Because we cannot always fully tighten tapered thread fittings, and because even after fully tightening the fitting a small spiral leak path remains along the full length of threads, a thread sealant must be used during assembly. Sealants appropriate for use on aircraft NPT fittings are Tite-seal, Permatex #2 and Locktite 565. Do not use RTV, Teflon tape or Fuel Lube on NPT fittings.
When installing the fitting, be sure the threads on both parts are clean and dry since most sealants will not tolerate any oil contamination. First determine the clocking position by installing it finger tight and marking the desired clocking. Remove the fitting and apply a small amount of sealant to 2 - 3 threads of the male fitting. Leave the first 2 bare to prevent contamination inside the fluid path. Remember, this is an interference fit so not much sealant is required.
Thread the fitting in with your fingers until you just begin to feel resistance and then an additional 1.5 - 2 turns. This is a general guideline… you must still use judgment to not over tighten and damage the threads, but a properly installed fitting is quite tight. If the installation requires a specific clocking, when approaching the correct position you must determine whether you will be able to make another full rotation and still be within the 1.5 - 2 turns stopping range. You must avoid turning the fitting backwards in the loosening direction because it will have a high probability of leaking. If you must do this, it is best to completely remove the fitting, clean up the threads on both parts, and try again.
