I suddenly developed a much louder than the normal Toyota knock so I pulled the valve cover and found that the two rocker arms closest to the firewall on the exhaust side were dry. No oil on the rockers or camshaft just metal on metal. I have already sourced new rockers and a camshaft. My question is what caused this. It is a 89 pickup with the 22re and 160,000 miles. It is well maintained and no sludge in the engine that would have gunked anything up. I am not intimate with the Toyota oiling system and want to make sure that I am not replacing parts just to have the same problem happen to my new parts.