You have two options for obtaining a leakproof seal on threads.
Tapered threads seal the thread but tend to be better suited to cheap
rugged components like galvanised pipe. They require little skill to obtain a leakproof seal and are tolerant of people using a myriad of sealants.
Parallel threads require a flange seal or tapered seat seal. They are better for positioning outlets and work
well with more 'petite, assemblies that take up the least amount of space. They are far less tolerant of grit and poor choice of sealants. Racor units are a precision engineered component so parallel thread fittings are appropriate.
Note that sealants don't seal either tapered or parallel fittings but they may avoid twisting and tearing flange seals
and often aid disassembly.
There are many thread, seat and flange types and forms. The name brands are well engineered and have many decades of quality in mission critical applications.
I never mix and match and only buy name brands. Life is much simpler. Lowest cost or nearest supplier are not criteria I ever use in sourcing fittings. With a long background in motorsport I have seen many $$$ failures due to penny part failures.
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