6 For the do-it-yourself crowd, be very careful. Not all parts are interchangeable, such as the countershaft. Early-model 3650 ('01 through early '02) is TCET1265 and it uses countershaft PN TCCF1652. Tremec also uses 1652 in model TCET1789. Countershaft TCCCF1848 (shown) is used in the other six 3650 models (TCET2070, TCET1858, TCET1903, TCET1925, TCET1986, and TCET5852). You have to make sure the proper First gear, Reverse idler, and Reverse-driven gear are matched to the same cluster, either TCCF1652 or TCCF1848.