The rear screw on the sprocket covers will misalign if the sprocket cover is from a different engine. For example: K0-K2 engines were all cast the same, in the same molds, as were their side covers (like the sprocket cover). Partway thru the K3 era, there were new injection molds made, first for the side covers and then, by the K4, for the engine cases. These new covers were about 1mm different in hole placement of the sprocket covers, in particular the rear one. If the upper screw is inserted and tightened in one of these covers before the rear one is, there will often be about 0.5 to 0.75mm misalignment, tilting the rear screw. If the owner then just muscles that screw in, it either strips or damages the case threads, or can even break the whole boss right off the back of the engine(!). I get to make new ones on these engines a lot: 4 times in this last year, for example, because the top of the boss was broken off and missing entirely.
