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Most, if not all threaded automotive fasteners have specified torque, which is ensured by using torque wrenches. But how about threaded pipe, galvanized steel for water or black pipe for gas, especially the latter? Should it be torqued to spec using something like a torque version of a pipe wrench, to ensure a proper joint?

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No. Automotive parts (generally anything you'd find a torque spec on) and fasteners are engineered to a degree that residential galvanized and gas piping simply isn't.

  • Automotive parts are designed to accommodate disassembly and maintenance - water pipes are typically not.
  • It's possible to get a bolt on, say, a brake caliper or a wheel hub on tight enough that it interferes with its safe function.
  • There's a minimum torque on head bolts to minimize the chance of gasket failure or blow-by.
  • It's also a good way to ensure that parts don't warp if they aren't tightened symmetrically around a perimeter.
  • Overtightened bolts can shear off or accumulate stresses that could cause pre-mature failure.

None of these are issues with water and gas lines (except with soft metals like copper). The proper amount of torque is somewhere between "it doesn't leak anymore" and "I can't move it with a wrench anymore" (assuming, of course, that the former comes before the latter).

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  • ..but short of stripping the threads or snapping a fixture off. ;-) Commented Feb 5, 2016 at 4:27
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Torquing a bolted joint serves a very specific purpose - to set the clamping force exerted by the bolt on the joint. By applying some specific torque to a bolt with a certain thread pitch, the effect is to stretch the bolt by a specific amount. The bolt then acts like a spring, clamping the joint together. Overtightening the bolt applies too much clamping force and can actually deform the joint. Undertightening means the joint can't take as much force before it "floats" and fails.

Threaded piping is completely different. In this case, the threads aren't used to apply force to a joint, the threads are the joint. Their primary purposes are to keep the pipes from separating under pressure, and to seal the cavity inside from the outside. For strength, tightness is of little importance. For sealing, tightness matters, but no matter how tight, the pipes will never seal without a soft interface material like dope or tape. Therefore, thread pipes need to be tightened until they seal under pressure, and no more.

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    Also note that fittings like tees or elbows need be tightened to align with the new direction of the pipe run, irrespective of any torque level at that point. And the pipe-fitters who re-plumbed my hot-water home heating system used both tape and dope on each and every joint...
    – DJohnM
    Commented Feb 5, 2016 at 5:47

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