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I recently purchased a Grohe tub spout and it requires a MNPT fitting on the copper. The MNPT fitting must be placed behind the CBU and the tolerance is 1/8". Since I haven't chosen my tile yet, I'm still not sure how long to cut the copper pipe.

My question is: can I just sweat on another female npt fitting on the copper in the wall, then just make a smaller intermediate pipe with two male ends (basically a copper nipple)? I'm just thinking that way, I can cut the intermediate copper pipe & screw it in after the cbu and tile is done.

Here's a diagram of what I'm thinking:

diagram of spout fitting

Here's the spout in question: https://imgur.com/a/Ou43OW6

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Typically you would use a copper sweat by FIP 90° elbow (often called a wing back elbow, as it has tabs on it to allow for securing to backing). Then screw in a test nipple (or use a nipple and cap) while the tile is being done. Once tile is done, use a brass nipple of the correct length (best to buy a few of different lengths and just return the ones that were the wrong length).

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You can but, I would get a spout for copper. They clamp on with an Allen wrench. More connections = possible leaks. Also when removing the spout it could all come out with it (not a big deal). Make sure everything is copper. Galvanized pipe leaves rust stains.

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