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Aluminum reduces rotational mass, but you still have to be aware of the critical speed where any shaft begins to vibrate and bend. As the shaft gets longer, it needs to get larger in diameter to raise the critical speed to a safe level. This one is 3.5 inches in diameter, 61.125 inches long, and fits a '65 El Camino with an M20 four-speed and a 12-bolt rearend. More horsepower also requires more diameter, up to 4 inches in some cases.