Sonnax SmartShell 4L60, 4L60E Heavy Duty Reaction Sun Gear Shell The original reaction sun gear shell is a major weak spot in the 700-R4 and 4L60E transmissions due to the splines being worn down or completely stripped out and we highly recommend you update it! The Smartshell by Sonnax is an updated heavy duty reaction sun gear shell that is designed to be stronger than the original shell. Save the Planetary Bearing- In the OE sun gear shell, the thrust load passes from the shell through the sun gear, then overloads the small captured planetary bearing. To protect this bearing, the Smartshell features a custom bearing assembly and roller clutch race that re-routes the thrust load directly to the carrier, completely bypassing the delicate bearing. Stop Sun Gear Shell Breakage- The smartshell is a stronger shell that avoids the weight found in traditional, heavy duty aftermarket shells with some unique improvements: Slightly thicker shell than OE, The hub area is reinforced with a laser beam welded collar and undergoes a final machining process for precision alignment. The shell splines and lugs are heat-treated for outstanding durability.