From the factory 1-piece rear main seal small block Chevy 350, LT1 and Vortec engines are balanced internally in the front and externally in the rear. When building these engines with an Eagle rotating assembly the balance is changed to internal front and internal rear. Automatic transmission applications use the included 153 tooth internal balance flexplate. Manual transmission applications will require an aftermarket internal/nuetral balance flywheel assembly (not included).