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Left side case races coming loose was a problem that started happening, IIRC, around '89. The MOCO simplified the design of the steel race insert that was cast into the aluminum case. This was allegedly to increase quality, but was actually to decrease the reject rate that was relatively high on the older designs because the aluminum pour didn't always completely fill the mold around the older, more complex steel instert. (Lower reject rate equals more profit.) The simplified insert lacked the finger like details that extended out into the casting and locked it in place. These simple inserts would sometimes work loose over time. Their eventual fix was to eliminate the cast in feature completely, and go to an insert pressed into the case after casting. The only Shovelhead or prior cases that I have ever heard of having loose inserts were the result of EXTREME abuse.
