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Gas Cylinder/Piston/Nut question |
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M1WAM21 
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Joined: Jan 20 2022 Location: 35614 Status: Offline Points: 11 |
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 Topic: Gas Cylinder/Piston/Nut questionPosted: Jan 22 2022 at 9:38am |
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Greetings -
Fortunate to have inherited an imported (Blue Sky) Inland carbine a few months ago, and have been learning all I can from this forum...it is great. It shoots fine, but I noticed the slide was hitting the hand guard a bit, so I studied as many pictures as I could, and tried to come up with the right solution for the pointed portion of the slide making a small dent in the center of the hand guard.. The slide is a Type V slide, marked 7160091. It appears to me that the forward slide stop is actually against the gas cylinder. Couple of rookie questions. The gas piston nut had been staked, but it came off easily, and I cleaned all the threads as well as the gas piston. I made the decision to order a new piston and type II nut from Amherst, as their website indicated they were both USGI equipment. My thought being, a new piston and and nut would stop the slide a little sooner, so that the point of the deflector portion of the slide, would not contact the hand guard. First question is what is the actual length of the gas piston supposed to be? Original: 0.635" overall length 0.398" Head diameter 0.299" Shaft diameter New: 0.693" overall length same head and shaft diameters as the original. Does the gas piston itself wear down that much over time? I'm sure it depends on how much it has been shot. Next question is, does the piston protrude a bit out of the nut, such that the slide stops against the piston, or the piston nut? It appears there is quite a bit of impact when the slide moves forward and stops, and I just don't want to damage the slide, or the gas cylinder at all, if I put in the new components. Thanks... |
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floydthecat
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Joined: Oct 13 2016 Location: Mississippi Status: Offline Points: 1998 |
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Posted: Jan 22 2022 at 10:09am |
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Piston lengths on the one’s I have measured run from .690 to .698. The piston will protrude out past the front of the nut slightly when seated no matter which type nut is installed. I don’t think I have ran across a piston any shorter than around .690, but I have certainly not measured all of them. Slide contact with the hand guard is often reported with the late issue slides. Early slides were void of the angular shape that was designed to change ejection-direction of fired brass. Some shooters have reported they polish that section of the slide to prevent contact, but in your case….the longer piston might correct it.
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jackp1028
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Joined: Jan 01 2016 Location: Cloudcroft, NM Status: Online Points: 1276 |
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Posted: Jan 22 2022 at 12:45pm |
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Hi and welcome to the Forum from New Mexico!
The Dept of the Army Ordnance specification for the gas piston OAL is .695" min. to .705" max. The slide should be stopping against the protruding piston which is much harder (Rc 48-53). Your piston must have originally been out of specification. It's hard to imagine it could have worn that much (over .060"). Check to see if there's any significant wear of the inner surface of the operating slide. Otherwise I would not expect the new parts to cause any damage to the slide or gas cylinder. |
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JackP
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floydthecat
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Posted: Jan 22 2022 at 12:54pm |
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.635 is really, really short. They are very hard little rascals and I’ve never seen one damaged or worn that far. That much really might solve (or at least diminish) his slide to hand guard contact.
Like Jack says, the slide stops upon contact with the piston after it bottoms. A short piston or serious depression in the slide contact area will allow the nose to bang the hand guard.
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New2brass
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Dan Pinto, Photo Editor Joined: Nov 29 2015 Location: CT Status: Offline Points: 4656 |
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Posted: Jan 22 2022 at 1:39pm |
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The wall where the piston contacts is also machined further back on the later slides. Jack, do you have a spec for that for him to check?
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jackp1028
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Joined: Jan 01 2016 Location: Cloudcroft, NM Status: Online Points: 1276 |
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Posted: Jan 22 2022 at 2:17pm |
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The rear wall of both the 7160091 (Late Type V) and 7161843 (Type VI) operating slides should be .480" to .485" thick. The part number is usually stamped on the outside of the inertial block of these slides, but not always. I do not have drawings of the earlier Type V that has the thicker rear wall, but I would expect it to be approximately .04" thicker. Here's a link that explains the difference.
http://www.uscarbinecal30.com/slides.html Just for reference, my Inland with the later Type V slide almost touches the handguard. An early Type V slide would not be able to touch the hand guard, even if well worn. |
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JackP
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New2brass
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Dan Pinto, Photo Editor Joined: Nov 29 2015 Location: CT Status: Offline Points: 4656 |
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Posted: Jan 22 2022 at 2:52pm |
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M1WAM21
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Posted: Jan 22 2022 at 7:03pm |
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Thank you ALL, for the detailed reply with the pertinent information! It is extremely hepful.
I will attempt to apply some pics with my response, to help make it cogent. Here is a shot of the slide hitting the hand guard with the original piston.  And here is a pic of the rear slide...no significant wear at all. The next pic shows the thickness of the rear wall of the slide...it measures .482" thick.  And here is the thick rear wall...It looks good and measures to what was indicated.  I will put in the new piston, and nut, and send a pic of the front slide and handguard relationship soon.. Here is a pic of the old (bottom of view) and new (top) pistons...not sure how the old piston was so worn down, as there is not visible mushrooming of either end of it, it is just clear, it is way too short and worn. I will continue my research, and thanks again...  Best regards - |
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floydthecat
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Posted: Jan 22 2022 at 8:12pm |
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Just a few thousandths short in piston length can make a big difference in how the action cycles. The type II nut was counterbored a few thousandths to lengthen the piston stroke. Makes me think that carbine might be picky about what it eats with a piston that short.
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New2brass
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Posted: Jan 22 2022 at 9:02pm |
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What make carbine and what make barrel? could this be a fix on a barrel that was altered for a headspace issue? Maybe take some pictures of the chamber.
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floydthecat
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Posted: Jan 23 2022 at 7:40am |
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One of those things we will be curious about until we figure it out. The type II nut was counterbored about .060 as I recall, which added that much to the piston-stroke. A piston this short used with a type II nut would bring the stroke closer to a +-.700 piston used with a type I nut.
Working with caliber conversions, I learned that altering the length of a piston-stroke seriously matters in what the carbine will cycle. The designers and users obviously had a reason to increase the piston-stroke with the type II nut and I suppose it was to achieve stronger cycling. What does all this matter….I don’t know, but am just seriously curious about how/why a piston this short got in a carbine and it still functioned. The carbine is fairly liberal in what it will shoot, but .040 off the piston length seems to be stretching quite a bit.
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M1WAM21
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Posted: Jan 23 2022 at 9:52am |
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As I've only put about 60 rounds through it, I really can't vouch for how particular it is to the ammo that will feed. At this point, I'm just trying to study it, to make sure it is safe and reliable, before I really take it out with various rounds. To be honest, what I have shot, were cast reloads and JHP reloads. I did measure all of them on a chrono, and they were moving at about 1800fps, with no signs of high pressure on the casing, ejection or primer pockets.
Anyway, I did put in the new piston and nut, and it does make a difference on the slide deflector and handguard relationship:  Regarding the other question, it is an Inland make with an Inland barrel, imported by Blue Sky, and I believe it was purchased in teh 80's.    And finally, here is a shot of the chamber. Hope this is what you wanted. I plan to size a few spent casings today, disassemble the bolt, and do the scotch tape measurement of the headspace in the next few days, to make sure it is good. Will report back in to verify. Thanks again all!  |
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floydthecat
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Posted: Jan 23 2022 at 10:11am |
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Check the actual head space for sure. I think the thing to really observe is how the action locks up using that longer piston. Head space checking will not reveal if the longer piston is holding the slide back far enough to prevent full rotation and lockup. Just a thought.
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jackp1028
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Posted: Jan 23 2022 at 4:05pm |
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Based on your last picture, the slide deflector is exactly where I would expect it to be with the correct gas piston. Also, there does not appear to be anything unusual with the fit of the barrel to the receiver. There is plenty of slide over travel both in the forward and rearward direction for the bolt to be completely in battery even if the piston were to make early contact due to any alteration of the barrel fit for a headspace problem. During normal operation expect the slide to move at least 1/4" rearward before the bolt begins to open. The bolt should return to battery as the slide moves forward, again, over 1/4" before it makes contact with the piston.
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JackP
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M1WAM21
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Posted: Jan 25 2022 at 11:49am |
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Thanks all for the review, and commentary on the new gas piston being installed. And the behavior of the slide and the locking of the bolt, is exactly as Jackp1028 describes above.
So I have since disassembled the bolt, and resized a few cases to check the headspace, with the scotch tape process described in a separate post by FloydTheCat. The resized case was exactly 1.290", and the bolt easily fell into battery. With a couple of layers of trimmed scotch tape, at 1.295" it was tight, but with just a nudge of my index finger on the right lug, it locked into battery. At 1.296", the bolt would not close. So I presume, my headspace dimension then would be 1.295" , and would be correct and within specification? If that is indeed correct, it would then appear to me, for any reloads, I could never use a case that was longer than 1.294? And any that were under 1.290", there would be too much space between the bolt and the case head for the firing pin to engage? I've had a couple of light primer strikes that did not fire is the reason for my question, and they were all reloads. Thanks again for the replies so far...it has really helped. Best regards -
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Matt_X
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Posted: Jan 25 2022 at 12:12pm |
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After seeing various acceptable ranges, I went to the the US Tech Manuals. Yours is certainly in the acceptable range for a carbine thats been used. 1.290 and 1.300 are the minimum and maximum field gages. The 5th echelon used a 1.295 gage for use with newly rebarreled carbines. Not your situation. Others may see it differently but thats my reading. |
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floydthecat
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Posted: Jan 25 2022 at 12:57pm |
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If it head spaces at 1.295, as long as your case does not exceed 1.295 you will be safe on the tight side. I would suggest that you stick with the max of 1.294 like you mentioned. However, you are only limited on the lose side by what the action can actually reach and fire. The SAAMI spec allows .020 of head space using the shortest case of 1.280 and the field gauge of 1.300. 1.300 is not a sacred number, 1.302 is often used taking allowable “slack” out to .022.
The short of this is, most commercial brass measures closer to 1.286-or-so. Don’t limit yourself to brass of exactly 1.290. You can very safely shoot brass down in the 1.285 area. Your 1.295 head space indicates that you would still be within SAAMI spec of .020 using brass as short as 1.275 (1.295-.020) All this is pretty academic, you have a nice tight action and are never likely to find brass any shorter than 1.286–or-so. All you have to do is watch the tight end and no longer than 1.294. In reality, the carbine will shoot much sloppier than that. I think jack would agree that as long as the firing pin can strike the primer…..it will keep shooting well past a head space of 1.300 and more likely out to 1.310. I have actually handled and fired carbines that were a tad loser than that. As in a factory new Iver Johnson. The “math” looks good. You might have other issues with the lite primer strike problem.
Edited by floydthecat - Jan 25 2022 at 1:22pm |
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Rebel92
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Posted: Jan 25 2022 at 3:37pm |
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Good information right here, Floyd and I were just chatting about this.
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M1WAM21
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Posted: Jan 26 2022 at 10:01am |
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Thanks again FloydTheCat, for the detailed response regarding case length...as that is now what I'm focused on. I'm confident of the max length for a sized case being 1.294" for my action, so I'm now trying to determine what my shortest case could be, and still fire.
So I measured my firing pin, and it is 2.950", which from my review of the various forum posts here, is within acceptable range. Just a rookie question for the experts, as I'm trying to determine what my pin protrusion length is. When I put the firing pin in the bolt, I use my thumb to hold the rear tang to keep it tight. Here is a picture with the rest of the question below the picture:  Notice the gap where the red arrow points to. When released, does the hammer rest against the firing pin all that way to it's stop in the front of the bolt, or does the firing pin rest against the face pointed to by the yellow arrow? Just want to make sure I'm measuring the correct length of pin protrusion, so I can determine how short a case I can reload? The firing pin itself, does not appear to be worn on either end, nor does the hammer. Never seen any reference to OBB marks, so not sure what manufacturer that indicates.  And just for a visual, here is the pin protrusion, with my thumb pushing the rear tang completely to it's stop in the bolt.  Best regards - |
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W5USMC
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Posted: Jan 26 2022 at 11:02am |
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"Never seen any reference to OBB marks, so not sure what manufacturer that indicates."
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Wayne
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