: Discs coated with high-grip material that grab onto the steel plates when hydraulic pressure is applied.

: Excessive heat is the primary enemy of these components, causing the friction material to glaze or burn, leading to "slipping" gears.

: Oil pressure from the transmission's pump is the force that physically "packs" these plates together to transfer power.

In an automotive context, "tranny pacs" (more formally known as ) are the critical internal components of an automatic transmission responsible for shifting gears . These packs consist of a series of alternating friction and steel discs stacked within a drum. When the transmission’s "brain," or valve body, directs hydraulic fluid to a specific drum, a piston squeezes the pack together. This pressure locks the discs, engaging the corresponding planetary gear set to change the vehicle's speed or direction.

A Closer Look at the Components of an Automatic Transmission

Professional mechanics often use comprehensive (sometimes colloquially referred to as "tranny packs" or kits) to restore performance when these internal parts wear out. These kits typically include new friction plates, steel plates, gaskets, and seals to ensure the transmission remains leak-free and shifts smoothly. Key Functions and Components