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AN fitting B-nut no safetying?

Kalibr

Well Known Member
Everything on the engine seems to be safety wired (or otherwise retained by lock nuts, lock washers, thread lockers, etc). Everything except fuel and oil line AN fittings...not even on the lines that are subject to constant relative movement and vibrations such as on the fuel line between the firewall bulkhead fitting and the engine fuel pump (or alike). No safety wire, no thread locker, no provision for any retention device whatsoever.
So, those AN fittings never get loose?
 
No

In short, no, they don't. You should be using only steel AN fittings on the engine to combat fatigue failures of the fittings themselves. But they do not loosen if properly torqued per specification. We use AN tubing fittings in lots of places on the vehicles I build at work. We've tested them to over 20G RMS in all three axes for several minutes (their entire service life in this application) and while we've had the brackets they are bolted to loosen, the aluminum tubing they are holding snap completely off at the end of the sleeve, and sensors and solenoid valve actuators break off, we have never, that I know of, actually had an AN tubing nut loosen.
 
It's a very, very good idea to use some form of witness paint to allow quick visual verification that fittings have been torqued and have not come loose. One technique is to use nail polish to draw a line on the stationary fitting (typically the male AN nipple) and a matching line on a flat of the female B-nut. These witness lines are added ONLY after the fitting has been torqued, thus it becomes an immediate flag to the eye - no nail polish means that fitting wasn't torqued! Using fine, straight lines also allows the human eye to quickly detect a B-nut that might not be as tight as it once was - lines that don't line up do catch the eye.
 
I think the biggest threat is overtightening, with the thought they could come loose because the lack of safety wire.

I lost an entire hydraulic system at the start of a North Atlantic crossing because of an overtightened B-nut plssing fluid out.
 
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