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Old January 06, 2013, 12:36   #62
westcliffe01
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FALaholic #: 62696
Join Date: Jan 2011
Location: SE Michigan
Posts: 76
Given that the gas cylinder support nests into the crescent notch on the barrel flange, is it not, in fact, impossible for the barrel to ever unscrew ?

I understand that the preload on the barrel threads is reduced at 40ftlb compared to 120ftlb, so that joint might be a little less stiff at 40ftlb than at 120ftlb, but one is surely at a zero backlash condition already at 40ftlb ?

In looking at torque specifications for other fasteners of a similar size yesterday, it seems that around 60Nm is a common torque spec, which is close to 45ftlb but in most cases they use a lubricant whereas the barrel is assembled dry. For fasteners that have to be "tight" it is far more common that a specific angular rotation be specified, which translates to a defined "stretch" in the fastener. The Fal barrel torquing procedure in fact follows this definition, but the threaded portion if the barrel is very short which is why there is so much discrepancy in the preload value (compare the length of a cylinder head bolt to the length of thread engagement of the barrel). Varmint Al's web page demonstrates nicely how non uniform the stress is on a threaded joint.
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