As with all Chinese made fittings, the tolerances are not very good. I've found that to get a good seal that doesn't leak, you need to use teflon tape and pipe sealant together to prevent leakage. I usually wrap three layers of tape around the nipple and apply sealant to the tape and internal threads before assembling them. I have found this to stop leaks and have tested my fittings up to 120psi.
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