Re-reading the 1979 report might be helpful here. This isn't my field, but it seems that the engine is attached "hard" to the pylon, then the pylon is attached via a bearing mount system to the wing frame. The bearings wear out, and hence have to be replaced (not sure how often, but they were doing it on the entire fleet prior to the 1979 crash). The 1979 investigators thought that the fatigue cracks were caused by removal of the entire pylon/engine assembly as one unit (because that put excess stress on the aft bearing, they suspected due to support being provided from below by a fork lift). After the 1979 accident engines had to be removed first, then pylon, supposedly removing that cause for mount cracking. Perhaps there was another cause.
Before the 1979 accident, engines also had to be removed first.
Airlines have to follow the approved maintenance manual procedures; that manual called for engine removal and installation from a pylon that was on the wing. American was improvising a maintenance procedure without the legal authority to do so, resulting in 191.
