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Thread: Marking cam pins?

  1. #91
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    Quote Originally Posted by lysander View Post
    There is an error in your logic.

    The cam angles for locking and unlocking are equal, the bolt carrier is a cylinder and the cam pin is a cylinder. This means the contact areas as the pin moves forward are exactly the same as the contact areas as the pin moves backwards only rotated 180 degrees. That means the wear areas are exactly the same.

    The increase "slop" is caused by and increase in clearance between the parts, so if you want to reduce your shock loading, you want to reduce the amount of wear. The pin is softer than the carrier so most all the wear in on the pin.

    The loading during the unlocking camming is much greater that the loading during locking camming, therefore the "front" of the pin will wear faster than the "back". If you keep one side towards the front all the time, the front wear will accumulate faster than the back. If you switch pin orientation regularly, or randomly, the wear will be distributed equally on the front and the back of the pin, so on average the amount of wear on any one face will be less.

    Also, the cam pin does not bottom out in the cam track during locking, the carrier stops on contact with the barrel extension well before the pin reaches the back end of the cam slot, so this side doesn't wear at all. The highest load on the cam pin is at the end of unlocking when the pin bottoms out at the front of the cam slot. Again, randomly switching the orientation of the cam pin will reduce the wear on any one side.

    If you want to reduce the loads on the hole in the bolt, the way to achieve that is find a way to reduce the amount of wear on the face of the cam pin that faces forward. Flipping it 180 degrees every now and again is a better way to do that than leaving one face always facing forward....

    Personally, I think liberal use of a good lubricant in the cam slot is a most cost effective way of achieving the desired end.
    Agreed a good lube that stays put like a light grease is ideal there. But the error here is thinking this is about cam pin life and not bolt life. The carrier track is not the only wear point on the cam pin, the twisting open action wears the cam pin inside the bolt hole. That is where the slop can be real bad especially if you started with a wiggly cam pin inside the bolt hole to begin with. The bolt is holding onto the pressurized case and carrier is flying rearward, the connection point between the two is a shaft inside a hole 90* to the momentum force, when cam pin hits bottom in the carrier track during unlock, the cam pin applies pry forces to the bolt at the bottom-muzzle-side and the top-buttstock-side of the bolt cam pin hole via force applied on the top-muzzle-side of the cam pin by the carrier track (this same principle is why Knights reinforced the E3 bolt at the cam pin hole by reducing the diameter of the cam and cam pin hole hole). The wear on the reverse side of the cam pin at the carrier cam pin track is not really at issue, like you said because the cam pin only bottoms out in the track on unlock. The slop issue is the wear of the cam pin inside the bolt hole and the pry forces that happen during the shock loading of unlocking.
    Last edited by jerrysimons; 08-18-20 at 09:52.

  2. #92
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    Quote Originally Posted by Ned Christiansen View Post
    I can't help but agree that some data would be nice..... right now it's just logic and theory, but pretty strong in those areas-- but that's just my opinion.

    Thing is, to get actual data on this or anything takes a tremendous amount of time and money, same as with any of the many other incremental, theoretical improvements that we fuss over at times (that applies to gun people car performance people, bike / bicycle people, et al).

    So my take is "can't hurt, might help". For you guys talking about marking your own, I would absolutely not do it with a punch, don't go hitting a hardened part with a hardened tool. Dremel a little arrow into it, or a dot or something, lightly.
    In establishing mil spec the government who originally ordered the M16 established a standard for some reason. They must have done testing. Just assuming a product is better than mil spec because it is harder or has a harder coating may be wrong. An AR is a machine. All parts have to work together. If you make one part harder in an effort to make it better it may wear down connecting parts quicker. Metallurgists know this so do engineers who design rifles. But aftermarket guy sometimes do not get this. This miss-match happens in the automotive world where, for instance, someone builds an oil pump with a harder slot to fit into a cam dish. Then the cam dish wears away causing failure but the pump is still intact. Mil spec is mil spec for a reason. If this were to happen in this case the hole in the bolt may actually wear faster with a hardened cam pin. For my money, if you advance a component as better than mil spec you really need proof for this, not logic, not opinion.
    Last edited by Dr. Bullseye; 08-18-20 at 12:55.

  3. #93
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    Quote Originally Posted by jerrysimons View Post
    Agreed a good lube that stays put like a light grease is ideal there. But the error here is thinking this is about cam pin life and not bolt life. The carrier track is not the only wear point on the cam pin, the twisting open action wears the cam pin inside the bolt hole. That is where the slop can be real bad especially if you started with a wiggly cam pin inside the bolt hole to begin with. The bolt is holding onto the pressurized case and carrier is flying rearward, the connection point between the two is a shaft inside a hole 90* to the momentum force, when cam pin hits bottom in the carrier track during unlock, the cam pin applies pry forces to the bolt at the bottom-muzzle-side and the top-buttstock-side of the bolt cam pin hole via force applied on the top-muzzle-side of the cam pin by the carrier track (this same principle is why Knights reinforced the E3 bolt at the cam pin hole by reducing the diameter of the cam and cam pin hole hole). The wear on the reverse side of the cam pin at the carrier cam pin track is not really at issue, like you said because the cam pin only bottoms out in the track on unlock. The slop issue is the wear of the cam pin inside the bolt hole and the pry forces that happen during the shock loading of unlocking.
    The ultimate cause of a "wiggly" cam pin will be cam pin wear. So, if you want to reduce the wiggle, you have to keep the surfaces as close to original shape as possible, which means you have to reduce the wear on the cam pin, i.e., prolong the life of the cam pin.

    That wear is going to be the three places in red in the below drawing as the carrier tries to twist the cam pin out of the bolt. After a time the areas in red will become below the original surface and the amount of play will increase

    Rotating the cam pin 180 degrees on a regular bases distributes the wear over multiple areas and prolongs the time before the slop becomes a problem.

    Attachment 63440

    Knight's idea of reducing the diameter of the cam pin and its hole in the bolt is just a simple way of increasing the amount of material in the bolt to resist the twisting force. And, you pay for that with increased stress on the cam pin. But, cam pins are cheaper than bolts, so there is net gain there.

    Quote Originally Posted by Ned Christiansen View Post
    So my take is "can't hurt, might help".
    From my thumbnail analysis, I come to the exact opposite opinion, "can't help, might hurt" . . .
    Last edited by lysander; 08-18-20 at 15:04.

  4. #94
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    Well..... good logic going both ways here...... I can't say that I am totally convinced one way or the other. I'm going to do it, mark a couple cam pins, but it won't be a legit test because there are so many other factors (among them starting with used parts), and really, I'm not gonna get round counts on multiple guns that would constitute real testing. The only solution here is half of us do it and half don't, everyone do your shooting in white lab coats, keep rounds counts and other conditions noted, and report back in five years.... :-)

    I note that LanTac is also doing something similar--- FWIW. When I think about how the rearward-moving carrier impacts the cam pin I still think better-mated surfaces at that impact point (front of cam track) could be an advantage...... mated surfaces between the cam pin and cam track as the bolt is rotated also seem advantageous.... I have not as yet looked one over having this conversation in mind, I sorta need to work that in I guess.

  5. #95
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    If the cam helices were different between the opening and closing ramps, or the carrier was square, then, yes there would be a benefit to keeping the cam pin in one orientation.

    However, the cam helices are at the same angle and the carrier is round. That means the surface presented to the cam pin is identical, coming or going.

    In either case, I highly doubt it make any difference one way of another.

  6. #96
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    Quote Originally Posted by lysander View Post
    If the cam helices were different between the opening and closing ramps
    Is this the case with the LMT Enhanced carrier?

  7. #97
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    Quote Originally Posted by Disciple View Post
    Is this the case with the LMT Enhanced carrier?
    No, They changed the helix angle and altered the length of the straight portions, but the opening and closing helices are the same angle.

    This is an example of a non-symetrical cam path:
    Attachment 63575

    The locking cam is steeper than the unlocking cam. And, yes the lead-in helix is the same, but at 67degrees, 6 minutes rotation the two cam curves diverge.
    Last edited by lysander; 08-26-20 at 21:28.

  8. #98
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    Quote Originally Posted by lysander View Post
    No, They changed the helix angle and altered the length of the straight portions, but the opening and closing helices are the same angle.

    This is an example of a non-symetrical cam path:
    Attachment 63575

    The locking cam is steeper than the unlocking cam. And, yes the lead-in helix is the same, but at 67degrees, 6 minutes rotation the two cam curves diverge.
    Much appreciate the education sir...

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