Main Bearings

Replace the main bearings on the AMC 390ci V8 engine, requiring engine removal and complete bottom-end disassembly to install new crankshaft bearing shells.

Warnings

Tools required

Parts

• Main bearing set (standard or appropriate oversize) × 1 — Verify crankshaft journal size before ordering
• Thrust bearing set × 1 — Usually included with main bearing set
• Complete engine gasket set × 1 — Includes oil pan, timing cover, intake, valve covers
• Oil pan gasket × 1 — Use OEM specification
• Timing cover gasket × 1 — Use OEM specification
• Front and rear crankshaft seals × 1 — Use OEM specification
• Engine oil filter × 1 — Use OEM specification
• Engine assembly lube × 1 — High-pressure moly lube
• Main bearing cap bolts × 10 — Recommended replacement on reassembly
• Motor oil (10W-30 or appropriate viscosity) × 6 — 6 quarts minimum

Fluids

• Engine oil (10W-30 or appropriate viscosity) — 6 qt
• Engine coolant — 12 qt

Preparation

1. Disconnect negative battery cable
2. Drain engine oil and coolant completely
3. Remove hood by unbolting hinge bolts (mark hinge position for reinstallation)
4. Disconnect all electrical connections, vacuum lines, fuel lines, and throttle linkage from engine
5. Remove radiator, fan shroud, and fan assembly
6. Disconnect exhaust pipes from manifolds
7. Remove transmission bellhousing bolts and separate transmission from engine
8. Label and disconnect motor mounts
9. Clear adequate workspace and position engine hoist with load leveler

Procedure

1. 1

   Remove engine from vehicle
   Attach engine hoist chains to intake manifold or cylinder head lifting points. Verify all connections and hoses are disconnected. Slowly lift engine while adjusting load leveler to maintain balance. Once clear of engine bay, carefully move engine to workbench and mount securely on engine stand by bolting to bellhousing mounting surface.
2. 2

   Remove external components
   With engine on stand, remove valve covers, intake manifold, exhaust manifolds, distributor, fuel pump, water pump, harmonic balancer, and timing chain cover. Remove oil pump and distributor drive shaft. Mark position of distributor drive gear before removal for proper reassembly timing.
3. 3

   Remove timing chain and camshaft
   Remove timing chain tensioner if equipped. Remove camshaft timing gear bolt and slide timing chain and both gears off as an assembly. Carefully remove camshaft thrust plate bolts and withdraw camshaft straight out through front of block, rotating slowly to avoid damaging cam bearings. Support camshaft to prevent it from dropping.
4. 4

   Remove oil pan and windage tray
   Remove all oil pan bolts and carefully separate oil pan from block. Remove windage tray or oil pump pickup screen if installed. Clean all gasket surfaces thoroughly. Inspect oil pan for cracks or damage and clean completely.
5. 5

   Remove rod caps and mark components
   Using a marker or number stamps, clearly mark each connecting rod cap and its corresponding rod with cylinder number (1-8) and orientation (ensure marks face same direction). Remove all rod cap nuts and carefully remove rod caps. Push pistons up into bores to provide access to main bearings. Keep rod caps with their respective rods.
6. 6

   Mark and remove main bearing caps
   The AMC 390 uses five main bearing caps. Using a marker or stamps, clearly mark each main bearing cap (1-5 from front to back) with its position number and orientation arrow pointing toward front of engine. Note that cap #3 typically contains the thrust bearing. Remove all main bearing cap bolts in a cross-pattern, loosening gradually. Use main cap puller or carefully tap caps with soft-face hammer to break them free. Keep caps in order.
7. 7

   Remove crankshaft and inspect journals
   Carefully lift crankshaft straight up and out of block. Inspect all main and rod journals for scoring, discoloration, or damage. Using micrometer, measure each main journal diameter at multiple points (thrust surfaces and 90 degrees apart) to check for taper and out-of-round condition. Record all measurements. Journals must be within 0.001 inch round and taper, or require machining.
8. 8

   Inspect block and main saddles
   Remove old bearing shells from block and caps. Clean all bearing surfaces thoroughly with solvent and lint-free cloths. Inspect main bearing saddles in block for cracks, scoring, or damage. Check for debris in oil passages and blow out with compressed air. If available, use dial bore gauge to measure main bearing bore diameters.
9. 9

   Install new main bearings and check clearances
   Install new upper bearing shells (with oil holes) into block saddles, ensuring tang is properly seated in notch. Install lower bearing shells into main caps. Place strip of Plastigage on each crankshaft journal. Carefully lower crankshaft into position. Install main caps in correct order and orientation. Install bolts and torque to specification (typically 95-100 ft-lbs for AMC 390, but verify for your application). Do not rotate crankshaft with Plastigage installed.
10. 10

    Measure bearing clearances
    Remove main cap bolts and carefully remove caps without rotating crankshaft. Measure Plastigage width using scale on package. Proper main bearing clearance for AMC 390 is typically 0.001-0.003 inches. If clearances are out of specification, verify crankshaft journal measurements and bearing size. Repeat Plastigage check after corrections.
11. 11

    Check crankshaft endplay
    With all main caps torqued to specification, use pry bar to lever crankshaft fully forward, then fully rearward while measuring movement with dial indicator against crankshaft snout. AMC 390 crankshaft endplay specification is typically 0.003-0.008 inches, controlled by thrust bearing at #3 main. If out of specification, verify correct thrust bearing installation or replace thrust bearing set.
12. 12

    Perform final bearing installation
    Remove main caps one final time. Apply engine assembly lube liberally to all bearing surfaces on both upper and lower shells and to crankshaft journals. Install crankshaft and main caps in correct order. Install new main cap bolts if available. Torque main cap bolts in three steps: first to 30-40 ft-lbs, then 60-70 ft-lbs, then final torque in cross-pattern from center (#3) outward. Verify crankshaft rotates freely by hand with smooth, consistent resistance.
13. 13

    Install connecting rods and check rod bearing clearances
    Apply assembly lube to rod bearings and journals. Pull connecting rods down onto crankshaft rod journals, ensuring rods are on correct cylinders and proper orientation. Use Plastigage to verify rod bearing clearances (typically 0.001-0.0025 inches). Once verified, apply assembly lube and torque rod cap nuts to specification (typically 45-50 ft-lbs for AMC 390).
14. 14

    Reinstall camshaft and timing components
    Lubricate camshaft lobes and journals with assembly lube. Carefully slide camshaft into block, rotating slowly to avoid damaging bearings. Install thrust plate. Apply assembly lube to timing gears and install timing chain with gears, ensuring timing marks align correctly per AMC specifications (typically both marks adjacent on centerline). Install timing cover with new gasket. Install harmonic balancer.
15. 15

    Install oil pan and external components
    Install new rear main seal if applicable. Install oil pump with new gasket, ensuring proper engagement with distributor drive. Install windage tray if equipped. Apply gasket sealer to oil pan gasket and install oil pan, torquing bolts in cross-pattern. Reinstall all external components including water pump, fuel pump, intake and exhaust manifolds, distributor, valve covers, and all accessories.

Reassembly

1. Prime oil system by rotating oil pump with drill motor on pump drive shaft, or by cranking engine with ignition disabled until oil pressure registers
2. Install engine into vehicle in reverse order of removal, ensuring motor mounts align properly
3. Reconnect transmission, exhaust, all electrical connections, fuel lines, and cooling system
4. Refill with fresh engine oil and new filter, coolant to proper level
5. Reinstall hood using alignment marks made during removal

Verification

• Before first start, verify oil pressure builds immediately on cranking (disconnect ignition and crank until oil pressure shows)
• Start engine and listen for any unusual noises; knocking or rattling indicates bearing installation problem
• Monitor oil pressure at idle (should be 10+ PSI minimum) and at 2000 RPM (40-60 PSI typical)
• Check for oil leaks around pan, timing cover, and all gasket surfaces
• After initial 20-minute break-in period at varying RPM (avoid sustained high RPM), change oil and filter, inspect for metal particles in old oil
• Test drive vehicle and monitor oil pressure and engine temperature under load
• After 500 miles, change oil and filter again and retorque cylinder head bolts if accessible

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