Aeroflow Performance brand new, cast iron housing is built to take the abuse of the street, strip or track without the fear of breaking. Case has forged caps and extra reinforcement where it's needed most. Most stock differentials use the 3.062" O.D. bearings with 1.781" I.D.AF5076 -1000 has forged 3.062" diameter bearing caps. Accepts OEM and aftermarket Ford 9" pinion supports and fits a 28 or 31 spline.AF5076 -1001 has forged 3.250" diameter steel bearing caps. Upgraded 5/8" Heavy Duty Cap Bolts which are a grade 8 material. This case ONLY works" with 35 and 40 spline spools and will not fit a 28 or 31 spline spool.