Which substance should be used to lubricate coupling threads?

Using ivory soap to lubricate coupling threads is a method utilized primarily to reduce friction and facilitate easier assembly and disassembly of fittings or connections. Ivory soap specifically has properties that make it suitable for this purpose, as it can provide a reduction in wear and prevent seizing of threads without attracting dirt or grime.

Light oil, while it can provide lubrication, might not adhere as well to the threads and could become diluted or washed away, especially in wet environments. WD40, known primarily as a water displacement agent, can serve as a lubricant but is not specifically formulated for use on threads meant for repeated disassembly. Grease, on the other hand, can be too viscous and can hinder the ease of threading due to its thicker consistency.

Using ivory soap allows for a clean and effective way of lubricating threads, maintaining ease of operation and ensuring that the tools remain in good working condition without the downsides that can accompany the other options.