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A drop or two of oil on the threads only, help it screw on better. Pipe dope should not be used as the only purpose of these untapered threads is to draw the compression ferrule tighter against the pipe. Pipe dope could actually be counterproductive as it makes it harder to tighten-up.
*Jim: Pros are that:You can undo the fitting and reconnect it later (not possible with sweated fittings unless you include a union).No flames are required.No flaring tool is required.The fitting parts, except for the ferrule, can be reused if you buy extra ferrules.Cons:More expensive than sweat fittings. Way more.Are they allowed inside of walls? I've only used them in the open. -David
*I've used them with good success on small diameter, soft copper lines that need to be removable: refrigerator ice maker and dishwasher. Both quicker and less expensive than sweat unions. (And about 1/3 of the unions I sweat wind up with solder flowing onto the threads so I can't put the unions together! You can tell I'm not a plumber.)I have seen compression type repair couplings but I think they may have neoprene seals rather than brass ferruls.
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