High pressure gas cylinder safety cap. This will work with all high pressure cylinders with FINE threads which most cylinders now have. This cylinder cap is designed to protect the cylinder valve in case the cylinder would fall. Diameter = 3-1/8", thread size 11 UNF (11 threads per inch). To comply with the Department Of Transportation requirement, the main valve must be in the off position in transportation. The Griftan safety cap can be used for transportation with the valve closed.