Remove the transmission, drain the oil, and take off the side cover assembly. Next, remove the drive-gear bearing retainer and gasket, followed by the drive-gear bearing stem snap-ring, then pull out the gear to use a large screwdriver to lever the drive-gear bearing from its location. Take out the speedometer driven gear from the rear extension and remove the extension retaining bolts, then remove the reverse idler shaft E-ring. Withdraw the drive-gear, mainshaft, and extension assembly together through the rear casing. Detach the drive-gear, needle bearings, and synchronizer ring from the mainshaft. Expand the snap-ring in the rear extension that retains the rear bearing and withdraw the rear extension. Use a dummy shaft or special tool to drive the countershaft out of the rear of the transmission case, carefully remove the dummy shaft, and extract the countergear, bearings, and thrust washers from the interior of the transmission case. Drive the reverse idler shaft out of the rear of the transmission case using a long drift. Dismantle the mainshaft only if a press or bearing puller is available; otherwise, take the assembly to a Chevrolet dealer, ensuring all components are kept separated and in order for easier reassembly. Remove the 2nd/3rd synchro hub snap-ring from the mainshaft, being careful not to mix up the synchro unit components, as they are matched in production. Remove the synchro unit, 2nd speed blocker ring, and 2nd speed gear from the front end of the mainshaft. Depress the speedometer drive-gear retaining clip and remove the gear from the mainshaft, then remove the rear bearing snap-ring from its mainshaft groove. Support the reverse gear and press the mainshaft out of the rear bearing and snap-ring from the rear end of the mainshaft. Remove the 1st/reverse synchro hub snap-ring from the mainshaft and the synchro unit, followed by the 1st speed blocker ring and 1st speed gear from the rear end of the mainshaft. Clean all components in solvent and dry thoroughly, checking for wear or chipped teeth; replace the appropriate synchro unit if there has been a history of noisy gearshifts or if the synchro facility could easily be beaten. Extract the oil seal from the rear end of the rear extension and drive in a new one, then clean the transmission case inside and out, checking for cracks, particularly around the bolt holes. Extract the drive-gear bearing retainer seal and drive in a new one. Begin rebuilding the transmission by reassembling the mainshaft, installing the 2nd speed gear so that the rear face butts against the flange on the mainshaft, followed by the blocker ring and the 2nd/3rd synchro assembly, ensuring the notches of the blocker ring align with the keys of the synchro assembly. Fit the snap-ring that retains the synchro hub to the mainshaft, then install the 1st speed gear and blocker ring to the rear end of the mainshaft, followed by the 1st/reverse synchro unit, again ensuring the notches of the blocker ring align with the keys of the synchro unit. Install the snap-ring, reverse gear thrust washer, and spring washer, then install the mainshaft rear ball bearing with the outer snap-ring groove nearer the front of the shaft, followed by the rear bearing shaft snap-ring. Fit the speedometer drive gear and retaining clip, then insert a dummy shaft through the countergear, sticking the roller bearings, needle retainer washers, and transmission case thrust washers in position using thick grease, noting that the tangs on the thrust washers are away from the gear faces. Install the reverse idler gear and shaft with Woodruff key from the rear of the transmission case, avoiding installation of the idler shaft E-ring at this time. Install the countergear assembly from the rear of the transmission case, then insert the countershaft so that it picks up the roller bearings and thrust washers, displacing the dummy shaft or tool if used, ensuring the countershaft slot is at its rear end when installed. Expand the snap-ring in the rear extension and locate the extension over the rear end of the mainshaft and onto the rear bearing, ensuring the snap-ring seats in the rear bearing groove. Insert the mainshaft pilot bearings into the clutch gear cavity, then assemble the 3rd speed blocker ring onto the clutch drive gear. Locate the clutch drive gear, pilot bearings, and 3rd speed blocker ring over the front of the mainshaft, ensuring the notches in the blocker ring align with the keys in the 2nd/3rd synchro unit, and stick a new gasket to the rear face of the transmission case using grease. From the rear, insert the combined clutch drive gear mainshaft and rear extension, ensuring the 2nd/3rd synchro sleeve is pushed fully forward for engagement with the countergear anti-l sh plate. Install the rear extension to transmission case bolts and torque tighten to specifications. Fit the outer snap-ring to the clutch drive gear bearing and install the bearing over the drive gear and into the front of the transmission case, followed by the clutch drive gear bearing shaft snap-ring and the clutch drive gear bearing retainer with its gasket, ensuring the oil return hole is at the bottom. Finally, install the reverse idler gear ring to the shaft, and with the synchronizer sleeves in the neutral position, install the side cover, gasket, and fork assembly, torque-tightening all the bolts, and install the speedometer driven gear in the rear extension.
Posted by GMPartsGiant Specialist 
