Abstract: A pipe coupling has a main body including (a) a threaded portion adapted for threaded engagement with an inside-coated pipe having a mating threaded end portion, (b) an annular groove and (c) an annular projection, the members (a), (b) and (c) being located on the inside circumferential surface of the main body in this recited order from each open end of the main body toward the central portion thereof, a short cylindrical body having one end abutting the annular projection and being mounted within the main body, and an annular packing mounted within the space defined by the threaded portion (a), said annular groove and the peripheral surface of the cylindrical body. A pipe joint connection is provided by threadedly engaging inside-coated pipes with the threaded portions of the coupling. The pipe coupling of the invention permits a leakage-free pipe joint connection on the job by simply threading inner-coated steel pipes and threadedly connecting these pipes through the coupling.

Abstract: A mold for the production of a reinforced plastic tubular body having an annular groove such as a pipe coupling, the mold having a mold body and detachably mounted on a predetermined position thereof, an annular groove former consisting of a surface layer of an elastic material and a core covered with the surface layer. The reinforced plastic tubular body is produced by forming a layer of a molding material around the aforesaid mold, curing the layer of molding material, separating the mold body from the cured layer of molding material, and then separating the annular groove former from the cured layer of molding material by removing the core and then removing the surface layer.