If an engine problem was noted during the roll-test, the car would go to Heavy Repair, on a hoist, and the engine would be pulled, stripped of dress components, and sent back to the engine plant for credit; no internal engine repairs were permitted in the assembly plants. A fresh engine would be run down the engine dress line, VIN-stamped to match the car, delivered to Heavy Repair, and would be installed in the car. The car would then be re-inspected, re-roll-tested, and shipped. A good Heavy Repairman could do a complete engine change in about three hours.
Nearly a thousand cars a day went through the plant, and there wasn't time, space, or manpower to deal with time-consuming engine teardowns or repairs, and the engine plants wouldn't allow it; they were responsible for engine warranty, and didn't want anyone but their own people inside the engine, at their own plant.