Which one is the correct tool?
The task at hand is connecting PVC adaptors (male brass threads on one end, PVC/CPVC fitting to receive glue on the other) to valves with brass (?) bodies.
If it's a fitting similar to this.
I'd go with an adjustable spanner (also known as a crescent, or adjustable wrench). As it has smooth jaws, and will not mar the surface of the soft metal fitting.
If the fitting that this is threading into, is also a hex fitting. I'd use another adjustable spanner, to hold that in place while tightening the joint. If this is threading into a pipe, or fitting that is smooth. I'd use a pipe wrench to hold the smooth pipe/fitting in place, while tightening the joint. Wrapping a rag around the pipe before grabbing it with the pipe wrench, will reduce the marring of the pipe being held.
If you're transitioning from one type of pipe to another, you might want to consider installing a union. This will allow you to disconnect the pipes, without having to cut the PVC/CPVC. A union that goes from brass to PVC/CPVC, might be just what you need.
If you're threading a brass fitting into a PVC/CPVC fitting, you might want to rethink that idea. If you're threading PVC/CPVC into a brass fitting, you might also want to rethink that.
With the fitting posted, I'd probably; very gently, use tongue and groove pliers (also known as Channellocks). Though if it were me, I'd probably avoid that fitting. I'd use a PVC/CPVC to brass union instead on one side of the valve, and a hex adapter on the other.
I'd use a strap wrench.
It will also come in handy if you ever need to unscrew or tighten finish pieces, like a plated pipe.
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