I wish I could answer that with authority but I just can`t. It may be because the 489 case used a crush sleeve versus a hard spacer. It could be that the sure-grip unit in the 489 case was a cone-style versus a clutch style in the 742, or it could be simply the failure rate of one or the other at the track. For whatever reason the 742 case seems to appear behind most cars with enough HP to break something. In my experience twisted axles and broken ring/pinion teeth seem to be more of a problem than pinion shafts.