A little history on this engine. In 1967 or 68 I bought a used 1965 400hp JA engine to replace the 261 in my 1958 2 ton grain truck. (the 409 owner had a Chevy garage replace it with a new crate 427) I took out the 11 to 1 pistons and put std truck pistons in the car block. I replaced the solid cam with a hydraulic 340hp cam. It has run good and been trouble free for 50 years. Last week it got a tick on the left side. It quickly got louder and started to miss. Yesterday I pulled the valve cover, not easy with 2 speed vacuum lines too close to the valve cover at the firewall. Not to mention stiff back and bad knees. No 5 intake rocker was loose. I'll get a new nut and re-adjust, but I thought it odd that after this long one rocker got loose.