Cast aluminum versus stamped steel: when it comes to the TH400's clutch apply pistons, aluminum is the clear winner. Here's why: the stamped steel unit consists of two individual pieces that press together. Sometimes this press fit loosens over time, and fluid pressure loss is the unavoidable result. The cast piston on the left is a single part, and leaves no room for a seal to leak or gap to develop in the first place. Another argument in favor of the aluminum-type piston is that of rigidity. The aluminum piston will flex much less than a steel unit, thus preserving the outer diameter seal and sealing surface from wear. When changing out pistons, builders must take note that a check ball exists in either the piston or the forward/direct drum; without one, you are asking for trouble.