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Old 09-28-2004, 07:49 AM   #1
 
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Op rod in spec? How can you tell?

When purchasing an Op-rod, how can you tell that it is a good one in spec? What type of measurements should be done in order to verify?

I was looking at some op rods from online dealers that have them listed in excellent condition, but how do you know? What do you ask?

I wanted to buy several, just to have on hand, since they are a very specialized part.

Thank you.
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Old 09-28-2004, 09:56 AM   #2
 
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There are four critical dimensions on an op rod. First, the piston should measure at least .525" and not be out of round. Second, the tab that fits into the receiver slot should be approximately .325" tall, .255" to .270" wide, and .070" to .100" thick. Third, the hooks that grab the op rod catch should not be rounded to the point they will not snag the catch. Fourth, the bends in the rod need to be correct so the rod does not bind up anywhere. You can only check this by putting it on a rifle. A good op rods lasts for thousands of rounds, so you should not need to have several spares on hand.
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Old 09-28-2004, 10:03 AM   #3
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Quote:
Originally Posted by aimit
There are four critical dimensions on an op rod. ... Second, the tab that fits into the receiver slot should be approximately .325" tall, .255" to .270" wide, and .070" to .100" thick....
where do you get the 0.070" spec?
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Old 09-28-2004, 10:46 AM   #4
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Is this what you are looking for?

Quote:
Originally Posted by Quagmire
The U.S. .30 Caliber Gas Operated Service Rifles (A Shop Manual Volumes I & II) by Jerry Kuhnhausen

Measure Gas Piston O.D. pg. 125:

Ordnance M1 Rifle gas piston O.D. specification is .5260"-.007" (or .5260" max. and .5253" min.). Nogo dimension (.525" O.D.) has become the defacto minimum.

1. Adjust 1" micrometer to .525" and reference check the gas piston. Adjust dial as needed to measure the piston.

2. Rotate the piston to check roundness.

3. If the piston cams tight or loosens, readjust the micrometer and measure the larger/smaller diameter area.

Gas cylinder/piston roundness note: Cylinder roundness is the other half of the piston/cylinder dimensional problem. Gas blowby can be 4 times greater when pistons and cylinders are both out of round, except in the rare case wherein piston/cylinder eccentricity agrees. See cylinder gauging on pg. 113.

4. If the piston measures less than .525" dia. at any point and/or out of roundness exceeds .00035", replace the operating rod (or the gas piston). See piston replacement on pages 126~130.
Buy the book, it's worth every penny.
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Old 09-28-2004, 10:49 AM   #5
 
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Old 09-28-2004, 11:04 AM   #6
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Thanks Cornbread, you saved me the trouble.

The Tab can't be more than .320 tall. Anymore and run into cycling problems as it will drag on the receiver slot until worn down. Also, the rear dimension is important, you want it to be as close to .058 as possible to align the Op-rod in the receiver-slot correctly.
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Old 09-28-2004, 11:58 AM   #7
 
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Wow

Wow! A lot of information. Specifically what kind of tool would I need to make these measurements?

I did not realize that it had to be so precise. I guess it is true - nothing is easy that is worthwhile.
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Old 09-28-2004, 02:20 PM   #8
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A Dial-Vernier.
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