Simple thing; good fits, robust steel housing, connections are reliable, no leaks. When assembling the pneumatic system you will use many of them compatible with all types of nylon tubing. Although I wouldn't use them for high flow situations as the inner diameter limits the amount of gas flow, they are great for small appliance projects, appliance air dryers, small regulators for special applications and dedicated use. There is some teflon on the threads of the fittings, but I usually add a couple of turns of teflon tape to make sure it's tight. These fittings are well made, but what they screw into (usually brass, stainless steel, or galvanized steel) can be a messy thread. The best price for this amount on the internet!
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