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Old 04-08-2013, 09:06 AM
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A 12 bolt housing is a 12 bolt housing. The 2,3 and 4 series comes from the location of the ring gear flange on the differential. A 2 series differential has the flange offset to the left more than a 3 series and a 3 series is offset to the left more than a 4 series. This is to make up for the difference in the diameter of the pinion gear head for 2.56 to 6.xx gears. The car 12 bolt uses a 2 series differential for 2.56 and 2.73 gears, a 3 series for 3.07 to 3.73 gears and a 4 series differential for 3.90 to 6.xx gears. There are special thick 3.90, 4.10, 4.56 and 4.88 gears that are made to fit the 3 series and there is a special thin 3.73 that is made to fit the 4 series.
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