A cylindrical casing (12) is disclosed which is of glass reinforced plastic and has an end cap at each end thereof. In the drawing the illustrated end cap is designated 42.2. Each end cap has at least one opening (52) therein through which water can flow. The illustrated end cap 42.2 is held in place by a mounting ring (28) embedded in the wall of the casing and encircling the casing internally. An internal groove (32) extends circumferentially of the mounting ring (28). A first locking ring (34) having a peripherally extending external rib (36) is within the ring 28. The rib (36) is in the groove (32). A second locking ring (38) fits into the locking ring (34) to expand it outwardly and press the rib (36) into the groove (32). Stud holes (40) in the end cap (42.2) and in the locking ring (38), receive studs (44) which secure the locking ring (38) to the end cap (42.2).