Both stainless steel and no-lead brass are much harder materials than standard brass because of the elimination of the lead. The lead would allow the fittings threads to "flow" together and seal. This is not so with stainless steel and no-lead brass.
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Follow these steps for the best results:
Photo 2 Apply a quality pipe thread sealant on top of the gray PTFE. Stainless steel and no-lead brass products are very hard and the threads are very rigid. A quality thread sealant is needed to flow and fill any gaps between the threads.
Photo 3 Thread the two pieces together and hand tighten plus one half to one full turn at the most. Do not overtighten, it will cause leaks. The more you tighten, the more it will leak. If overtightened a lot, you may not be able to use the fittings again.
Do not overtighten - Will cause more leaks, not less.
When working with stainless steel and no-lead brass, you need to seal them differently. NOTE: Manufacturers can tell by measuring threads when they have been overtightened. Do not try to claim defects and return overtightened fittings.