The recent discussions about proper torque on such things as engine mount bolts reminds me that over the years I have heard from several normally reliable sources that using a torque wrench on locking nuts (either plastic or all metal) is not only a waste of time but possibly even a bad idea.  The logic is that because of the built in interference of the nut either a lower than desired reading will be read or upon approaching the desired torque the nut will "slip" and result in higher than desired torque.  It's been pointed out that truly torque-critical items, like connecting rod bolts and case half connecting bolts never use locking nuts.  In other words, if the torque is really important don't use lock nuts.  I am curious what others on the list might have to say about this.  I am not an engineer, nor do I play one on TV, but I've been using nuts and bolts since way back in the Erector Set days and have absorbed "do" and "don't do" advice from people whose expertise I respect, including the above observations on torquing fasteners.  I probably won't change what's worked well for me over the decades but perhaps someone has some compelling argument to the contrary, or perhaps confirm what I've heard.  The floor is open.

Leighton,

 

Proper torque........Hmmmm...

 

Since I am a simple minded person, all I know is that there are many bolt-type fasteners that must be tight enough.  Ignoring my heritage, where "Gut und Tite" was the rule (just short of stripping the threads), there has to be a method/standard for consistently tightening a bolt to properly perform its design function in the place it is being used.  Usually there is a reference based on the bolt size, quality and thread spacing.  This reference sometimes contains notes about lubricated threads and the effect on the torque. 

 

Aircraft reference tables don't mention using fiber locking nuts but it is hard to find a fastener in an aircraft that does not use them - including locking nut plates.  Also, there is no special reference to high temp steel locking nuts.  I use the AN bolt torque tables unless the use is only for shear or some other special application.  For engine bolts/nuts, I reference the appropriate overhaul manual. 

 

I rationalize my use by noting that fiber/steel locking nuts do not add that much resistance - maybe a few percentage points above that of a non-locking nut.  Good enough for me since I do not regularly re-calibrate my torque wrenches. I prefer that the bolts be slightly off torque rather than grossly off torque since my arm hasn't been calibrated in years either.