This is fitting that gets pressed into the back of the passenger side head for the heater to work. The short heater hose connects to . These are usually destroyed when the engine is -built and are a must for the head. This is made exactly the same as the original with plastic insert to restrict flow high rpm and gives full pressure low rpm to keep constant heat and pressure. Correct 1.188 dia or 1 3/16. There are others on the market that do not have the pressure valve or look totally incorrect. There is no comparison between the two. This is exact inline tube reproduction. is the correct size and shape and plated in silver zinc.