From WOW! to Bitter Disappointment
- Thread starter sobo4303
- Start date
NOpe.
When I assembled the rifle I did a few checks.
Firstly after lapping and cleaning the bolt, I placed into receiver and checked for clearances. Roller to side of receiver and bottom of the receiver had about .020" - 0.025" clearance all round. The bolt did not wiggle to much, but I checked clearance by pushing right lug to receiver and then the left lug to ensure no contact. I also measured the headspace clearance with the go gauge and that was approximately .001".
Next, I only assembled the bolt and op rod (no spring) to check for tolerances and binding and/or tightness. Pressing the op rod tight to the receiver (bolt closed position) the bolt was not tight in any direction and wiggled freely. That included the roller as well, it was not tight. As a comparison, I checked it against the Norky bolt and while there was less wiggle, there was no binding and the bolt moved freely. I then relaxed the pressure against the op rod, and again, no binding or tightness of any sort.
I then slowly pulled back on the op rod and tested the bolt movement along it's travel, one with the op rod tight against the receiver and the other more relaxed. While I did notice the bolt shift in the receiver, there was no binding. The bolt got tighter (relatively speaking) toward the rear of the receiver, but no binding of any sort. I did however notice when firing that there were a few misfeeds, in that the bolt seemed to be moving quite hard once a mag with dummy rounds was inserted. The bottom of the bolt shows that contact was being made with the casings throughout travel. I chalked it up to a magazine issue. In hindsight, I compared the two bolt diameters at the "rub" spot. The Norky was 0.572" and the 7.62 was 0.626". That explained the energy loss from the action which caused the misfeeds.
At all times, again, the bolt moved freely without magazine, but it was tighter in comparison than the Norky bolt both in "wiggle" and when chambered with resized dummy rounds.
When I assembled the rifle I did a few checks.
Firstly after lapping and cleaning the bolt, I placed into receiver and checked for clearances. Roller to side of receiver and bottom of the receiver had about .020" - 0.025" clearance all round. The bolt did not wiggle to much, but I checked clearance by pushing right lug to receiver and then the left lug to ensure no contact. I also measured the headspace clearance with the go gauge and that was approximately .001".
Next, I only assembled the bolt and op rod (no spring) to check for tolerances and binding and/or tightness. Pressing the op rod tight to the receiver (bolt closed position) the bolt was not tight in any direction and wiggled freely. That included the roller as well, it was not tight. As a comparison, I checked it against the Norky bolt and while there was less wiggle, there was no binding and the bolt moved freely. I then relaxed the pressure against the op rod, and again, no binding or tightness of any sort.
I then slowly pulled back on the op rod and tested the bolt movement along it's travel, one with the op rod tight against the receiver and the other more relaxed. While I did notice the bolt shift in the receiver, there was no binding. The bolt got tighter (relatively speaking) toward the rear of the receiver, but no binding of any sort. I did however notice when firing that there were a few misfeeds, in that the bolt seemed to be moving quite hard once a mag with dummy rounds was inserted. The bottom of the bolt shows that contact was being made with the casings throughout travel. I chalked it up to a magazine issue. In hindsight, I compared the two bolt diameters at the "rub" spot. The Norky was 0.572" and the 7.62 was 0.626". That explained the energy loss from the action which caused the misfeeds.
At all times, again, the bolt moved freely without magazine, but it was tighter in comparison than the Norky bolt both in "wiggle" and when chambered with resized dummy rounds.
The roller failure was really bugging me so I examined the failure under a magnifying glass. I suspect a defect in the roller, as the failure occurred along the groove where the retaining ring holds the roller in place. Either the ring was oversized, the roller groove was cut too deep or the roller was too brittle and the forces exerted on the roller in the cycling action was too much for the metal to handle.
I have a couple of replacement rollers on order. 7.62 are also very concerned and left my number for Chris to contact me and resolve. I will keep y'all posted as to the progress.
I have a couple of replacement rollers on order. 7.62 are also very concerned and left my number for Chris to contact me and resolve. I will keep y'all posted as to the progress.
An update, Chris contacted me about an half hour after writing the last post. He was very surprised with the roller failure and was prepared to send out a new bolt. After having spent the time to lap it and get it "just right", I suggested that it may just be a 1 in million metal failure or something. He is sending me some replacement rollers and wants to be kept informed in case there is an issue with the roller mechanism.
I told him once I remove the roller, I will inspect and measure the roller axle for deformities or irregularities and proceed from there.
I am pretty impressed by their prompt response, as this type of failure seems quite rare, from what I have been told.
I told him once I remove the roller, I will inspect and measure the roller axle for deformities or irregularities and proceed from there.
I am pretty impressed by their prompt response, as this type of failure seems quite rare, from what I have been told.
You need to smear grease all in around hte rolled and inside the oprod hump. I assume you did so, but even if you did not, the roller would have peened before shattering like that.
I suspect it was over-hardened at manufacture and failed from being too brittle.
I suspect it was over-hardened at manufacture and failed from being too brittle.
Agree but if the roller was never "PACKED" with grease it can cause such a failure.
Please just watch around the 2 minute mark were he shows a product he baught from brownells to pack grease inside of the roller.
[youtube]6m4GQSH-5Ek[/youtube]
Brand new manufactured bolts from 7.62 are not packed with grease and with use failures with the roller like such that have been posted are possible. As from your photo's it looks like there might be grease around the out side in the op rod channel but not inside the roller. Since you are a mechanic I will use this anolgy. The wheel barings are pack with grease to prevent what?
Yup on both counts. I learned as a youngster how to pack wheel bearings...applied the same principles to my M14. Even showed a few last year at the clinic how to do it.
I am fanatical about lubrication and the bolt roller was packed.
I looked at the failure and there are spots on the roller where the failure is angled outward. I believe that either the bearing surface was not even, had a burr or something or the retaining ring was oversized as compared to the track in the the roller.
The roller, prior to installation, did not bind and felt quite smooth when turning, mind you it was full of grease too.
When I remove the roller(s), I will be able to better inspect the bolt. But remembering my metalurgy class and the characteristics of certain failures, this one was either in the roller metal itself or something on the underside (such as the groove) to have such a uniform circumferential failure.
Thanks for input, it really is appreciated.
I am fanatical about lubrication and the bolt roller was packed.
I looked at the failure and there are spots on the roller where the failure is angled outward. I believe that either the bearing surface was not even, had a burr or something or the retaining ring was oversized as compared to the track in the the roller.
The roller, prior to installation, did not bind and felt quite smooth when turning, mind you it was full of grease too.
When I remove the roller(s), I will be able to better inspect the bolt. But remembering my metalurgy class and the characteristics of certain failures, this one was either in the roller metal itself or something on the underside (such as the groove) to have such a uniform circumferential failure.
Thanks for input, it really is appreciated.
1 more question for ya sobo4303.
Does were the roller seperated matches the grove on the reciver on witch it rolls on?
The grove is shown here;
Just wondering if it they made the roller wall to thin on yours as mine never has shown this issue yet.
Does were the roller seperated matches the grove on the reciver on witch it rolls on?
The grove is shown here;
Just wondering if it they made the roller wall to thin on yours as mine never has shown this issue yet.
I thought about that, so ordered two sets from Brownells too!
Satain,
My roller only comes into contact with the op-rod. There is about 0.030" to 0.040" between the back of the roller to the face of the receiver wall. It crack sorta reminds me of the splits that occur when cutting glass, ceramic tiles or those concrete sidewalk blocks. They will break nice and cleanly when scored.
When I get the new rollers, one thing that I will check a little more closely is how the roller is sitting in the op-rod hub. I know that the Norky bolt was slightly narrower and perhaps there may be a resultant stress point being applied too the roller from the perpendicular planes of the roller surface and the op rod hub. I did check for that issue when I assembled the bolt because I knew that the roller axle was slightly wider, but there was ample free play and no binding whatsoever when I cycled through all the movements (no springs).
We'll see what happens when I get the new rollers.
