Synchros - Single Gear

Replace a single worn synchronizer assembly in the NV4500 manual transmission, requiring complete transmission disassembly and rebuild.

Warnings

Tools required

Parts

• Synchronizer assembly (specify gear) × 1 — Use OEM specification
• Transmission gasket set × 1 — Use OEM specification
• Transmission output seal × 1 — Use OEM specification
• Input shaft bearing retainer seal × 1 — Use OEM specification
• Shift fork pads (if worn) × 1 — Use OEM specification
• Drain plug washer × 1 — Use OEM specification
• Transmission assembly lubricant × 1 — Use OEM specification

Fluids

• 75W-90 synthetic gear oil — 4 qt

Preparation

1. Park vehicle on level surface and engage parking brake
2. Disconnect both battery cables (negative first)
3. Raise vehicle on lift or jack stands to working height
4. Remove skid plate if equipped
5. Drain transmission fluid completely and note condition for contamination
6. Remove transfer case (retain all shims and note orientation)
7. Disconnect shift linkage at transmission, noting positions
8. Disconnect speedometer cable or vehicle speed sensor
9. Support engine with fixture or hoist to prevent dropping when transmission is removed
10. Mark driveshaft-to-yoke orientation and remove rear driveshaft
11. Disconnect clutch hydraulic line at transmission slave cylinder
12. Remove transmission crossmember after supporting transmission with jack
13. Prepare clean workspace with large drain pan for transmission disassembly

Procedure

1. 1

   Remove transmission from vehicle
   With transmission supported on jack, remove bellhousing mounting bolts. Carefully slide transmission rearward off input shaft, ensuring it clears clutch assembly without binding. Lower transmission onto work surface. The NV4500 has multiple mounting bolt sizes - verify all are removed before attempting to separate.
2. 2

   Remove external components and shift tower
   Remove shifter tower assembly bolts and lift off carefully, noting detent ball and spring positions. Remove backup light switch. Remove front bearing retainer bolts and carefully pry retainer off input shaft. Remove the vehicle speed sensor (VSS) or speedometer driven gear assembly as equipped for this application. Clean all external surfaces thoroughly before disassembly.
3. 3

   Split transmission case halves
   Remove case bolts in crisscross pattern. The NV4500 case uses multiple bolt lengths - note locations for reassembly. Carefully pry case halves apart using designated pry points only. Lift rear case half straight up to separate. All shift forks and synchronizer assemblies will remain with mainshaft in front case.
   ⚠Do not pry between machined sealing surfaces. Use only designated pry boss locations to avoid case damage.
4. 4

   Remove countershaft and reverse idler
   Remove countershaft rear bearing retainer. Drive countershaft rearward out of case using brass drift. Remove countergear assembly with thrust washers and bearing spacers - keep all components together. Remove reverse idler shaft retaining bolt and slide shaft out. Remove reverse idler gear and thrust washers.
5. 5

   Remove mainshaft assembly
   Remove mainshaft rear bearing snap ring. Use bearing puller to remove rear bearing from mainshaft. Tilt mainshaft assembly and carefully lift out of front case with all gears and synchronizers attached. Place on clean workbench. Input shaft will remain in front case bearing.
   ⚠Support gear clusters while removing to prevent dropping and damage. Gear teeth are hardened and will chip if dropped.
6. 6

   Disassemble mainshaft to access synchronizer
   Remove mainshaft front snap ring. Slide off appropriate gear cluster to access the worn synchronizer. Note exact orientation of synchronizer sleeve, hub, and shift fork groove position. Remove synchronizer snap ring. Slide synchronizer assembly off mainshaft. Remove blocking ring and inspect cone surface on gear for scoring or damage. If replacing 3rd-4th synchronizer, it is located at front of mainshaft. If replacing 1st-2nd, continue disassembling toward rear.
   ℹ️Take photographs during disassembly to ensure correct reassembly orientation. Synchronizer sleeves have specific front/rear orientation.
7. 7

   Inspect and measure all components
   Thoroughly clean all parts in solvent and dry with compressed air. Inspect all gear teeth for cracks, chips, or excessive wear. Measure gear endplay specifications with dial indicator. Inspect blocking ring-to-gear cone clearance (should be 0.020-0.050 inch). Check shift fork pad wear - replace if grooved more than 0.015 inch. Inspect all bearings for roughness or pitting. Replace any questionable components now to avoid repeat disassembly.
   ⚠Do not reuse blocking rings if clearance is below specification. Insufficient clearance prevents proper synchronization and causes grinding.
8. 8

   Install new synchronizer assembly
   Install new blocking ring on gear cone with alignment slots properly positioned. Lubricate mainshaft with assembly lube. Slide new synchronizer hub onto mainshaft with shift fork groove in correct orientation. Install snap ring ensuring it is fully seated in groove. Install synchronizer sleeve over hub, aligning internal splines. Verify blocking ring tabs engage synchronizer sleeve slots. Check synchronizer operation by hand - sleeve should slide smoothly with slight resistance from the synchronizer insert (strut) springs.
   ℹ️Synchronizer sleeve and hub are matched sets. Do not mix components from different assemblies.
9. 9

   Reassemble mainshaft
   Reinstall remaining gears, thrust washers, and snap rings in reverse order of removal. Verify all snap rings are fully seated in grooves. Install mainshaft rear bearing using bearing driver and press or careful hammer blows on inner race only. Install rear bearing snap ring. Measure mainshaft assembly endplay with dial indicator - should be within 0.002-0.010 inch. Adjust with selective snap rings if necessary.
   ⚠Never drive on bearing outer race or rolling elements. This will destroy the bearing. Always press or drive on inner race only.
10. 10

    Install mainshaft and countershaft in case
    Apply assembly lube to all bearing surfaces. Lower mainshaft assembly into front case. Install shift forks in proper positions on synchronizer sleeves. Install countergear with thrust washers in case. Slide countershaft through case and countergear from rear, aligning shaft flat with retainer bolt hole. Install reverse idler gear and shaft. Verify all components rotate freely without binding.
    ℹ️Thrust washers have specific orientation with tabs that must align with case notches. Incorrect installation causes immediate failure.
11. 11

    Assemble case halves
    Apply thin bead of RTV sealant to rear case mating surface (not front case). Install all shift fork shafts and detent balls/springs in proper positions. Carefully lower rear case half onto front case, ensuring all shafts align properly. Install case bolts finger-tight, then torque in crisscross pattern starting from center working outward. Verify input and output shafts rotate freely after case assembly.
    ⚠Excessive RTV sealant will squeeze into case and contaminate bearings. Use thin, continuous bead only on sealing surface.
12. 12

    Install external components
    Install new front bearing retainer seal. Install bearing retainer with new gasket, torquing bolts evenly. Install shift tower with new gasket, ensuring detent balls and springs are properly positioned. Install the vehicle speed sensor (VSS) or speedometer gear assembly as equipped for this application. Install backup light switch and torque to specification. Rotate input shaft through all gear ranges to verify proper shifting.
13. 13

    Reinstall transmission in vehicle
    Apply small amount of high-temp grease to input shaft splines and pilot bearing. Support transmission on jack and align with bellhousing. Carefully slide transmission forward onto input shaft, ensuring clutch disc splines engage properly. Transmission should slide fully into place with minimal force. Install bellhousing mounting bolts and torque in star pattern to prevent case distortion.
    ⚠If transmission will not mate to bellhousing, do not force with bolts. Remove and check for clutch disc misalignment or obstruction.
14. 14

    Reconnect all components and refill
    Install transmission crossmember and torque bolts. Reconnect clutch hydraulic line and bleed system. Install rear driveshaft aligning marks made during removal. Reconnect shift linkage and adjust to specification. Reconnect speedometer cable or sensor. Reinstall transfer case onto the transmission adapter, retaining any original spacers/gaskets and torquing mounting nuts to OEM specification. Remove fill plug and fill transmission with specified synthetic gear oil until level reaches fill hole. Install fill plug with new washer and torque to specification.
    Torque spec

    Fill Plug34 Nm (25 lb-ft)

    Drain Plug34 Nm (25 lb-ft)

Reassembly

1. Torque all fasteners to specification using proper sequence
2. Use new gaskets and seals throughout - do not reuse old gaskets
3. Verify all snap rings are fully seated in their grooves
4. Check that all shift forks engage synchronizer sleeves properly
5. Ensure transmission shifts smoothly through all gears before final installation
6. Reinstall transfer case, driveshaft, and all connections in reverse order of removal
7. Reconnect battery cables (positive first, then negative)

Verification

• With engine off, verify clutch pedal has proper feel and returns fully
• Start engine and verify transmission shifts into all gears without grinding
• Check for fluid leaks around case seams, front seal, and drain plug
• Test drive vehicle through all gear ranges under light load first
• Verify smooth engagement of the repaired gear under acceleration and deceleration
• Listen for any abnormal noises indicating improper assembly
• Recheck transmission fluid level after test drive and top off if necessary
• After 500 miles, drain and inspect fluid for metal particles indicating problems

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