When a pipe or fitting is threaded the wall thickness is decreased. Also, most plastics are notch sensitive, which basically means it will have a greater tendency to break in places that have been notched. When a pipe or fitting is threaded (notched) it will lose a significant percent of its strength.
See link below for pressure ratings.
Title: Why do threaded fittings have lower pressure ratings then socket fittings?
Description: Threaded connections have lower pressure ratings than socket connections.
Last Edited: 9/18/2013
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