Why? Each bell 1/2 is stamped, welded together over the center section. They then machine the inside using the machined center section as center. The boring rounds out the inside of the bells. The outside remains as stamped and with poor alignment to center. If you measure the thickness of your bells, you will wide variation in thickness. If the clutch is .015" thicker on one side than the other (and they usually are) then there is more metal one side than the other which = gross out of balance. Out of balance does not feel good and it gobbles up horse power. It takes energy to shake the shit out of things.