TM 9-4940-444-14 & P
An exhaust system shall be constructed with materials recommended and shall be installed in a permanent manner.
The interiors of the pipes and fittings shall be smooth and free of any obstructions to minimize the resistance to air
flow. The system shall be as free as possible from air leakage either into or out of the system except at points where
air is taken into or discharged from the system.
Galvanized sheet steel shall be used except where corrosive fumes, vapors, high temperature and other elements
which will attack galvanized sheet steel are handled. (Where these conditions are encountered, special instructions
The material for various round and rectangular pipes and ducts shall not be less than the following thicknesses:
RECOMMENDED GAGE FOR PIPE & ELBOWS
Diameter of Round Pipe Or Greatest
Dimension of Rectangular Pipe
Up to 8" inclusive
Over 8" to 18" inclusive
Over 18"t to 30" inclusive
C. Pipe Joints.
1. Longitudinal Joints:
All longitudinal sheet metal pipe Joints or seams shall be lapped and riveted, the rivet centers shall not be more
than three inches and the amount of lap and size of rivets shall correspond to the following table:
U. S. Gage No.
Width of Lap
Size of Tinner
Joint - Inches
Rivets to be Used