Synchros - Single Gear

Replace a single worn synchronizer assembly in the manual transmission by disassembling the transmission, removing the main shaft, replacing the damaged synchro, and reassembling.

Warnings

Tools required

Parts

• Synchronizer assembly (specific to gear position) × 1 — Use OEM specification
• Transmission gasket set × 1 — Use OEM specification
• Input shaft bearing retainer O-ring × 1 — Use OEM specification
• Countershaft thrust washers (if worn) × 2 — Use OEM specification
• Snap rings (replacement set) × 1 — Use OEM specification
• Transmission fluid (80W-90 GL-4) × 1 — Use GL-4 rated gear oil for manual transmission

Fluids

• 80W-90 GL-4 manual transmission fluid — 3.5 qt

Preparation

1. Park vehicle on level surface and chock rear wheels securely
2. Disconnect negative battery cable
3. Raise front of vehicle and support on jack stands at frame rails
4. Remove drain plug and drain transmission fluid into suitable container
5. Mark driveshaft and rear axle yoke relationship with paint for proper reinstallation
6. Prepare clean workspace with adequate lighting for disassembly and inspection

Procedure

1. 1

   Remove transmission from vehicle
   Disconnect clutch linkage at the bellhousing or clutch fork. Remove the speedometer cable from the transmission tailhousing by unscrewing the retaining ferrule. Support the transmission with a transmission jack positioned at the center of the case. Remove the 4 bolts securing the transmission to the bellhousing. Mark the driveshaft-to-yoke orientation, then remove the 4 U-joint strap bolts at the rear axle. Slide the driveshaft rearward to disengage from the transmission output shaft. Remove the transmission mount-to-crossmember bolts, then remove the crossmember-to-frame bolts. Slide the transmission rearward until the input shaft clears the bellhousing, then lower carefully from the vehicle.
2. 2

   Clean exterior and remove shift cover
   Clean the exterior of the transmission case thoroughly with solvent to prevent contamination during disassembly. Position the transmission with the shift cover facing up on your workbench or in a vise with soft jaws on the case (not the cover). Remove the 8 bolts securing the shift cover to the transmission case. Lift the shift cover assembly straight up, noting the position of all shift forks and interlocks. Place the cover assembly aside on a clean surface.
3. 3

   Remove extension housing and output shaft
   Remove the 6 bolts securing the extension housing to the rear of the transmission case. Tap the housing gently with a rubber mallet to break the gasket seal, then slide it rearward off the output shaft. Remove the speedometer drive gear snap ring from the output shaft if present. Using snap ring pliers, remove the large snap ring at the rear bearing retainer inside the case. The output shaft assembly can now be removed by pulling it straight rearward out of the case. Support the shaft carefully as the rear bearing may come with it.
4. 4

   Remove input shaft and bearing retainer
   Remove the 4 bolts securing the input shaft bearing retainer to the front of the case. Pull the input shaft assembly forward and out of the case. The input shaft bearing will come out with the shaft. Inspect the bearing for roughness or excessive play. Remove and discard the O-ring from the bearing retainer groove.
5. 5

   Disassemble output shaft to access synchronizer
   Secure the output shaft in a bench vise using soft jaws on the shaft itself, not on the gears. Identify which synchronizer corresponds to the grinding gear. For 2nd gear synchro: remove the 2nd-3rd synchronizer snap ring using snap ring pliers. For 3rd gear synchro: remove the same 2nd-3rd synchronizer assembly. Use a brass drift and rubber mallet to carefully drive the synchronizer hub off the shaft, keeping track of the keys and springs. Remove the synchronizer sleeve, blocking ring, and inspect all components. The blocking ring is the bronze component that contacts the gear cone.
6. 6

   Inspect and measure worn components
   Clean all disassembled components in solvent and dry thoroughly. Inspect the gear cone surface for scoring, heat discoloration, or wear grooves. Measure the blocking ring-to-gear cone clearance by placing the ring on the gear cone and measuring the gap with feeler gauges. Minimum clearance should be 0.020 inches; less than this requires replacement. Check the synchronizer sleeve teeth for wear, rounding, or chipping. Inspect the hub for spline wear. Check output shaft splines for damage or excessive wear.
7. 7

   Install new synchronizer assembly
   Lubricate the output shaft splines and gear cone with assembly lubricant. Install the new blocking ring onto the gear cone, ensuring the ring slots align with the synchronizer keys. Install the synchronizer hub onto the output shaft with the correct orientation (longer boss or machined groove should face the direction noted during disassembly). Install the 3 keys into the hub slots. Install the spring behind the keys, ensuring the spring tabs engage all 3 keys. Install the second spring on the opposite side, offset 120 degrees from the first spring. Slide the synchronizer sleeve over the hub, aligning the sleeve internal teeth with the keys. Verify the sleeve slides smoothly and engages the blocking ring properly.
8. 8

   Reassemble output shaft
   Install the synchronizer retaining snap ring into the output shaft groove, ensuring it is fully seated in the groove with no gaps. Install any remaining gears, thrust washers, and snap rings in the reverse order of disassembly. Verify all snap rings are fully seated. Check that all gears rotate freely on the shaft and that thrust washers are correctly positioned. Measure output shaft endplay if possible using a dial indicator (specification typically 0.004-0.018 inches).
9. 9

   Install countershaft and cluster gear
   If the countershaft was removed, coat the countershaft thrust washers with assembly lubricant and position them in the case recesses with the tangs engaged in the case notches. Insert the cluster gear with its bearing rollers into the case. Use a dummy countershaft or long drift to hold the bearings in place. Align the cluster gear and thrust washers, then drive the countershaft into the case from the rear, pushing the dummy shaft out the front. Ensure the countershaft tang or flat engages the case recess to prevent rotation. Install the countershaft retaining roll pin if your transmission is so equipped (verify during disassembly).
10. 10

    Install input shaft
    Coat the input shaft bearing and pocket with assembly lubricant. Install a new O-ring on the bearing retainer. The input shaft pilot tip must engage the roller bearing or bushing in the output shaft front recess. Insert the input shaft assembly into the front of the case, aligning the bearing with the case bore. Install the bearing retainer with any oil return slot or drain hole positioned at the bottom (6 o'clock position) if present. Install the 4 retainer bolts and tighten in a crossing pattern to secure the bearing. Verify the input shaft rotates freely.
11. 11

    Install output shaft assembly
    Coat the output shaft rear bearing and bore with assembly lubricant. Carefully insert the output shaft assembly into the case from the rear, ensuring the gears mesh properly with the cluster gear and that the input shaft pilot engages the output shaft bearing pocket. The rear bearing should enter the case bore. Install the rear bearing retainer snap ring into the case groove, ensuring it seats fully.
12. 12

    Install extension housing
    Clean the extension housing mating surface and case rear face thoroughly. Install a new gasket on the case or apply a thin bead of RTV sealant to the housing flange. Slide the extension housing over the output shaft and onto the case, ensuring the housing aligns with the case dowel pins. Install the 6 extension housing bolts and tighten in a crossing pattern. Do not overtighten as this can distort the housing and bind the output shaft.
13. 13

    Install shift cover assembly
    Clean the shift cover and case mating surfaces completely. Position all shift forks in their neutral positions on the shift cover. Apply a thin bead of RTV sealant to the case mating surface or install a new gasket. Carefully lower the shift cover assembly onto the case, ensuring all shift forks engage their respective synchronizer sleeves or gear collars. The interlock components must align properly in their bores. Install the 8 shift cover bolts and tighten in a crossing pattern. Verify all shifts operate smoothly through all gear positions before final installation.
14. 14

    Reinstall transmission in vehicle
    Apply a thin coat of high-temperature grease to the input shaft splines and pilot bearing. Position the transmission on the transmission jack and raise into position behind the bellhousing. Align the input shaft with the clutch disc splines, rotating the output shaft by hand to facilitate engagement. Slide the transmission forward until the bellhousing and case mate completely with no gap. Install the 4 transmission-to-bellhousing bolts and tighten securely. Install the crossmember and transmission mount, then remove the transmission jack. Install the driveshaft, aligning the marks made during removal, and torque the U-joint strap bolts. Reconnect the speedometer cable and clutch linkage. Adjust clutch free play per specifications.
15. 15

    Fill transmission and test operation
    Fill the transmission with approximately 3.5 to 4 pints (roughly 2 quarts) of GL-4 rated 80W-90 gear oil through the fill plug on the side of the case until oil reaches the bottom of the fill hole threads. Install the fill plug securely. Reconnect the battery negative cable. Start the engine and verify the clutch disengages properly. With the engine running and clutch depressed, shift through all gears slowly, pausing in each position. Listen for any grinding, clashing, or abnormal noises. Test drive the vehicle through all gears under light load, then moderate acceleration. The repaired gear should engage smoothly without grinding or clashing.

Reassembly

1. Ensure all snap rings are fully seated in their grooves with no gaps
2. Verify synchronizer springs are properly installed with correct orientation and offset
3. Torque all fasteners to specification if torque values become available
4. Use only GL-4 rated gear oil; GL-5 will damage brass synchronizers
5. Adjust clutch free play to specification after installation to ensure proper synchronizer engagement

Verification

• Test shift quality in all gears with engine running and clutch depressed; no grinding should occur
• Verify smooth engagement of the repaired gear under light throttle during test drive
• Check for fluid leaks around extension housing, shift cover, and bearing retainer after test drive
• Confirm transmission operates quietly in all gears with no abnormal noises
• Re-check fluid level after test drive and top off if necessary

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