Main bearing studs
While rebuildng the lower end, I fit the upper pan without a gasket and noticed that the rear main studs contacted the oil pan. I put a level on all the studs an found that the rears were longer by about 3-4mm. I double nutted them and gave them each about three turns. The studs bottomed out and were then even with all the others. None of the bearings were unusually worn and the low end 'miced' out within normal ranges. I don't know if they were not fully insterted at the factory or had loosened over time. Just something else to check during your next rebuild.