can be sometimes.

Back in the day, I ordered up several sealed cans of US GI M14 gas cylinders.
A few weeks later, I got a really interesting message from Canada Customs on my answering machine telling me that,
" the import of gas cylinders is forbidden because they might explode in transit."
We sorted that one out eventually, but there was always more ...
TTFN
LAZ 1
[;)
 

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Question about the pistons

This is a great thread Gus. It's good too have fact and not suspect information, something's can get costily with wrong good meaning information I have all of the tools needed for the cleaning but was afraid of damageing something, now I have the correct info.

Now the question is do you need diffrent pistons from the same makers, and diffrent pistons from diffrent makers. I have been wondering about this for some time.
Thank's
Packing
 

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Variable elimination

Were you able to determine whether it was the variation in the Major O.D (precision ground diameter) that was the biggest influence on accuracy?

Other things that come to mind that may have changed how the rifles shot:

1: Port size in the piston
2: Concentricty of the major and minor (tail) O.D.s
3: Timing (piston tail length/ OAL)
4: Internal gas volume capacity of the piston (i.e., piston bore diameters and depths)
 

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Adding to the unknown

This subject is one of the most interesting, it results in more questions than answers.

Adding to the mystery, here is another situation.

Using the Piston change out method to find the most accurate piston for a particular rifle works. The question arises, is it possible that because different pistons of different sizes were used the process itself wore scrapped or chaffed the cylinder walls into uniformity of the desired fit for the piston that gave the best accuracy?

If you enjoy this topic you will be delighted when somebody introduces the mystery of sling pressure on accuracy.. The Black Hole, please avoid it.
 

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I believe the properly dimensioned piston, in a properly dimensioned cylinder, rarely contacts the walls of the cylinder. Rather, the gas will flow around the 0.002" or so of space between the two components. In other words, the piston will float in the cylinder, protected from contact with the walls. Now, if the op rod guide is not aligned properly, then the op rod misalignment will apply a non-longitudinal force on the piston and will destroy this fine balance.

I say this because I have 2 USGI pistons... the one with 12,000 rounds measures 0.4970", the new one 0.4973". Even within tolerances, the worn piston has virtually zero wear. The worn cylinder measures on 0.501"interior diameter, whicj is also very slight wear.

It seems to me that, accuracy considerations aside, the M14 gas system is almost perfect from the durability standpoint. Which is why I question the need at all for any special coatings.

The 6 grooves in the piston are also self-cleaning, as the piston moves to the rear, the gasses in the barrel blow out any residue in the grooves as the grooves pass by the vent hole in the cylinder.

Perhaps it's the interaction of the piston with the op rod that determines some portion of accuracy for the M14. If the piston is on the smaller end of tolerance, it allows more variability in the motion of the op rod from shot to shot. Lots of dynamics here...

Pretty amazing system, IMO... a significant improvement over the Garand.

JWB
 

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Discussion Starter · #14 · (Edited)
This has to do with the three impact areas of the system. The front of the piston, the face of the plug and the end of the piston stem. In my rifles I notice that over time these impact areas have a tendency to flair. this is a small amount of mushrooming so to speak. Using high quality tools I can detect this on an otherwise normal [new] condition.. The open end of the piston, which I refer to as the head, and to a lesser extent the open end of the plug change diameters, very little, but they get bigger in time., just like many of us.
Art, by golly you do come up with some great questions that come from your long experience with the M14 system. I don't believe anyone has ever asked those questions before. I guess it is best to answer "part by part" so to speak.

1. When we found the end of the gas plug mushrooming, we just radiused the end of the plug and found it didn't really affect the accuracy. Of course, as often as we cleaned and inspected the gas systems, the mushrooming never got very big before we chamfered it. Edited to add: Come to think of it when we shortened the Gas Plugs to time the op rods, we always chamfered the plugs BEFORE the piston test was done. So maybe that's why it was never a big deal for us.

2. As far as the tail of the piston mushrooming, we chamfered it when it mushroomed and found no problem with accuracy as the very end of the tail was still the same length and that part of the tail stuck out from the end of the cylinder when fully forward anyway. If perchance it did mushroom enough to shorten the tail, there would have been a noticeable shift in group size and/or accuracy and that would have required a new piston and test.

3. The front end of the piston head mushrooming was altogether different, as I'm sure you found. For most of the time I was around the RTE Shop at Quantico, we did not have a lathe/precision grinding machine set up that was accurate enough to regrind the mushroomed portion of the head back to or close to the original diameter. Though I was never a machinist, I'm sure Mike Gingher having been a long time Tool and Die Grinder could have done it if we had had a machine capable of doing it.

I don't ever remember us trying to slightly chamfer that mushrooming piston head, though some of the Armorers over the years may have done it on a belt sander with a very fine grit belt. The problem with that is it was never going to be anywhere near as accurate as the original piston head.

This is purely speculation on my part that slight chamffering may have worked for a short while in some rifles, but would not have lasted very long as much as we shot the rifles. How does that compare with what you found?

We had PLENTY of pistons and such a mushrooming head would have caused accuracy to suffer when it got bad enough. When we identified an accuracy problem that was most likely or even possibly Gas System related, the rifle went back for a new piston test and the mushroomed head ones were replaced.
 

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MGySgt USMC (ret)
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Discussion Starter · #15 ·
Outstanding information from you, as usual. Thanks a million from all of us M14 nuts...GI1
Haven't noticed you posting for a while. Glad to see you back. Your Avatar cracks me up every time I see it, especially after having met you in person. (No guys, he does not look like the picture in the Avatar. Grin.)
 

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Discussion Starter · #17 ·
Good post, Gus. Regarding Chicom pistons, they are carbon steel, and the head exterior only is chrome plated, not the interior nor the stem. These pistons will rust when exposed to moisture. I have only measured a few, the pistons tend to be over USGI spec. About 0.499" , where US spec is 0.4970" to 0.4975" . But the Chicoms have been very uniform, and have clean sharp corners on the grooves.

So if used in a new USGI gas cylinder, the Chicom piston will be "too tight" and not allow the proper amount of "blow-by" when the piston is moving, since the min. spec on the cylinder is 0.500".

OTOH, the Chicom gas cylinder, which is also carbon steel and will rust, tends to be a bit oversize. If you keep the chicom gas piston and cylinder together, things will be OK. BTW, the gas cylinder spindle valve and spring also will rust.

I wonder how the hardness of the chrome plating on the chicom cylinder compares to the SADLAK TiN plating? It's not clear to me that any plating is necessarily a good idea.

JWB
GREAT info on the Chinese pistons and cylinders. Thank you. That pretty much cinches that we in the U.S. will never have enough Chinese pistons to do a piston test until/unless they change the import laws. Not sure if the guys in Canada would be able to make use of it on the Chinese rifles, though.

I haven't done enough testing with Tn plated pistons to have much comment, yet. For years, Brookfield and Sadlak plated pistons cost enough more than G.I., that I could not afford to buy enough pistons to make up a piston test set.

This is purely speculation on my part, but I think the plating would be best for areas where there was a lot of dust or sand in the air and where the plating would be more for lubricity for operation in those environments. However, please don't take that as "chiseled in stone," because I have not worked much with the plated pistons and I could easily be missing something.
 

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Gus,

For the civvies with limited or no access to piston inventory, how should we approach this issue of piston dependency for accuracy? Other than try the one you got and pray.

nez
 

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MGySgt USMC (ret)
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Discussion Starter · #19 ·
One brand new Chinese rifle would not function at all. Simply swapping in the Chinese gas assembly from my personal rifle got that one shooting fine. And the funny thing was, his gas assembly worked just fione in my rifle.
Go Figger??

LAZ 1
[;)
Laz, neat post, thank you.

I have seen this with maybe five or six Garands over 35 plus years of working on them. It took taking them to the range and finding the good parts that would work in the rifles.

Believe it or not, I saw the EXACT same thing with a BRAND NEW M16A2 when we shot them the first year at Division Matches and this one was on Hawaii. The first batch of A2's were sent directly to The Basic School and Quantico and we got the very next 1,500 A2's to "introduce" them to the Corps at the Division matches that year. We had inspected all the rifles at the RTE shop before we has sent them out, so it was not a case where something was "out of spec" at least as we could test them with the M16 Gages.

One poor shooter came up four times in the first two practice days reporting functioning problems with the A2 he was issued. By the time the A2 came out, of course all Marine Corps Armorers were well versed with the A1. We followed the suggestions in the TM to the letter and nothing really worked.

My role in that match was as the Head Armorer and it was more supervisory than "worker bee." They did consult with me on the third time the shooter brought the rifle up and they were doing what they were supposed to do to fix it. On the fourth time he came up, I apologized for the rifle and told him we would either get if fixed or get him a replacement rifle as I wasn't sure how many of the new A2's would do this and we did not have many extra A2's to issue. Also, the shooter wanted to keep the rifle if we could fix it as he liked the groups it shot. I hoped we were not seeing the beginning of a trend of rifles that did not work. That would have been VERY UGLY at a Divison Match.

So this time I took the rifle and my Armorers asked me what was I going to do that they had not done? I told them we most likely were dealing with the rare rifle that while everything gaged correctly, tolerance stack up was keeping the rifle from functioning correctly. I told them, "I'm going to GUT this thing of anything having to do with the gas system and replace the parts." (We did not have a lot of repair parts as these were BRAND NEW rifles and if there was going to be a trend of malfunctions, I was already planning on going to the 3rd Echelon Maintenance Shop and pleading for a whole BUNCH of spare parts.)

So I grabbed another Brand New rifle that we had not issued and swapped the charging handle, bolt carrier groups (after checking headspace of course) and even the gas tubes between rifles. The shooter came back shortly afterwards and was very happy to report his rifle was now "fixed." I put the parts from the 1st rifle in the 2nd rifle and was able to do a rather "strenous" function test after we got done shooting that days. The second rifle also worked perfectly with the parts that had not worked in the first rifle.

We also did the same thing with a NM AR15A2 at the Nationals about three years ago. One of the guys worked on the rifle twice and the third time he asked me what to do. I told him this had happened to me in Hawaii. We put a new bolt carrier group and gas tube in that rifle and also permenantly fixed it. The parts from his rifle worked flawlessly in another rifle we tried them in.

So indeed, these things happen and my explanation has always been it is just a case a tolerance stack up during manufacture.
 

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Discussion Starter · #20 ·
Were you able to determine whether it was the variation in the Major O.D (precision ground diameter) that was the biggest influence on accuracy?
Yes, though it was from empirical data.

Other things that come to mind that may have changed how the rifles shot:
1: Port size in the piston - This one was thing we absolutely could rule out because we bought our barrels without the gas ports pre-drilled. After we shimmed the unitized cylinders to the correct position, we ran a specially ground center drill bit into the barrel through the Gas Cylinder and that centered the port on the center of the hole in the Gas Cylinder. Then we drilled and final reamed the gas port to size with one of only two special reamers of the same size. We did not find that one reamer over the other caused any demonstrable difference in accuracy.

2: Concentricty of the major and minor (tail) O.D.s - We measured the head diameters over three areas all around the piston head (Front, back and behind the gas port in the head) and then averaged the diameter size to "size" the piston. As we were working with tenths or more lilely hundredths of thousands of an inch in measurement, that was the best we could come up with and it did prove out on the test rack.

3: Timing (piston tail length/ OAL) - We timed the Gas plugs instead of the tails as we found that was much more important and consistant.

4: Internal gas volume capacity of the piston (i.e., piston bore diameters and depths) - We bought an $ 800.00 precision inside micrometer to check and record the inside diameters of the gas cylinders as well as pretty expensive telescoping gages. An $800.00 precision inside micrometer was top of the line and SUPER expensive in the early/mid 80's. It almost sent our Ordnance Officer into apoplexy when we first asked to buy it, but after explaining why, he approved it. Even after we had used the gages to test hundreds of gas cylinders and the pistons that had shot best in them, we could find no correlation as to "this size cylinder" needs "this size gas piston."
 
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