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Discussion Starter · #1 ·
I've noticed that the piston at the end of my M1 Garand's operating rod has a small nick / gouge on the edge of its face. I've only shot my rifle a few times and haven't experienced any cycling problems, but I'm concerned that "nick" might become an issue. I've stoned it to smooth it out some, but I didn't want to "bevel" it.

Should I be concerned that "gas cutting" will eventually eat into the nick and make is worse?
 

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"Death From Above"
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Not sure the of the right answer. If it make you feel any better you can always have the piston replaced on the op rod.
 

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Concerned?

I don't know if concerned is corrected, it would depend on how much you shoot the rifle. Since there is nothing to be done to the edge of the piston, short of placement, why not enjoy the rifle and shoot it all you want. There is a certain amount of gas that gets past the piston anyway, the worst case is, you may wear a little more a little sooner. This condition will wear out the Power Thrust area in the gas Cylinder a fraction sooner, thousands of are good rounds are still in the rifle. Art
 

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Art, it's not unusual, could have been there since manufacture. I'm sure you verified there's no burring against the gas cyl. They were made to lose gas and they will, no matter what.
 

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MGySgt USMC (ret)
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As long as the nick did not raise a burr that would gouge the cylinder, go on and use it until you wear it out. If there's a burr, CAREFUL stoning should be done just in that area to remove the burr.
 

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M1 rod piston head

If the burr on the piston has been there and the rifled fired, there may be a corresponding mark on the cylinder wall in the power thrust area, neither can really be repaired, only polished.

If I recall correctly, the NM M1 rods has a larger dia. piston, very little, but wider. If the M1 in question is loosing too much gas, try a NM rod, assuming you can find a real one. None of this is good news, but short of replacing both piston and cylinder it is an option.. If it was my rifle I would shoot and be happy just to have a Garand. art
 

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Discussion Starter · #7 · (Edited)
Appreciate your thoughts on this. The piston seems to be perfectly round and my vernier calipers show the diameter is right at .526" measured in several places. I think I'll simply shoot the rifle and not worry about anything until a problem arises.

Here is a photo of a portion of my rifle's operating rod showing its only marking. I think this is one of the "national match" type rods (though there is no "NM" anywhere on it). Maybe someone could confirm that for me too.

 

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Everything I've read on this issue states there were no op-rods NM mfg. as such. Op rods were gauged at the piston to insure the OD was the correct diameter, and then stamped NM.
 

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Piston on the NM Op Rod is to closer specs where the standard piston is to have "sharp corners and free of burrs"

Dont know if its true but from talking to guys who work on Op Rods I'm wondering if the steel isnt softer as they say they are harder to keep them bent to spec.
 

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NM opt rods markings

Everything I've read on this issue states there were no op-rods NM mfg. as such. Op rods were gauged at the piston to insure the OD was the correct diameter, and then stamped NM.
All the NM M1 opt rods I have seen, the Drawing No. ends in 722. art
 

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The only Garand op rods I've seen that were marked "NM" were late Remington-manufactured replacements of the "77" series. An older SA NM rifle would have had a GI SA op rod carefully fitted to it.
 

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MGySgt USMC (ret)
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The Type I NM Garands that Springfield Arsenal built all had the 77 series rods because they took the best parts from the production line to assemble the rifles. There were no other NM Modifications made.

The Type II NM Garands had the half minute discs and 1/2 minute RS Base and Windage Knob.

Springfield Arsenal did make NM "SA" op rods. I've owned three and I've seen four more, though they are rather rare. I am not sure when they used them, though.
 

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OP Rod

As long as there are no burs it will be fine. The other important thing is the tab. If is with in spec your good to go.
CBMCORPS1MCORPS1

Operating Rod Specifications

Piston Diameter: .5260 to .5253 (under .5250 calls for rejection)
Piston Thickness: .190 to .155
Tab/Lug Height: .320 to .317 (under .310 calls for rejection)
Tab/Lug Thickness: .100 to .095
Tab/Lug Width: .265 to .235
 

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Appreciate your thoughts on this. The piston seems to be perfectly round and my vernier calipers show the diameter is right at .526" measured in several places. I think I'll simply shoot the rifle and not worry about anything until a problem arises.

Here is a photo of a portion of my rifle's operating rod showing its only marking. I think this is one of the "national match" type rods (though there is no "NM" anywhere on it). Maybe someone could confirm that for me too.

That's a Springfield Armory Op rod. It say it after the serial # SA. If it is National Match it will have NM after the numbers.
 

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Discussion Starter · #17 ·
Thanks for all of the information. Yes, I was pretty certain it was an "national match" type rod made by SA but not marked "NM." I've read these were used beginning sometime during the 1960s for national match and other M1 rifle builds.
 
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