Fitting a Replacement M-1 Gas Lock

[Raptor]

I recently acquired a 1955 vintage M1. The gas lock was not a high hump that is correct for that period. So I got one on gunbroker.

The former [incorrect} lock tightens to the 6:15 position and needs to be backed off only slightly to install the gas plug. Certainly a great fit. With that installation there is good play between the front hand guard and the gas cylinder (about the thickness of a credit card)

The "new" hump style lock tightens to the 2 o'clock position. When it is backed off to the 6:00 position to install the plug - the movement of the front hand guard seems excessive - and is disconcerting in any event.

I was thinking of working on the bevel in the mouth of the gas lock (which bottoms on the barrel shoulder bevel) to get from 2 o'clock to 6 o'clock. My measurements suggest that I will still have about the same clearance between the gas cylinder and hand guard.

Does this sound like a reasonable approach.

I'm new to M-1's, but have done a fair amount of amouring on modern firearms.

[Major Tom]

I'm no expert, but, you may want to try other gas locks for proper fit.

[Dave Waits]

Here's the unwritten rule on Gas Locks, as long as the entire barrel port is inside the gas cylinder port, you're good to go. So, you have a little movement in there to adjust with forward or back..

[Maury Krupp]

Does this sound like a reasonable approach.

It may be "reasonable" but it doesn't seem particularly "reasoned" to me :icon_scratch:

What are you doing? Restoring the rifle or building a shooter?

Changing the whole lock to be "correct" based on the receiver date is a common collector thing to do.

Using a lock that times to 5-6 o'clock is a common accuracy step.

The Army didn't care about lock timing so any "correct" lock should be fine for a restoration.

Most shooters don't care about "correct" parts so the "incorrect" lock you've already got should be fine for a shooter.

There's really no such thing as "excessive" movement of the Front Handguard on a Service Grade M1.

So exactly *why* do you want to do this?

Maury

[Devil Dog]

Maury has the right idea and the right question.
Finding the correctly timed lock is not easy. I had the opportunity to go through a pile of locks to get each of my guns correctly timed. Some were lucky and I found the correct lock after 5 or 6 tries. Others took as many as twenty tries to find the correct fit. Statistically, your chances of finding a lock that tightens between 5 and 6 o'clock are one in twelve. so, get ready to try more than a few. I'm glad I did it and it was worth the time. You could take your "barreled receiver" to a gun show or a parts supplier and ask if you can try one till it fits. If you are shooting this gun and want to be as accurate as possible, I think the lock timing is very important and worth correcting. Having a 6 o'clock lock up and a handguard with clearance are TWO separate issues. Do not adjust a good fitting lock to create H.G. clearance.

[bd1]

There's no substitute for trying different locks. Unfortunately, unlike w/ the M14 clones, you can't flip a Garand lock and try it from the other side out.

[JohnF]

Mechanical thingsPosted By: Gus Fisher
Date: Wednesday, 19 April 2006, at 9:55 pm

What I find wrong with most rifles is that the gas cylinder is loose and the locks don't fit the cylinders properly. This alone can really improve a rifle's performance when these are out of whack.

To tighten a gas cylinder on a JC Garand Match rifle, one is allowed to peen the splines just as military armorers did it. For other than a JCG Match rifle, Loctite "red" is good to use along with spline tightening, but again it is not legal for JCG rifles.

The fit of the lock is very important. The further around from 6 O'clock the lock hand tightens on the barrel, the more room the gas cylinder has to move back and forth and loosen quicker. As the gas cylinder loosens, your front sight does not remain in the same place, so accuracy goes down the drainn. As the gas cylinder bangs back and forth as the bullet is traveling down the barrel, it also sets up a negative node of vibration and that hurts accuracy as well. Some folks do not seem to understand how that works, so I will explain.

The gas cylinder lock tightens toward the angled front shoulder on what is known as the gas cylinder seat. The barrel diameter is larger in this point to keep the cylinder aligned an in place. If you take the cylinder off the barrel, the lock will still tighten down to some point where it is stopped by the angled front side of the gas cylinder seat. If we didn't have to worry about the gas cylinder lock screw going through the bottom hole in the lock, then we could tighten down any lock so it is tight. However, as the bottom hole has to have the lock screw go through at 6 o'clock, then the lock has to be turned back to 6 o'clock if it goes beyond 6 o'clock. This means the lock is no longer against the shoulder of the gas cylinder seat on the barrel. The further away from 6 o'clock it is, the further away the lock is from tightening down against the angled shoulder on the gas cylinder seat. If for example, your lock tightens down at 3 o'clock, you have to turn it quite a ways around to get it back to 6 o'clock so you can put the screw in it. That keeps the lock and cylinder pretty far away from a solid position against the front angled shoulder of the gas cylinder seat and out "loose" on the barrel with no support. So, it is important to remember the closer to 6 o'clock the better. The Technical Manuals called for trying different locks until you found one that hand tightened to between 6 o'clock and 8 o'clock as the correct "range" for the fit of the lock. OK, so why was that necessary?

The angled front corner of the gas cylinder seat was not as exact as the shoulder for the gas cylinder on the M14 barrel which was "square to" or perpendicular to the diameter. Further, that corner wears with time, so that's why the range of 6 to 8 o'clock is allowed. They didn't "time" the threads on Garand Gas cylinder locks when they made them, but rather just ran a Gun Tap through them and weren't worried about where the threads started. This too was corrected on the M14.

So the only option available to them and us is to try different locks until we find one that tightens as close to 6 o'clock as possible. If I find one that stops just a bit short of 6 o'clock and have to use a bit of force to get the lock to line up at 6 o'clock, I prefer it. On NM rifles, we lined them up between 5:00 and 5:30 and then tighten them down with a special wrench. This can also be done on JCG Match Rifles legally, btw, and in no way harms functioning.

[USSR]

IMHO, the concept of trying various locks until you find one that stops at 6 o'clock is totally unnecessary. Drive your gas cylinder on and insert a business card (.015" thickness) between the handguard ferrel and the rear ring of the gas cylinder. Remove the business cand and turn the lock on to see where it stops. If it stops past 6 o'clock but less than 12 o'clock, remove the lock and tap your gas cylinder a little towards the muzzle until the gas lock stops at the 5 or 6 o'clock position. If it stops at 12 o'clock but less than 5 o'clock, remove the lock and tap your gas cylinder a little toward the breech until the gas lock stops at the 5 or 6 o'clock position. This will ensure that your front handguard is neither too tight or too loose. Just MHO.

Don

[bd1]

You're right, that's a good locating technique. But I'd go ahead and peen the top spline to give the GC the desired lift when torqued on.

[chriskat]

The problem with moving the gas cylinder to get the appropriate lock position is that the gas cylinder is not supposed to stop the lock. The lock should bottom against a shoulder on the barrel. If the gas cylinder is stopping the lock you don't have the lock tightened far enough for certain.

[Gus Fisher]

IMHO, the concept of trying various locks until you find one that stops at 6 o'clock is totally unnecessary. Drive your gas cylinder on and insert a business card (.015" thickness) between the handguard ferrel and the rear ring of the gas cylinder. Remove the business cand and turn the lock on to see where it stops. If it stops past 6 o'clock but less than 12 o'clock, remove the lock and tap your gas cylinder a little towards the muzzle until the gas lock stops at the 5 or 6 o'clock position. If it stops at 12 o'clock but less than 5 o'clock, remove the lock and tap your gas cylinder a little toward the breech until the gas lock stops at the 5 or 6 o'clock position. This will ensure that your front handguard is neither too tight or too loose. Just MHO.

Don

Don,

There is one possible and one real problem with doiing it that way.

The possible problem is that you are assuming the front handguard is the correct length. I've seen way too many commercial handguards that were too long and even some of the NOS G.I. replacement handguards that were too long. That could move the rectangular hole in the gas cylinder too far forward and close off part or all of the gas port hole in the barrel, though that is not real common.

Second, you are relying on the threads of the barrel to hold the gas cyinder in place by themselves to keep the front to back recoil movement of the gas cylinder down. The threads of the barrel wear out quicker that way. I realize the WWII Garands were probably not assembled with a lock that fitted between 6 and 8 o'clock on a G.I. Garand as manufactured, but the government replaced the barrels when the barrel threads for the gas cylinders wore out.

I have only been able to talk to a couple of WWII armorers on how they did things. Most of the older Armorers I've spoken with and who worked in 3rd or higher echelon Ordnance Shops (where they had issued gages to check on wear of Garand rifle parts) were from just before, during and after the Korean War period or later - so I have to make that clear. I also have to mention that most of the worn out barrel threads I've seen were either from Garands that came back from Korea or the Garands they use at the Marine Barracks at 8th and I streets in Washington, DC. (They never fired liive rounds in the Barracks Garands - only blanks and yet they had a LOT of Garands with worn barrel threads when I inspected their rifles three times in about 10 years.) There have only been a very few DCM or CMP Garands I've seen with worn out barrel threads for the GC lock - most liikely a half dozen or less in the 35 plus years I've been inspecting them. This because most of those Garands had been inspected/rebuilt in U.S. Arsenals and if the threads weren't good, they replaced the barrels.

The guns that came back from Korea were were WWII rifles that had been inspected and/or rebuilt WWII rifles. They were used and abused heartily and most had worn out springs that would have allowed greater wear from recoil. When the Barracks in DC got their Garands from the supply system, they all had been inspected and rebuilt by the 5th Echelon Standards to "as near as new as possible" condition. MOST of them had NOS replacement barrels on them. They do go out on a lot of burial details and practice blank firing more for the burials, but I was a bit surprised to see the way the barrel threads wore. They had plenty of gas cylinder locks so each time I went to DC, I taught the armorers to choose locks that fit close to or slightly before 6 o'clock. IOW, they had to put just a little pressure to get the locks down to six o'clock or they were just beyond six o'clock. That kept down the wear on the barrel threads.

I am NOT an engineer, but a real engineer on this forum explained after I retired, that when the gas locks don't quite hand tighten to 6 o'clock, that pressure on the lock to barrel fit relieves a good bit of the stress on the barrel threads. I adimit I forgot the engineering term he used. What I do know is that NM Garands were fired a whole lot more than most G.I. Garands as the barrels were shot out and replaced before the barrel threads wore out. The gas cylinder locks almost universally were fitted so they would hand tight to 5 to 5:30 and then torqued down to 6 o'clock.

Even though I know of no hard and fast evidence or studies on fitting GC locks; from my training by those older Armorers, my own experience with working the rifles, and what I have seen as empirical evidence - I prefer to fit the locks the way I wrote and JohnF posted for us. Thanks, John, that saved a lot of typing.