M14 Forum banner
1 - 20 of 29 Posts

·
Registered
Joined
·
742 Posts
Discussion Starter · #1 ·
I discovered I have an unusual gas piston, looks modified or maybe wear?

Took a trip to the range yesterday and was zeroing a new to me AR TEL scope. I experienced a few failures to extract and a couple that did extract/eject, but failed to load the next round, these were intermittent failures. Cases that didn't extract required a stout pull of the op rod to remove. Rifle has an SAK NM barrel, I'm assuming a fairly tight chamber. Ammo was hand loaded, Hornady 168gr HPBT with 41gr of IMR 4895 and CCI #34 primers. These loads worked well in my other M1As.

Once home I cleaned the rifle and investigated for the possible culprit. Gas system was tight, but when I disassembled it I noticed the gas piston had a bevel in it I hadn't noticed before (on the left in the pics). To my knowledge this is original to the rifle, which is a 1973 vintage Devine. Could this gas piston be the cause of my failures or was it my hand loads? I also shot a M1A five liner which ran flawlessly with these loads, this one has a regular gas piston (middle on in the pic) and SAI barrel.

I'll swap out the gas piston before my next range trip, but that might be awhile before I can test it.

Gas piston in question is on the left, middle is from my five liner, right is from my parts bid (all pictured for comparison)
Wood Cylinder Household hardware Font Auto part


Writing implement Pen Office supplies Writing instrument accessory Wood



Close up
Wood Automotive tire Material property Cylinder Gas


Rifle 1973 Devine with AR TEL Scope
Air gun Military camouflage Trigger Camouflage Shotgun
 

·
Registered
Joined
·
1,571 Posts
Reaper6,

I am waiting to hear from the more experienced M1A/M14 gurus, but your gas cylinder port appears to be out-of-alignment.

That "gas stain" above the port on the left piston makes me think "misalignment" of the gas cylinder.

Regards,
D1
 

·
Registered
Joined
·
14,398 Posts
Your handload is anemic, but it cycles most rifles.

The piston in question appears to be too far in, which is the opposite of the usual problem. The gas cylinder itself may be worn.
 

·
Registered
Joined
·
742 Posts
Discussion Starter · #9 ·
I am waiting to hear from the more experienced M1A/M14 gurus, but your gas cylinder port appears to be out-of-alignment.

That "gas stain" above the port on the left piston makes me think "misalignment" of the gas cylinder.
Gas cylinder seems to be in alignment. When mounted and without the piston, I can insert a thin Allen wrench straight through the gas cylinder vent hole and into the bore.

Did you measure the diameter?
.498 dia. Same as my other piston.

Have you tried that piston in a different rifle? That way you can tease apart is it the piston vs cylinder vs something else.
No yet. I ran out of time, but will try that for my next range trip, might be a couple weeks before I get out again.
 

·
Registered
Joined
·
4,135 Posts
Your gas piston doesn't align with the gas port hole based on the white arrow on the bottom pointing to the location where I think the combustion gases are landing (dark circular area). It seems about 1/8" off, and the bevel cut on the gas piston's inlet hole was clearly done to facilitate gas flow into the gas piston, but apparently that isn't working given the issues described. I am not surprised that it is having functioning issues. The level of carbon deposits on the first ring shouldn't be there either at that level (see arrow on left), it should be more confined to the wider pad around the hole.

Your rifle needs to be inspected by a competent M1A gunsmith - as the mis-alignment is not insignificant. The gas piston appears to be sitting way too far rearward relative to the gas cylinder port. Why that is happening is a mystery. One possibility is a 're-welded' op rod that is a little too short?...or perhaps the gas cylinder is not aligned properly? or its internally worn allowing the gas piston too much rear-ward motion? Never seen this level of misalignment, so I'll defer to others re ideas.
Wood Bicycle part Material property Auto part Tints and shades

Again, a careful inspection is required to determine the root cause as to why the combustion gas is being dumped about 1/8" too far forward relative to the gas piston's inlet hole.

BTW, that's a nice looking rifle and that AR TEL scope looks very nice too.
 

·
Registered
Joined
·
3,271 Posts
Have you measured the distance from the front of the piston back to the rearmost portion on the hole? From the photos, it looks like the hole is a little more rearward than on the other 2 pistons.
Also measure the location of the hole relative to the tail of the piston - on all 3.

The gas soot around the hole on the FTE piston does look 'troubling'.

Does the gas piston rattle with bolt closed on empty chamber?
 

·
Forum Jester
Joined
·
8,198 Posts
Your handload is anemic, but it cycles most rifles.

The piston in question appears to be too far in, which is the opposite of the usual problem. The gas cylinder itself may be worn.
might need to get a shim kit and align the gas cyl bottom hole with the barrel hole
 

·
Registered
Joined
·
742 Posts
Discussion Starter · #14 ·
Thanks for the inputs, gave me a few things to check out and rule out. Here's what I've found so far:

Your gas piston doesn't align with the gas port hole based on the white arrow on the bottom pointing to the location where I think the combustion gases are landing (dark circular area). or perhaps the gas cylinder is not aligned properly?
View attachment 468418
Again, a careful inspection is required to determine the root cause as to why the combustion gas is being dumped about 1/8" too far forward relative to the gas piston's inlet hole.
I don't think the dark carbon spot is from a direct blast from the gas port, but maybe the bevel is redirecting some gas there. I've gone through the gas system and all looks properly aligned.

Gas cylinder appears aligned with gas port, here a small Allen wrench goes cleanly through the vent hole and through the gas port.
Tire Bicycle frame Automotive tire Bicycle tire Bicycle handlebar


With everything assembled, I put a small dot of white paint on the on the piston through the vent hole.
Automotive tire Musical instrument accessory Material property Bumper Bag


Once the paint dried, I removed the piston and compared the location of the paint mark to another piston.
Wheel Tire Automotive tire Household hardware Cylinder


As far as I can make certain, the gas system seems to be in proper alignment.

The gas piston is sitting way too far rearward relative to the gas cylinder port. One possibility is a 're-welded' op rod that is a little too short?
Op rod is SAK, I don't think its a re-weld. I compared it to a TRW op rod I had laying around and dimensionally, they matched.
Office supplies Wood Metal Bicycle part Tints and shades



Op rod alignment to gas system
Cylinder Wood Bicycle part Office supplies Gun accessory


Does the gas piston rattle with bolt closed on empty chamber?
Probably.
No, nothing rattles.

Try a new piston...
^^^This is what I'll do. Seems the most logical step at this point.

I was kind of hoping someone here would have recognized the mod and could tell us what it was for. It will remain a mystery for now.
 

·
Registered
Joined
·
2,020 Posts
Folks have done some weird stuff trying to increase dwell time.
Me too on a lot of things, lots of things go in the scrap barrel too

You should have seen the look on one guy's face when he walked up behind me during testing in a bay at the range. I told him I wasn't going to be liable if something blew up while testing to please step away 😲
 

·
Registered
Joined
·
5,776 Posts
Folks have done some weird stuff trying to increase dwell time.
That particular modification would decrease the dwell time, and increase the velocity of the reciprocating parts. By allowing more gas to enter the piston after it has moved backwards you would get increased bolt speed, possibly out running the magazine spring.
 

·
Registered
Joined
·
4,135 Posts
I think Kurt’s idea about the gas cylinder spindle valve is interesting, and I guess it might be worth looking at the spindle as well to see if it could have been re-directing the gas slightly off angle? The subsequent info and pics gives the impression everything is properly aligned.
Good luck with this issue. It’s a neat rifle.
 
1 - 20 of 29 Posts
Top