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Gas Cylinder/Piston/Nut question

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Smokpole View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Smokpole Quote  Post ReplyReply Direct Link To This Post Posted: Jan 26 2022 at 11:03am
OBB and OB-B are early IBM firing pins.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote W5USMC Quote  Post ReplyReply Direct Link To This Post Posted: Jan 26 2022 at 11:41am
Originally posted by Smokpole Smokpole wrote:

OBB and OB-B are early IBM firing pins.

Not necessarily early, they were used throughout IBMs production.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote floydthecat Quote  Post ReplyReply Direct Link To This Post Posted: Jan 26 2022 at 12:11pm
Just judging from the picture, pin protrusion looks plenty good but should measure .044 minimum to .060 maximum. The pin is driven by inertia after the hammer blow and will travel a little deeper into the bolt than the hammer nose. All that will depend on how hard the primer is and pin-protrusion.

Backing up to the math, a firing pin with the correct protrusion should ignite a primer in a case length down to 1.275 (1.295-.020) in your case, but toss most of the scientific stuff out the window. If pin-protrusion is long enough, it could fire on a shorter case. 

The thing here is, there should be no reason to trim a case that short just to see if it will fire, unless one just wants to test it. Carbine brass usually grows after firing, but how much might depend on the strength of the load and the quality of the brass. I think one doesn’t need to be too concerned with anything but long brass….longer than the carbine head spaces. I don’t think one would ever run across commercial loaded or even fired brass as short as 1.275, unless someone trimmed it. Firing with long head space is really not good for the chamber and I think trying to keep head space as tight as possible without going too tight is the correct thing to do. Military head space specs tend to be liberal, they want the gun to fire under adverse conditions and design them lose so a grain of sand, mud, blood or some guts won’t stop them.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote M1WAM21 Quote  Post ReplyReply Direct Link To This Post Posted: Jan 26 2022 at 1:55pm
Thank you ALL, for the detailed information and guidance.  This forum, with its knowledgeable, experienced and articulate members, is invaluable to those wanting to learn and be safe with a firearm so rich in history.  Thanks so much...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote floydthecat Quote  Post ReplyReply Direct Link To This Post Posted: Jan 26 2022 at 3:20pm
A suggestion that will save you some time. Some reloader's like to trim their brass to a specific length after each firing, which allows the construction of an identical round as it’s run through the tooling. While it’s good practice, might not be necessary. If you check and trim to like 1.285-86, you might find that you can use that brass again without trimming and maybe more than once. It might not stretch far enough to get too close to your head space. All I really care about is OAL and long brass. I just measure and let anything at 1.290 or shorter after sizing bypass the trimmer. If I happen to catch one at or close to 1.280 after sizing, I just toss it.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote M1WAM21 Quote  Post ReplyReply Direct Link To This Post Posted: Jan 26 2022 at 6:15pm
Thanks FTC, I completely understand what you're saying.  And what I'm going to determine, is what case length I need to "just toss it"!  You have helped me be confident it the process, and knowing now, that my headspace is 1.295", lets me focus on determining how short a case I can engage.  The first thing I'm going to do, is to pull the bullets from the 4 light primer strikes I had, and measure the cases.  There were indentations on the primers, they just did not fire, so I now have 4 rounds that I can pull the bullet, measure the cases, and then make an hypothesis that the length of those cases are future candidates "to just toss" !  Again, thanks.....
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Post Options Post Options   Thanks (0) Thanks(0)   Quote floydthecat Quote  Post ReplyReply Direct Link To This Post Posted: Jan 26 2022 at 6:51pm
I leave you with one more thought. Tight head space can cause light primer strikes as well. The bolt might even be closed enough to look rotated, but in a correctly built carbine, the firing pin will be blocked until the bolt is fully rotated and deliver light strikes. You can ask me how I know…..but I know. I have had two personal memorable instances over the years. Luckily one did not fire, but the other one only cracked a bolt when it did fire. Both instances were due to brass longer than head space in carbines that spaced very tight, like 1.291 tight. Long brass can sneak in and bite you in the butt, or maybe some carbon build-up or some other reason that prevents proper chambering. I like a few thousandths of slack in my actions.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Matt_X Quote  Post ReplyReply Direct Link To This Post Posted: Jan 26 2022 at 8:49pm
Originally posted by floydthecat floydthecat wrote:

... pin protrusion looks plenty good but should measure .044 minimum to .060 maximum. The pin is driven by inertia after the hammer blow and will travel a little deeper into the bolt than the hammer nose. All that will depend on how hard the primer is and pin-protrusion.

I really appreciate your perspective on this. 
I was going to raise the same question (in a different thread) about whether the back of the pin should be flush when measuring protrusion.     When measured flush on the two bolts I checked, there was .052 and .053" firing pin tip protrusion.  But when held as far in as they could go, the protrusions were .070 and .083" respectively.

This photo showing the measurement technique suggested to me that the spec was intended to be done with the back of the firing pin flush with the back of the bolt.  But I still was curious about others interpretations and experiences.
from page 57 TM9-1276, 1947

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Post Options Post Options   Thanks (0) Thanks(0)   Quote W5USMC Quote  Post ReplyReply Direct Link To This Post Posted: Jan 26 2022 at 10:39pm
@Matt_X, 
Page 111 of Kuhnhausen's shop manual states 
"Firing pin protrusion is easily checked in disassembled M1/M2 Carbine bolts by installing the firing pin, pushing it hard forward, and measuring tip extension past the bolt face."
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Matt_X Quote  Post ReplyReply Direct Link To This Post Posted: Jan 26 2022 at 11:31pm
Yes!  His book was the one that got me worried that I was going to have punctured primers.
Then I checked the bolt I've been in using that carbine, and it too was way out of spec if done that way.  This is the carbine that's had some of Korean surplus fail to fire. 


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Post Options Post Options   Thanks (1) Thanks(1)   Quote floydthecat Quote  Post ReplyReply Direct Link To This Post Posted: Jan 27 2022 at 8:06am
My experience with punched primers has usually been sharp noses due to damaged pins or long (very long) head space. You can sometimes shorten the pin nose and get by with a long head space situation, but you then tinker with the carbine firing at all.
When fired, the case is suppose to expand and grab the chamber walls. With sloppy head space, the brass is rapidly forced back against the bolt face and pin nose (bolt thrust). The hammer is whacking the pin while the primer is being driven back into the pin nose. Of course this happens to some degree in any action regardless of head space, but long head space aggravates this condition and increases punched primer frequency.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sawbones Quote  Post ReplyReply Direct Link To This Post Posted: Mar 12 2022 at 9:21am
Newbie here.  Just acquired a Quality Hardware carbine in seeming pristine condition.  A closet queen, I suspect, from the collection of a recently deceased collector.  Essentially no wear in moving parts.  Scotch tape headspace method?  Please enlighten me. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote floydthecat Quote  Post ReplyReply Direct Link To This Post Posted: Mar 12 2022 at 9:34am
I will shoot you a PM.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote M1WAM21 Quote  Post ReplyReply Direct Link To This Post Posted: Mar 12 2022 at 11:01am
Just a quick follow-up, on the new gas piston and nut.  Now that it's been determined my headspace is 1.295, I ensured all cases were less than 1.294, and reloaded ~100 rnds with 15gr W680 and 113gr cast bullets (GC'ed).  Anyway, all rnds cycled flawlessly, with an Avg speed of 1885fps, and the M1 worked perfectly with the new piston/nut.  The slide deflector never budged forward, so my handguard is now free from that impact!  Thanks again for ALL the insight and guidance on this issue.

One additional question.  While I tightened the new gas piston nut, what I found when disassembling for cleaning, was that the nut was loose.  It had not backed out much at all, but was loose.  So, can I wrap that nut with a couple of wraps of teflon tape, or would some Loctite 243 be a better choice?  I'm really not inclined to attempt to stake it in with a punch, as I'm not a machinist, and would hate to mess up the gas cylinder itself.  

All the best....
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Post Options Post Options   Thanks (0) Thanks(0)   Quote floydthecat Quote  Post ReplyReply Direct Link To This Post Posted: Mar 12 2022 at 11:13am
Many here would suggest that you just tighten the nut (not extremely tight) and keep a check on it now and then. Some nuts were never staked in the first place. If it continues to work lose, then might be the time to consider staking or loctite. I never stake mine and have this thing about using glue on any firearm.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote W5USMC Quote  Post ReplyReply Direct Link To This Post Posted: Mar 12 2022 at 11:21am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote painter777 Quote  Post ReplyReply Direct Link To This Post Posted: Mar 12 2022 at 11:46am
Last summer while going over our Rifle club owned carbines, I ran across one with the late slide hitting the handguard. The HG was a replacement, I believe Asian Kapur. Upon inspecting and measuring I found the retaining tab was shorter, I believe the Tab had been field replaced. But don't know for sure.  Replaced with a marked GI and had no more contact to the rear of HG.

Curious if the OP's HG is marked?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote M1WAM21 Quote  Post ReplyReply Direct Link To This Post Posted: Mar 12 2022 at 12:05pm
And again, really appreciate the credible feedback, and was a bit relieved to hear that the nut not being staked is not a problem!  I'll just keep an eye on it from now on.

And no, the HG has no marks whatsoever on it...I have gone over it with a very close magnifying glass inspection, and there is nothing anywhere on the stock, or the handguard.  As noted earlier in this post, it is a Blue Sky import, and from the timeframe that I know it was purchased, I do believe that it came from Korea, and was purchased at a local gun show, back when there were plenty to be had at a reasonable price. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote New2brass Quote  Post ReplyReply Direct Link To This Post Posted: Mar 12 2022 at 12:44pm
The TN handguards can be troublesome at times to find the markings. There were plenty of post was HGs made without markings.
Is yours a 2 or 4 rivet HG/.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GotSnlB28 Quote  Post ReplyReply Direct Link To This Post Posted: Mar 12 2022 at 12:51pm
Loctite blue is a good choice and it doesn't do any harm. Inexpensive. I haven't had one come loose where I applied it.
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