In GM 4L60-E, 4L65-E and 4L70-E units, it is common for the plastic Forward accumulator piston to wear or crack and older aluminum piston bores to wear from repeated reciprocation. This causes a loss of fluid, resulting in delayed engagement, slips, burnt clutches and/or bands. Sonnax aluminum Forward accumulator piston 74927-AP eliminates the cracking problem and restores hydraulic integrity at the pin bore. The drop-in Zip Valve^™part installs quickly and easily.