Zero Gap Ring - Sheared Cross-Bolt

[SpaceAholic]

In the process of attaching a new BOR's/NF optics on my M82 to the rail, and using a newly purchased Sears torque wrench (which I set for 65 inch-foot pounds), the cross-bolt on one of my zero gap rings was sheared off

Im certain its my lack of familiarization with the use of the torque wrench which caused this. Does anybody know how to source a replacement cross-bolt (and the nut which went flying off and is lost) without having to purchase a complete zero-gap ring. Have tried the local ACE and Lows, and scoured Mcmaster-Carr online but couldnt locate.

Any assist would be greatly appreciated.

Scott

[SpecOpsScout]

Barrett? A ring bolt should not shear off at 65 INCH pounds...INCH pounds and FOOT pounds are not the same thing. Sixty five INCH pounds is something like five and a half FOOT pounds according to an online converter. Sixty five FOOT pounds is equivalent to about seven hundred and eighty INCH pounds.
I got a set of Badger Max Fifty Rings in trade with a stick years ago, that upon closer inspection has the cross bolt twisted and elongated. Badger said the they had probably been torqued by the last user to something in excess of a hundred and thirty inch pounds. It cost me about twenty dollars for new parts to make then right...

[SpaceAholic]

Correction - the torque wrench was set to 65 inch pounds. Used a newly purchased Craftsman 944593 (shown in the below link); engaged the cross-bolt waiting to hear the click, never happened (I still havent figured out why). Not knowning what 65 inch-pound torque should feel like and with no feedback from the wrench, well the rest is history. I wont make the same mistake again - so if anybody has a used zero gap ring or the crossbolt with its nut available please drop me a note.

http://www.travers.com/product.asp?RequestData=CA_Search&navPath=All+Products%2F%2F%2F%2FAttribSelect%3DBrand%3D%27CRAFTSMAN%27%2F%2F%2F%2FUserSearch1%3Dblock+id+197176+and+class+level3+id+28906&eaprodid=197176-94-034-730

[Bad Rat]

At this point assuming your Torque Wrench is calibrated to the +/- 4%, at 65 lb-in the max value would be 67.5 far below the 96 lb-in (8 lb-ft) recommended torque for a Grade 5 or Grade 8 1/4-28 Bolt. I would also assume the Fastener supplied with the Zero Gap Assembly is Grade 8 or greater thus 1 of 2 things happened:

1. The Torque Wrench is not calibrated properly.

2. The Fastener was predisposition to failure due to a flaw in the Material or Machining process.

To eliminate #1 take a 1/4-28 Bolt and Nut and assembly them together using a stack of 1/4 in Washers between them (use some oil on the threads to prevent galling). Without any torque pressure on the Assembly take a Caliper and measure the Bolt Length to within +/-.001. Record this. Then place the Torque Wrench on the assembly and Torque the assembly to 20 lb-in "Clicking" the Torque Wrench twice (don't pull past the "Click"). Then measure the Assembly length and record again. Keep doing this until you reach 65 lb-in going up by 10-15 lb-in increments. The Bolt should get longer. I am going to guess at around 65 lb-in it will be about .0045-.0060 longer (if using Grade 8, longer if Grade 2 or 5).

Keep doing this until the Bolt breaks noting the Torque Wrench setting when it does. If the Bolt breaks at greater than 96 lb-in I would think your Torque Wrench is OK and the fastener was compromised.
We used the Bolt Stretch Method whenever possible assembling Engines to eliminate many of the variables involved in applying Torque to a Fastener.

Now that Fastener Assembly after taking my 50 out of the Safe this morning looks like a Barrett component exclusive.

When you get the new fastener start the Torque process at 50% of the final Torque value and go up in 10-15 lb-in increments until reaching the final Torque Value (assuming you know your Torque Wrench is Calibrated and don't forget the Lube). Hope this makes sense and helps out.

[SpaceAholic]

BadRat - thanks but I dont have the tools to do as you recommend . However to support some forensic analysis here are a couple
of images of the sheared assembly:

[Bad Rat]

Keeping it simple, go to the Hardware store and purchase a 1/4-28 x 1 1/2 Grade 5 or Grade 8 Bolt and the matching nut and about 10-15 1/4 washers. Mount the Head of the Bolt in a Vise facing up so the threads are up. Place the stack of Washers on the Bolt and then using your fingers thread the nut onto the bolt making sure it is free of drag. Use a little Gun Oil to keep it from galling. Then set your Torque Wrench to 30 lb-in and "Click" the Wrench twice applying torque to the nut. Keep doing this going up by 10-15 lb-in until the Bolt snaps noting the Torque on the Wrench.

If the Wrench setting is less than say 100 lb-in when the Bolt snaps then I would say your Wrench is not calibrated correctly. If the Bolt snaps some where greater than 100 lb-in then I might think you Wrench is correctly calibrated and the component failed because of a defect.

The location of the break is interesting. Appears to have started at/on ther Minor Pitch Diameter. The Bolt will break/fracture at its' weakest point if too much Torque is applied...you have to test the calibration of the Torque Wrench.

Let me know how it goes.