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9/22/2017 12:11:25 AM
Posted: 7/4/2005 10:18:49 AM EDT
Believe it or not, I have a friend and that friend is having a problem with cam pins breaking in his AR. He has put GOD only knows how many thousands of rounds through this rifle. Recently he had to replace the bolt after the original broke. Shortly thereafter the cam pin broke. he didn't have a spare and since my AO is a desert in regard to gunshops, I made a new cam pin from 4140 and hardened it. That pin lasted exactly 10 rds before it broke in the exact same way that the previous pin broke. ( I suspect that the pin that I made my have been too hard) but, since it broke in the same place as the last one, I suspect that there is another problem.

If the cam pin hole and the locking lugs on the bolt were out of time, could that cause such a failure?
Have any of you had the same problem? ( cam pins breaking)
Link Posted: 7/4/2005 5:50:27 PM EDT
[Last Edit: 7/4/2005 5:50:47 PM EDT by Dano523]
Check the alignment of the bolt lugs to barrel extension lugs to make sure that they are not binding on lock/unlock.
Link Posted: 7/5/2005 12:25:13 AM EDT
Link Posted: 7/7/2005 3:49:49 AM EDT

Originally Posted By Tweak:
how did you harden the pin and how hard was it?




I heated the finished part with an acetylene torch until it was bright red/orange and dropped it into a cup of water. I did not do a formal hardness test, but I couldn't cut it with a file.
Link Posted: 7/7/2005 10:51:48 AM EDT
You stated that the bolt broke first and then the cam pin when a new bolt was installed.With God only knows how many rounds through the gun as stated I would have probably replaced the cam pin with the new bolt as they do wear in together. I would also check the gas port dimension to see if it has opened up beyond spec. Enlarged ports change the timing of the bolt unlocking.The bolt will open sooner under higher chamber pressures causing battering and eventually the bolt breaking at the cam pin hole the weakest part of the bolt. Tweak should know the gas port dimension for what length barrel you have , they are different for each length barrel.
Link Posted: 7/7/2005 11:40:11 AM EDT
Link Posted: 7/9/2005 9:05:34 AM EDT

I heated the finished part with an acetylene torch until it was bright red/orange and dropped it into a cup of water. I did not do a formal hardness test, but I couldn't cut it with a file.



anything thinner than .5" should be oil quenched to prevent cracking........but, the biggest thing is you need to temper it after hardening it!!!....i'd go to about 750 deg farenheit for a spring temper and max toughness on this part......wear resistance isn't that big an issue w/ this part

as previoulsy mentioned, i'd check for gas port erosion causing early opening....you'll see extensive impact marks just at the rear corner of the cam pin cut out in the upper reciever if that's the prob...

good luck, and keep us posted!
skintback
Link Posted: 7/9/2005 4:10:33 PM EDT
I am grateful for the input folks. Thank you very much.


Skintback: You mentioned 750 degrees. What is the proper procedure for tempering this part?
Link Posted: 7/10/2005 12:49:15 AM EDT
Link Posted: 7/10/2005 1:09:02 PM EDT

Originally Posted By Tweak:

Originally Posted By skintback:
you'll see extensive impact marks just at the rear corner of the cam pin cut out in the upper reciever if that's the prob...



ah, another recurring question on here.

how does early opening cause the wear at the rear of the cam pin clearance cut?



A correctly Mfg'd cam pin will have the corners cut at an angle and not left square ,if these corners are not quite cut back far enough they will catch the corner of the receiver cut and wear it down some what .
As to making your own cam pin , I would find out what metal is being used as I do not think it is your run of the mill tool steel. This will also dictate the type of heat treating process being used. The also have a dry film lubricant that is applied per mil spec which does impregnate the metal of the cam pin.
Link Posted: 7/10/2005 1:48:21 PM EDT
Link Posted: 7/10/2005 10:18:25 PM EDT
Link Posted: 7/11/2005 7:29:51 AM EDT
anytime you harden a martensitic steel like 4140/O-1/1095/etc, you take it to full hardness....this leaves the steel very wear resistent, but extremely brittle....... reheating the steel to a temperature less than the hardening temperature allows some of the stress to be relieved, and greatly improves toughness...this is at the expense of wear resistance.......


most non cutting applications of martensitic steel, 'cept for bearing stuff, is done so the part can flex and absorb stresses, but return to it's original shape.....like a spring.......... a non hardened part will flex and stay that way........

tempering is done by giving the steel a long soak at the temperature required to give the final characteristics desired......... slow heating, soak for 1-2hrs for small parts, and slow cooling is best ... since the temp is below 1250, you don't need to worry about atmosphere protection or anything like that....just pop that puppy in the oven!!

you can get the tempering charts on any steel from the supplier/manufacturer.....these will give you the hardness after tempering at different temps.......

i won't tell ya how many thousands of knife blades of O-1/A-2 i've tempered in a kitchen oven!!!..lol....

the american society for metals is the place to find just about anything on any metal...they are the source for heat treating info!
Link Posted: 7/11/2005 7:56:48 AM EDT

how does early opening cause the wear at the rear of the cam pin clearance cut?



if the bolt unlocks while chamber pressure is excessive, the bolt will be pushed rearward in the carrier a longer amount of time than normal........ when the cam pit hits the rear of the cam slot in the carrier, it slams the cam pin over towards the locked position......since the carrier has moved rearward enough to no longer be aligned w/ the relief cutout in the upper, the cam pin slams into the side of the carrier key slot in the upper, starting at a point that overlaps the rear of the cutout and stopping at a point when the chamber pressure reduces....


i began looking at this situation in response to probs i encountered w/ known good uppers starting to short stroke after adding my dispersion regulator ,a device that effected the gas release at the muzzle.. uppers w/ no evidence of early opening began showing it immediately.....

i would seriously suggest that anyone who runs a removable can, or any other device that effects muzzle gas timing on an AR should look at an adjustable port for it.....
Link Posted: 7/11/2005 11:17:50 PM EDT
Link Posted: 7/12/2005 8:22:37 AM EDT

I'll have to ponder awhile on that.



i know what you mean on that one!!!...lol...


i had seen a few shorties doing it, but they were doing it when they were brought to me... when i took some of the old airforce slab side/pencil bbl uppers that had no evidence of it and installed my dispersion regulator on them, they started showin probs.......some short strokin, and all showin marks on the inside of the upper that weren't there before....... the dispersion regulator does effect the gas release at the muzzle, but other than that, should have no effect on the gas system.......


my only explanation of how this could happen is that the regulator simulates a longer bbl, and effects the gas system loading...... if this is true, then uppers showing similar marks on the inside of the upper are getting excessive gas/early opening.....and would prob work alot better/longer w/ a proper adjustment on gas system.....


tweak, if you have any other ideas as to why this is happening, let me know!...... i've got a cheapy gas block drilled/tapped for a 6-32 adjustment screw.....all i gotta do is hook up w/ a bud w/ a legal lower to stick a shortie on, and i'll be able to tell ya if this works..... i obviously don't have the yrs of experience you and dano do, but i was an honor grad from the school at aberdeen on these, and worked w/ alot of them at the 7th special forces "contengincy weapons", and the special forces weapons pool on smoke bomb hill...but, that was all back in the stone age....alot has changed since then on these rifles!...lol... i'd really appreciate some feedback from folks w/ more experience...

many thx
skintback

ps: the A1 mod of the regulator is showin amazing potential!.........movement of the gun comes to a complete stop approx 25-30% into the recoil cycle, then pushes itself back to original position...the next mod will time the muzzle return to the cyclic rate of milspec rifles...... w/ a stock A2, my group size w/ the regulator is approx 1/3 of what i can do w/ a stock rifle........everyone that's shot it has had similar results........ in "infantry speak", instead of holding a house sized group on auto @400m, it will hold the front door!..........or, instead of a group the size of a deuce and a half at 200m, it will hold the drivers door......if the drs will leave me alone for a while, i may get this finished and into the military system...... i'm hopin...
Link Posted: 7/12/2005 9:52:43 AM EDT
Speaking of cam pins and such, this weekend i reassembled a shorty upper that i had dissasembled a while back pending a barrel swap out project. I put off the swap and reassembled the upper as it was. It is a colt slickside upper with an unkown 11.5 upper with 5.5 fh. Yesterday i went to shoot it to check out the sights and be prepared for when i have time to fine tune it. I fired 3 shots of wolf and stopped to check it out. I noticed it smoking from everywhere and figured it was from a light penetrant that i hosed out the gas tube with, i know your not supposed lube the gas tube but figured it might loosen up any buildup. When i engaged the bolt stop and looked at the inside the first thing i noticed was cam pin marks. The first3/16" starting at the cut out was a little more that slightly and kind of dug in. Moving rearward the next 3/16" shows just slight additional wear. after that the next 1/4" shows a shallow worn spot. The next 1 1/4" shows new wear not quite smooth. I continued with 10 shots of wal mart winchester 45 jhp's. then finnished the 20shot mag of wolf 62gr fmj. I shot a total of 30 shots. After the first 3 shot's i kept checking and it did'nt seem like the wear was progressing. The marks were not there beflore the first 3 shots. I used the same parts as before. I forgot to mic the gas port when i tore it down but i did soak the gas port with clp and made several strokes with a pipe cleaner and never wiped it out.

I don't have a digital camera so i hope i was discriptive enough. Also poi was a little low and left with wolf and still on with wwb 45's. at 50 yds.
Link Posted: 7/13/2005 7:42:16 AM EDT
hey gravy,
i'm sure no expert at these, but it sounds like you're seeing the exact same thing i am........i would bet money your gas port size is enough to allow the 11.5" to function w/ just a birdcage on it.......... adding the long flash suppressor is effecting the release of gas at the muzzle, and is allowing the gas sys to "over fill"...

the good news is that this obviously isn't a major prob for most folks/configurations......... it's hard to argue w/ 40+yrs in military svc around the world... it does look like another case of where the farther you get from standard milspec, the more probs you'll see.....

i would only consider adressing this as a repair issue if it was causing excessive wear to the gun, or function probs like short strokin..... i'm a believer in "if it works, don't fix it!!!"

i am a bit curious if folks that have installed a can/suppressor on their rifle are running into probs from this.......it would make a case for a gas control valve, where you could set it to off, norm low/high, or for the can.....

i'm beginning to get the impression you need to order bbls ported for the finished length of the bbl assembly, not just the bbl..... i imagine your bbl would function normaly w/ a port sized for a 16"bbl instead of an 11.5"

good luck, and keep us posted
skintback!

Link Posted: 7/28/2005 3:54:13 PM EDT
I just got back from 2.5 weeeks offshore! So anyway i will monitor the issue till' i get my sbr paperwork and drop the fh.
Link Posted: 7/30/2005 4:21:18 PM EDT
Just to add another data point and add further confusion. I had a standard postban BM 16" shorty with a A2 receiver. When the ban ended, I switched out the sixteen inch barrel for a 20" gov't profile barrel. After switching out the barrel, I noticed that the wear behind the cam pin cutout had gotten much worse, to the point that it had had pushed out some aluminum causing the charging handle to slightly bind in the receiver.

It should also be noted that I started shooting wolf around the same time that I switched out the barrel and is all I shoot in this rifle.
Link Posted: 7/30/2005 10:53:12 PM EDT
[Last Edit: 7/30/2005 10:57:50 PM EDT by Dano523]

Originally Posted By skintback:

if the bolt unlocks while chamber pressure is excessive, the bolt will be pushed rearward in the carrier a longer amount of time than normal........



As soon as the carrier gas sytem is charged, the carrier is moving reaward and starting to cam the bolt unlocked. The differance between a normal unlock, and a overcyle/ more pressure bound bolt lugs to the receiver extension lugs is that the cam pin will show more wear on the bearing unlocking side of the cam, not the back or front of the cam pin. Simple put, the bolt/cam never fully makes if back to the back of the carrier slot for anything more than a bounce on bolt closer (carrier in spec).


when the cam pit hits the rear of the cam slot in the carrier, it slams the cam pin over towards the locked position......since the carrier has moved rearward enough to no longer be aligned w/ the relief cutout in the upper, the cam pin slams into the side of the carrier key slot in the upper, starting at a point that overlaps the rear of the cutout and stopping at a point when the chamber pressure reduces....


again, mic a bolt fully pressed into the carrier (back of bolt lugs to the face of the carrier, then mic a bolt locked into the extension fully seated tight against the lugs. You will find that the is free distance still left in the cam slot when the bolt is lock into the barrel lugs, and this still leaves plenty of the room for any lug enlongation/compresion after the round has been fired, yet before the bullet has reached the gas port to start the carrier in movement for the bolt to be pressed more into the carrier without the cam pin making contact with the back of the slot (carrier, cam pin in spec).


My best guess without having looked at the rifle is still some sort of bolt binding that taking place as the carrier starts to unlock, and this is causing more torsion pressure to the cam pin than normal (read the carrier is moving back, the bolt still pressure bound/stuck to the lugs, and the key is the link that has to unlock it. Simple stated, it's not the rearward pressure of the bolt, but the tension pressure of the cam against the side wall slot of the is causing the problem (first bolt, with the second bolt being too brittel to take the lateral tension (read have some flex and not snap).
Link Posted: 7/31/2005 5:16:21 AM EDT
thx for the reply dano.......

the impact is occuring after unlocking, during the extraction, or possible the primary initial extraction, of the cycle........ it appears that when the bolt lugs unlock, there is enough chamber pressure to push the bolt rearward in the carrier w/ enough force to impact the rear side of the cam pin cam slot in the carrier.....this cams the pin over and impacts the side of the upper........ it has also left some marks on the side of the lugs of the bolt, the same distance from the front of the lugs as the delay time on the carrier slot.......

i had a chance to try adjusting the gas system on this shorty.......it's a 10.5" M1sales upper w/ <20rnds through it and the new carrier/bolt...ammo is white box winnie/imi 55gr ball ...............i added my dispersion regulator on the end, which added a shade over 2" of total length, but does effect the gas release at the muzzle............ it's got a cheapy gas block on it, so i drilled/tapped it for a 6-32 screw w/ a .0625 radius on the end to match the id of the tube..........i got 100 reliability/bolt hold open w/ a tube id of only .093..........that's 25% smaller than the .125 of the tube.....it actually functioned perfectly at only .0625, but i wanted a little extra for reliability.......

i realize i've created the prob by adding the regulator, but i don't think i'm the only person adding items to the muzzle of ar's that's seein probs........ i gotta wonder about the CAR15's bein made w/ the 11.5"bbls and the long break.......... i would fully expect them to work best w/ a port size for the 16"bbl instead of the 11.5......

thx again
skintback
Link Posted: 8/14/2005 4:04:22 PM EDT
Skint, am starting to think lower gas pressure is the culprit for wear around the cam pin cutout in the upper receiver. The only time I have really seen this problem is with my 20" shooting wolf (see previous post above). If not enough gas pressure is getting inbetween the carrier and the bolt, the carrier will not move back as violently, causing the bolt to rotate enough to disengage the lugs but not enough to move the cam pin all the way to the end of the cam pin slot in the carrier and out of the way of the cam pin cutout. Once the bolt lugs have disengaged, the carrier continues to move backward, pulling the cam pin into the rear edge of the receiver cutout.

The only other possible explanation I can think of is that if the barrel extension lugs are a bit out of alignment, it may allow the bolt lugs to prematurely disengage before the cam pin has the chance to move fully to the end of the cam pin slot.
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