42RE 42RH A904 Transmission Sonnax Rear Servo Piston 22912A Many Chrysler A904 and 42H/RE transmissions experience rear servo piston sticking/breakage, pin bore wear and reverse band slippage. The servo piston has a tendency to get cocked in the bore, inhibiting band apply. The high reverse line pressure continues to apply against a cocked piston, causing piston breakage. The bore of the piston also wears from side loading, resulting in insufficient band apply pressure. Sonnax billet aluminum rear servo piston 22912A addresses these problems. Modified seal ridges prevent piston from sticking in case bore Billet aluminum won’t crack like OE cast piston 2.540" O.D.