Can't say that I've ever heard that about plastic NPT ( National Pipe Thread).. I've heard don't use tape and don't over crank because of warpage.. Learn something new everyday
Same thing (torque) with lb/ft settings for tightening up nuts/bolts. Verify the specs - most are spec'd. dry and putting any type of oil or anti-sieze on the threads can cause the torque to be too great and the fastner will fail. Even a spec that is for steel to steel interface will change if the bolt is galvanized (either hot dip or electro galv.)
What you are measuring with the torque wrench is resistance to turning when what you really should be measuring is how much the fastner stretched. But, in most instances that is impossible to do, so that's why you have the lb/ft spec.
