Abstract: A test head useful in the testing of compressed gas cylinders capable of withstanding high pressure. The water jacket head includes a neck seal including a spud insertable into the neck of a compressed gas cylinder for permitting a high pressure seal to be made to the cylinder. The present neck seal incorporates a permanent quick-connect sleeve for which mating male spud insert units can be designed for nearly any size cylinder neck. Changing the spud insert can be accomplished by hand, without removing the head from its use location. A diaphragm support spring is installed between the moving spud stem assembly and the top plate of the head which biases the spud stem assembly, thus increasing diaphragm life and preventing increasing reassembly difficulty following spud changes. Studs and screws replace welds in holding the top, side and bottom plates together which greatly alleviates the task of servicing inside the head when leaks eventually occur in the moving elements.