An adapter assembly for securing a rear end cover to an automobile differential housing. The adapter assembly includes an annular-shaped plate having an inner diameter, an outer diameter, and a thickness in a direction transverse to the inner and outer diameters, and at least two and preferably three sets of holes. A first set of holes is through the thickness of the plate and countersunk, and a second set of holes is threaded. The optional third set of holes also extend through the thickness of the plate. The assembly further includes at least two and preferably three sets of bolts. A first set of bolts is configured and sized to be received in the first set of holes so that their heads are entirely received in the countersink and their shafts protrude from the plate. A second set of bolts is configured and sized to be threaded into the second set of holes. The optional third set of bolts is configured and sized to be received in and extend entirely through the third set of holes so that their shafts protrude from the plate.