Automotive drivetrains move around a lot when in use, and the L-Series is no exception. The only reason you don't feel these movements are the drivetrain mount insulators, which include the engine mounts, transmission mount, and differential mount. The engine mount insulators bear the bulk of the engine movement; however, the rubber used to insulate the engine from the body wears out over time. Heat cycling and the torquing of the motor put constant strain on the mounts from the moment you start the engine until you turn it off. Be sure to inspect your mounts and replace them before a complete failure occurs. Sold individually, 2 required per car.
70-73 models originally used a slightly different engine mount insulator that required additional hardware. The early engine mount simply had a pair of holes where the bolts fit through and were held in with a separate nut. It has since been superceded to this later style insulator, in which the nuts are part of the assembly.