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It's been long overdue for me to write instructions on shimming the gas cylinder but I have finally gotten around to it. I won't go too much into the "why's" of shimming the gas cylinder but if you want to know more on the subject, I suggest you read this thread by Gus Fisher:

http://www.m14tfl.com/upload/showthread.php?t=67322

I will say though, that if your front band is flappin' around when fully assembled, it will degrade accuracy. Shimming the gas system is one of the most beneficial fixes you can do to help extract accuracy from your rifle.

In short, if your gas lock snugs up past the 6 o'clock position, meaning that when you have tightened the gas lock up, you have to back it off a little to get the holes lined up to insert the gas plug, then you should insert shims between the barrel shoulder and the front band to make your gas lock snug up just before the 6 o'clock position (perfectly lined up for insertion of the gas plug) and require a little force by hand or by use of a gas lock/cylinder wrench to fully line up the lock with the gas cylinder.

If your gas system already has ideal lockup, then shims are not needed and you are good to go! Also, since some commercial parts are not as consistent as GI parts, commercial gas locks may lock up differently if the lock is flipped over (from front to back, not top to bottom). What this means is to reverse the lock so that the side which is now facing toward the front of the rifle is now facing the back of the rifle. This may give you more of an ideal lockup.

You can obtain shims from Claude, at RA parts or from Smith Enterprise. Here is what they look like..



You will need a set of castle nut pliers, a 1/16" allen wrench, a 3/8" boxed end wrench, BAD-T1 tool or a USGI multi tool to perform this procedure. You will also need a gas cylinder/lock wrench or a large crescent wrench to support the gas system when removing the flash suppressor.

Remove the castle nut set screw.


Using the gas cylinder wrench and your castle nut pliers, remove the flash suppressor.


Make a mark with some white-out or marking device of some sort to indicate your gas plug torque (allow to dry). Using a gas cylinder wrench and your multi-tool (3/8" box end wrench), remove the gas plug.


Loosen and remove your gas lock. Remove the gas cylinder and front band.




Now you should be able to see the barrel gas port. Test fit an allen wrench which can be fully inserted into the barrel. I found that a 1/16" allen wrench fit nicely. We will use this to verify that the gas cylinder is properly aligned with the barrel when done.


Slip the shims onto the barrel like so...


Now test fit your gas system by installing the front band gas cylinder and gas lock. Depending on how much slop you had in the gas system, you may need one, two or all of the shims. Your shim set will come with different thicknesses of shims so play with them until you find the correct lockup. It will be properly locked up when it snugs at the 5 o'clock position and can be tightened with finger force or light force with a gas cylinder wrench. When shimming, if you find that your gas lock locks up before the 4:30 position, it is too tight! it can have negative effects on accuracy. Try to obtain the 5 o'clock position. That's the "sweet spot"

Here is the correct lockup position:
Snug...


Locked...


Once you have good lockup, it is time to check your gas port alignment. Slip the 1/16" allen wrench into the gas port in the gas cylinder and insert it until you can see it in the barrel. It should go in with ease and move around freely. If it takes some force but you can still insert the wrench, your gas ports may not be aligned very well. If you can't insert it at all, your ports are not aligned and you will have cycling issues.



You should be able to see the wrench protrude into the barrel as so...


Once you have achieved proper lockup and gas port alignment, remove the allen wrench, install your gas piston and gas plug and you are ready to go.

Tip: If your gas system is not unitized, don't snug up your gas lock until your barreled action is installed in your stock. Once it is, snug the lock and finish assembling your gas system. This will allow your front band to find a neutral position against the stock ferrule (you want this).
 

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Gas shims

Thanks for the instructions.

I wish I had them when I did my first.

KG
 

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Wow! Tonyben Thank you for the fantastic post. I wouldn't have any reservations about going ahead with this myself now.

Later,

Hookster GI2
 

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Thanks

This is one of the best post I have seen great instructions and excellent photo demonstration, this is first class!!

thanks TONY


NRA LIFE MEMBER

U. S. ARMY VET
 

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Thank you for a very informative post!

is shimming something that should be done even if the gas cylinder is unitized then?
 

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Discussion Starter #10
Thanks for the feedback gents.

Thank you for a very informative post!

is shimming something that should be done even if the gas cylinder is unitized then?
Yes, it is recommended that you shim your gas system for ideal lockup even if your gas cylinder is unitized. Again, this will only benefit if your gas lockup is loose.
 

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yes its loose clocks about 8-9 o'clock . thought about sending it back to SAI I think it is causing my vertical stringing, looks like a good time to change out my FS for a lugged one at the same time.

Again a huge THANK YOU for this it really answered alot of questions I had
 

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Tonyben, I really appreciate you taking the time to post. I've wondered about this since I became a member and your thread made it crystal clear in five minutes! I hope this is made a sticky.
 

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Excellent Post ....Thank you.
 

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Ive had the shims and tools for 6months lacking that bit of confidence.Where the shims go etc etc.My loose front band
wont be loose much longer.Also the final tip finish final tightening/GC on the stock is the first time ive heard this!
Thanks much! Make it a STICKY!GI6
 

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What would be wrong with putting the shims between the front band and the gas cyl. body? Thanks, Mike
 

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Discussion Starter #19
What would be wrong with putting the shims between the front band and the gas cyl. body? Thanks, Mike
Porbably nothing but you want a gap to be between the front band and the stock ferrule (the vertical surfaces) and only contact on the bottom lip of the front band to the stock ferrule (the concave and convex horzontal surfaces). If you shim between the front band and gas cylinder, you could potentially mate the vertical surfaces which has been fond to degrade accuracy most of the time. Plus, I like for the front band to have the additional support of being pressed against the gas cylinder instead of "free floating" if you will...
 

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Excellent, great pictures. Needs to be stickied!
 
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