PURPOSE: To secure integrity of uniform coating by fixing a meter connecting nipple to the inside of an outside pipe body riser portion, and airtightly fixing an adapter having a shank portion and grabbing ridge to the inside of this meter connecting nipple.

CONSTITUTION: A riser assembly 10 is constituted with an outside structure comprising a riser pipe 12 fixed to an upper nipple 14. The top of the nipple 14 has a screw thread for connection to a gas meter or the like, and the bottom has a nipple neck 20 with a small diameter extending from a shoulder 22. The inner wall of the nipple neck 20 defines an important critical inner diameter for final assembling of the riser assembly 10. An inside diameter plastic pipe 30 and an adapter insert 32 are interconnected, and are inserted into the nipple 14 and the riser pipe 12. The adapter insert 32 is formed as a rotor having axial-line directional inner-cavity 34 and an upper collar 36.