Acetylene Cylinder Regulator Protector Safety Cap 3-1/2" x 11" Compatible With Acetylene Cylinder Regulator 3-1/2" X 11TP 1" X 6-7/8" Fine Thread. The Acetylene Cylinder Regulator Protector Safety Cap (3-1/2" x 11") is a protective cover designed to shield the regulator and valve assembly on an acetylene gas cylinder when not in use.