The message . com
from contains these words:

Plastic pipe in plastic coupling doesn't need any insert.

Those that I've used most assuredly did - the leaflet said so quite
emphatically.
YMMV [1]

[1] Which I've discovered means Your Mileage May Vary, not as I
originally thought You May Masturbate Vigorously. Though you may do that
as well if you like.

--
Skipweasel
Pay no attention to the man behind the curtain.