Recent studies have shown that micropitting initiated pitting appears to be the dominant metal fatigue mode in modern bearings and gears. If the formation of micropits can be controlled, the fatigue life of the bearings and gears can be readily lengthened, so the useful life of the engine or transmission can be radically extended. The lack of in-depth understanding of micropitting initiation mechanism hinders progress to control the micropitting-initiated pitting failure mode. In this study, we explore the initial stages of micropitting in relation to material differences that impact the crack propagation process. To investigate these mechanisms, we study micropitting and subsequent pitting using pinion and wheel pairs made from two different carburized steels, SAE 8620 and SAE 4027, in a 91.5mm helical back-to-back gear test rig using a typical transmission fluid. For comparison, a similar study is being carried out with the pinion and wheel pairs made from the same SAE 8620 steel. The preliminary results show that steel material differences may change the ultimate pitting fatigue life but may not significantly influence the micropit formation process and subsequent pitting failure mode.