During my pre-teens, my father often spent part of his day off working on one of our 1940s Chevrolets. Sometimes he would lift the valve cover off the top of the six-cylinder motor and ask me to hand him that 9/16th, opened end wrench. His purpose was to adjust the rocker arms, using that wrench and a feeler gauge. He would even do that with the engine running, oil splashing on his hands.