The intensifier tubes and cabin heat valves on all Model 18 airplanes should be inspected at each 100-hour inspection for evidence of cracks and holes which would permit exhaust fumes to enter the cockpit and cabin through the heating system.  These parts should also be inspected very carefully and thoroughly at each engine change.

Since cracks and holes in the intensifier tubes or heat valves create a dangerous condition, the airplane should not be operated until they have been replaced or repaired.  Small cracks or holes may be repaired by welding; however, the intensifier tubes should be sand-blasted and passivated after welding for best results.  Good judgment should be used before attempting repairs; for if the damage has progressed extensively, repairs will not prove satisfactory and it will be best to replace the damaged part.