Complete replacement of all synchronizer assemblies in the manual transmission, requiring full transmission removal, disassembly, synchronizer replacement, and reassembly with proper shimming and adjustment.

Warnings

Tools required

Parts

• 1st/2nd gear synchronizer assembly × 1 — Use OEM specification
• 3rd/4th gear synchronizer assembly × 1 — Use OEM specification
• Reverse synchronizer assembly (if equipped) × 1 — Use OEM specification
• Synchronizer blocking rings (set of 6) × 1 — Use OEM specification
• Synchronizer keys and springs kit × 1 — Use OEM specification
• Transmission gasket and seal kit × 1 — Use OEM specification
• Countershaft bearing set × 1 — Use OEM specification
• Input shaft bearing × 1 — Use OEM specification
• Output shaft rear bearing × 1 — Use OEM specification
• Mainshaft snap rings (selective fit set) × 1 — Use OEM specification
• Transmission assembly lube × 1 — Use OEM specification
• Anaerobic gasket maker × 1 — Use OEM specification
• Thread locking compound (medium strength) × 1 — Use OEM specification

Fluids

• 75W-90 GL-5 Gear Oil — 3.5 qt

Preparation

1. Verify transmission type (M20, M21, or M22) by checking side cover stampings and gear ratios before ordering parts.
2. Park vehicle on level concrete surface, engage parking brake, and place transmission in neutral.
3. Disconnect negative battery cable to prevent starter engagement.
4. Allow transmission to cool completely if recently driven (minimum 2 hours).
5. Raise vehicle and support on jack stands under frame rails, ensuring sufficient clearance for transmission removal (minimum 24 inches).
6. Remove driveshaft: mark yoke-to-flange alignment with paint, remove 4 U-joint strap bolts, slide driveshaft rearward to disconnect from transmission, and plug tail housing to prevent fluid loss.
7. Disconnect speedometer cable at transmission by unscrewing retaining ferrule.
8. Disconnect shift linkage at side cover by removing clevis pins or rod clips (varies by year and shifter type).
9. Disconnect clutch linkage: remove clutch fork return spring, disconnect clutch fork pushrod from clutch cross shaft.
10. Support engine from above using engine support bar or chain hoist to prevent engine from tilting when transmission is removed.
11. Position transmission jack under transmission pan with safety chain secured around case.

Procedure

1. 1

   Remove transmission from vehicle
   Remove 4 transmission-to-bellhousing bolts. Carefully slide transmission rearward on jack to disengage input shaft from clutch disc and pilot bearing. Tilt front of transmission slightly downward to clear bellhousing opening. Lower transmission and remove from under vehicle. Transport to clean workbench area.
2. 2

   Drain and clean transmission exterior
   Remove drain plug from transmission case bottom and drain remaining fluid into appropriate container. Clean exterior of transmission case thoroughly with degreaser and compressed air to prevent contamination during disassembly. Inspect case for cracks or damage that would prevent reuse.
3. 3

   Remove side cover and shift forks
   Remove 9 side cover bolts and carefully lift side cover away from case. Note position and orientation of shift forks and detent mechanism. Remove shift fork retaining pins or bolts that secure forks to shift rails. Withdraw shift rails from case, capturing detent balls and springs as they are exposed. Keep all shift mechanism components organized by position.
4. 4

   Remove front bearing retainer and input shaft
   Remove 4 front bearing retainer bolts and pull retainer from case (may require light tapping with plastic mallet). Remove input shaft bearing snap ring from inside front of case. Use brass drift through side cover opening to tap input shaft and bearing assembly rearward and out of case. Do not lose bearing rollers from input shaft pocket that pilot the mainshaft.
5. 5

   Remove tail housing and output shaft assembly
   Remove tail housing bolts (typically 6 bolts). Slide tail housing rearward off output shaft. If housing is stuck, tap gently with plastic mallet around perimeter. Remove output shaft rear bearing snap ring from inside tail housing bore. Use bearing puller to extract rear bearing from tail housing if replacement is needed.
6. 6

   Remove mainshaft assembly
   Tilt mainshaft assembly upward at rear and maneuver entire assembly out through top of case. This includes output shaft with all gears, synchronizers, and bearings as an assembly. Place mainshaft assembly on clean bench for disassembly. Mark gear positions with paint before disassembly to ensure correct reassembly order.
7. 7

   Remove countershaft and reverse idler
   Use brass drift to drive countershaft rearward out of case while supporting countergear to prevent it from dropping. Remove countergear, thrust washers, and bearing rollers (typically 28 rollers per end with spacer). Capture all rollers and keep organized. Remove reverse idler shaft retaining bolt, then drift reverse idler shaft from case and remove reverse idler gear with thrust washers.
8. 8

   Disassemble mainshaft - remove rear components
   Secure mainshaft in vise using soft jaws at rear bearing journal. Remove rear bearing snap ring. Use bearing puller to remove rear bearing from shaft. Remove 1st gear thrust washer. Slide 1st speed gear off shaft. Remove 1st gear blocking ring. Remove 1st/2nd synchronizer assembly snap ring. Use snap ring pliers to expand snap ring while tapping synchronizer hub with plastic mallet to remove 1st/2nd synchronizer assembly. Mark hub-to-sleeve orientation before separating. Remove 2nd gear blocking ring and 2nd speed gear.
9. 9

   Disassemble mainshaft - remove front components
   Flip mainshaft in vise to secure at splined front section. Remove 3rd/4th synchronizer snap ring. Remove 3rd/4th synchronizer assembly using same technique as 1st/2nd, marking orientation. Remove 3rd gear blocking ring and 3rd speed gear. Remove 3rd gear thrust washer and snap ring if present. Inspect mainshaft for wear, scoring, or damage at bearing journals and gear surfaces.
10. 10

    Inspect and measure all components
    Clean all parts in solvent and dry with compressed air. Inspect all gear teeth for chips, excessive wear, or pitting. Check synchronizer blocking rings for cone wear by attempting to fit ring on gear cone - minimum clearance should be 0.020 inches (use feeler gauge). Measure mainshaft diameter at bearing journals with micrometer - out-of-round limit is 0.002 inches. Inspect synchronizer sleeves for worn teeth and hubs for damage. Check countergear and reverse idler teeth. Replace any worn components.
11. 11

    Assemble new synchronizers
    For each synchronizer assembly: Place hub on bench with long side of internal teeth facing up (orientation critical). Install 3 synchronizer keys into hub slots. Place synchronizer sleeve over hub, aligning reference marks made during disassembly. Install one spring behind keys, with spring ends positioned in same key slot. Rotate assembly 180 degrees and install second spring with ends in different key slot from first spring (prevents both springs from unseating simultaneously). Verify sleeve slides smoothly on hub and keys remain seated.
12. 12

    Reassemble mainshaft - front section
    Apply thin coat of transmission assembly lube to mainshaft. Install 3rd speed gear with cone toward front. Install new 3rd gear blocking ring on gear cone with slots aligned for synchronizer keys. Install 3rd/4th synchronizer assembly with shift fork groove toward rear, ensuring synchronizer keys engage blocking ring slots. Install selective fit snap ring in mainshaft groove - choose thickest ring that fits completely in groove with no end gap visible. Measure 3rd/4th synchronizer endplay with dial indicator - specification is 0.004 to 0.014 inches.
13. 13

    Reassemble mainshaft - rear section
    Install 2nd speed gear with cone toward rear. Install new 2nd gear blocking ring on cone. Install 1st/2nd synchronizer assembly with shift fork groove toward front, engaging blocking ring slots. Install selective fit snap ring. Install 1st gear blocking ring on synchronizer cone. Install 1st speed gear with cone toward synchronizer. Install 1st gear thrust washer (tang fits mainshaft slot). Press rear bearing onto mainshaft using bearing installation tool - press only on inner race until bearing seats against thrust washer. Install rear bearing snap ring. Measure 1st/2nd synchronizer endplay - specification is 0.004 to 0.014 inches.
14. 14

    Reassemble countershaft and reverse idler
    Coat countergear bore with assembly lube. Install spacer in countergear center. Install bearing retaining washer in each end of gear. Load 28 bearing rollers into front bore using heavy grease to hold them in place, ensuring rollers are evenly spaced around full circumference. Repeat for rear bore with 28 rollers. Place countergear assembly into case with thrust washers (bronze face toward gear). Insert countershaft from rear of case through thrust washer, gear, and front thrust washer. Align countershaft so lock plate slot is accessible. Install reverse idler gear with shift lever groove toward rear. Insert reverse idler shaft and install retaining bolt with thread locking compound.
15. 15

    Install mainshaft assembly in case
    Carefully lower mainshaft assembly into case from top, tilting rear upward and maneuvering around countergear. Ensure rear bearing seats completely in case bore. Rotate mainshaft to verify it rotates freely and gears mesh properly with countergear. Check that synchronizer sleeves move smoothly on hubs and blocking rings are not binding.
16. 16

    Install input shaft assembly
    Coat input shaft bearing with assembly lube. Load 15 bearing rollers into input shaft pocket using heavy grease to retain them. Carefully align input shaft with mainshaft pilot (bearing rollers) and insert input shaft into case from front. Install input shaft bearing snap ring in case groove. Verify input shaft rotates freely. Install new gasket on front bearing retainer, coat retainer bolt threads with thread sealer, and install retainer with oil return slot at bottom. Install 4 retainer bolts.
17. 17

    Install shift mechanism
    Insert shift rails into case bores from front. Install detent springs and balls in case detent bores as each rail is inserted. Install shift forks onto rails in correct position for each gear range (1st/2nd fork, 3rd/4th fork, and reverse fork). Align fork slots with synchronizer sleeve grooves. Install fork retaining pins or bolts. Verify each rail moves smoothly through full travel and properly engages synchronizer sleeves. Check that detent balls properly locate each rail in neutral, forward, and reverse positions.
18. 18

    Install side cover and verify shift operation
    Apply thin bead of anaerobic gasket maker to case side cover mounting surface. Position side cover on case, ensuring shift forks remain engaged with synchronizer sleeves and reverse fork engages reverse idler. Install 9 side cover bolts and tighten in star pattern. Operate shifter mechanism through all gear positions, verifying smooth engagement and proper detent feel. Check that synchronizer sleeves move appropriate distance for each shift.
19. 19

    Install tail housing and output seal
    Press new rear seal into tail housing using seal driver, with seal lip facing forward. Apply assembly lube to seal lip. Install new tail housing gasket or apply bead of anaerobic gasket maker to case mating surface. Carefully slide tail housing over output shaft, aligning dowel pins with case holes. Install tail housing bolts and tighten in cross pattern. Verify output shaft rotates freely.
20. 20

    Perform final measurements and bench test
    Mount dial indicator on case with tip contacting output shaft end. Move shaft through full endplay range and measure - specification is 0.004 to 0.018 inches. If out of specification, adjust with selective thrust washers. Check all snap rings are fully seated. Rotate input and output shafts while shifting through all gears to verify smooth operation and proper synchronizer function. Listen for any unusual noise or binding. Fill transmission with approximately 2.5 quarts of SAE 80W-90 GL-4 gear oil through side cover opening before final installation.
21. 21

    Install transmission in vehicle
    Apply thin coat of clutch spline grease to input shaft splines and pilot bearing. Position transmission on transmission jack and raise into position under vehicle. Align input shaft with clutch disc splines using clutch alignment tool if necessary. Carefully slide transmission forward, ensuring input shaft enters clutch disc hub and pilot bearing smoothly. Avoid hanging transmission weight on clutch disc. Once input shaft is fully engaged and transmission is seated against bellhousing, install 4 transmission-to-bellhousing bolts. Remove transmission jack and engine support.

Reassembly

1. Reconnect shift linkage to side cover shift levers, ensuring correct rod orientation and adjustment for each gear position.
2. Reconnect clutch linkage: attach pushrod to clutch fork and cross shaft, install return spring, and adjust clutch free play to specification (typically 1 inch at pedal).
3. Reconnect speedometer cable to tail housing and secure retaining ferrule.
4. Install driveshaft by aligning reference marks made during removal, ensuring U-joint caps are properly seated in yoke, and install 4 strap bolts.
5. Fill transmission through side fill plug with 3.5 quarts of 75W-90 GL-5 gear oil until fluid reaches bottom of fill hole.
6. Reconnect negative battery cable.
7. Lower vehicle from jack stands.

Verification

• Start engine and verify clutch disengages properly without grinding noises.
• With engine running and clutch depressed, shift through all gear positions verifying smooth engagement without clash or grind.
• Test drive vehicle through all gears under light load, verifying smooth shifts and no jumping out of gear.
• Check for fluid leaks at front seal, tail housing, side cover, and drain plug after test drive.
• Verify proper clutch pedal free play and shift lever positioning in all gears.
• Listen for any abnormal transmission noise during operation - whining, grinding, or rattling indicates improper assembly.
• After 50-100 miles of operation, recheck transmission fluid level and inspect for leaks.
• Verify that all gears engage smoothly at various engine speeds and that synchronizers properly match shaft speeds without gear clash.

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