What tool is used to tighten a short 1/2” x 1 1/2” nipple that screws into a shower valve behind the wall? Well a nipple extractor work if you turn it in the opposite direction (clockwise )?
A coupling and another piece of pipe can be used to tighten a close nipple. You can't grip the threads with a wrench without damaging them. Some nipple extractors only work in one direction, but if the tool is called an "internal pipe wrench" it can work in both directions to tighten or remove a nipple.