Short Block Replacement

Complete removal and replacement of the 390ci V8 short block assembly, including disassembly of cylinder heads, timing components, and installation of new short block core.

Warnings

Tools required

Parts

• 390ci V8 short block assembly × 1 — Use OEM specification or aftermarket equivalent
• Engine gasket set (complete) × 1 — Use OEM specification
• Timing chain and gear set × 1 — Use OEM specification
• Oil pump × 1 — Use OEM specification
• Camshaft × 1 — Use OEM specification or performance equivalent
• Lifter set (16 hydraulic lifters) × 1 — Use OEM specification
• Main bearing set × 1 — Standard or oversized as required
• Rod bearing set × 1 — Standard or oversized as required
• Piston ring set × 1 — Use OEM specification
• Freeze plug set × 1 — Use OEM specification
• Oil pan gasket × 1 — Use OEM specification
• Timing cover gasket × 1 — Use OEM specification
• Valve cover gaskets × 2 — Use OEM specification
• Oil filter × 1 — Use OEM specification
• Engine oil (10W-40) × 5 — 5 quarts capacity
• Coolant/antifreeze × 4 — 50/50 mix
• RTV silicone sealant × 1 — High-temp rated
• Thread locker (medium strength) × 1 — Use OEM specification

Fluids

• Engine oil (10W-40) — 5 qt
• Engine coolant (50/50 premixed) — 16 qt

Preparation

1. Park vehicle on level surface and engage parking brake
2. Disconnect negative battery cable, then positive cable
3. Drain engine oil completely and remove oil filter
4. Drain cooling system at radiator petcock and engine block drain plugs
5. Remove hood and store safely (mark hinge positions for reinstallation)
6. Remove air cleaner assembly and breather system
7. Label and photograph all vacuum lines, electrical connections, and fuel lines for reference
8. Relieve fuel system pressure if equipped with fuel injection (carbureted application on 1970 model)
9. Place drain pans under work area to catch residual fluids

Procedure

1. 1

   Remove external components and accessories
   Disconnect radiator hoses (upper and lower) and heater hoses. Remove radiator and fan shroud. Disconnect throttle linkage and transmission kickdown linkage. Remove alternator, power steering pump (if equipped), A/C compressor (if equipped) - mounting brackets can remain. Disconnect exhaust manifolds at collector pipes. Remove distributor cap and mark distributor position relative to engine block.
2. 2

   Disconnect fuel and electrical systems
   Disconnect fuel line at fuel pump. Remove all electrical connections including starter solenoid, temperature sender, oil pressure sender, and ignition coil primary wire. Tag each connection for reinstallation reference. Remove spark plug wires and spark plugs.
3. 3

   Separate transmission from engine
   Support transmission with floor jack and wooden block. Remove transmission bellhousing-to-engine block bolts (typically six bolts on AMC 390). Mark torque converter position if automatic transmission. Carefully separate transmission from engine ensuring torque converter remains with transmission. Secure transmission on jack.
4. 4

   Remove engine from vehicle
   Attach engine hoist to factory lifting points or use chain around intake manifold mounting points. Take up slack in hoist chain ensuring balance point is correct. Remove engine mount bolts from both motor mounts. Slowly lift engine while checking for any remaining connections. Tilt engine as needed to clear firewall and radiator support. Once clear, move engine to work area and mount on engine stand using bellhousing bolt holes.
5. 5

   Remove intake manifold and valve train
   With engine on stand, remove intake manifold bolts in reverse of tightening sequence (center out). Lift intake manifold and carburetor assembly as unit. Remove valve covers. Using specific sequence, remove rocker arm assemblies and push rods. Store push rods in order for inspection and potential reuse. Remove lifter valley cover if present.
6. 6

   Remove cylinder heads
   Remove cylinder head bolts in reverse order of installation sequence (typically outward spiral from center). AMC 390 uses 10 head bolts per cylinder. Carefully lift cylinder heads from block - they are heavy (approximately 55 lbs each). Inspect heads for cracks or warpage. Place heads on clean workspace for transfer to new short block.
7. 7

   Remove front timing cover and components
   Remove harmonic balancer using puller tool - do not hammer or pry. Remove timing cover bolts and timing cover. Remove timing chain tensioner (if equipped). Note timing mark alignment between crankshaft and camshaft gears before removal. Remove camshaft bolt, timing chain, and camshaft gear. Slide camshaft carefully out of engine block, supporting to avoid damaging bearing journals.
8. 8

   Remove oil pan and oil pump
   Remove oil pan bolts and carefully separate oil pan from block. Remove oil pump mounting bolts and withdraw oil pump and pickup tube assembly. Inspect oil pump drive shaft condition. Clean all gasket surfaces on oil pan for reuse with new short block.
9. 9

   Complete disassembly and inspection
   Remove any remaining external components including fuel pump, motor mount brackets, and freeze plugs if being replaced. Remove all lifters and store in order if reusing. Thoroughly clean all reusable components including cylinder heads, intake manifold, timing cover, and oil pan. Inspect all parts for wear, cracks, or damage. Check valve springs, valves, and rocker arms for excessive wear.
10. 10

    Prepare new short block
    Inspect new short block for shipping damage or debris. Verify all freeze plugs are properly installed. Check main bearing caps are properly torqued and safety wired if applicable. Rotate crankshaft by hand to verify smooth operation. Apply assembly lube to camshaft lobes and bearing journals. Clean all gasket surfaces with solvent and verify they are smooth and flat.
11. 11

    Install camshaft and timing components
    Carefully slide camshaft into new short block, rotating slowly to engage bearing journals without damage. Install camshaft thrust plate if equipped. Install new timing gears on crankshaft and camshaft, ensuring timing marks align properly (typically dot-to-dot alignment). Install new timing chain. Apply thread locker to camshaft bolt and tighten securely. Install new oil pump with new gasket, ensuring proper engagement with distributor drive shaft.
12. 12

    Install oil pan and timing cover
    Install new oil pan gasket with RTV sealant at corners and end seals. Position oil pan and install bolts finger tight, then torque in cross pattern from center outward. Install new timing cover gasket and apply RTV at oil pan junction. Install timing cover ensuring proper seal alignment. Install harmonic balancer using installation tool, ensuring it is fully seated on crankshaft snout.
13. 13

    Install lifters and cylinder heads
    Install new or cleaned lifters into their bores with assembly lube. If reusing original lifters, install in original positions. Clean cylinder head gasket surfaces and block deck. Position new head gaskets on block, ensuring proper orientation and all coolant and oil passages align. Carefully position cylinder heads on block. Install head bolts with light oil on threads and under bolt heads. Torque head bolts in proper sequence and increments per manufacturer specification (typically 80-85 ft-lbs final torque in three stages).
14. 14

    Install valve train and complete engine assembly
    Install push rods in original positions with assembly lube on ends. Install rocker arm assemblies and adjust valve lash if using mechanical lifters (hydraulic lifters require zero-lash setting). Install new valve cover gaskets and valve covers. Install intake manifold with new gaskets, torquing in proper sequence from center outward. Reinstall all external components including fuel pump, distributor (in marked position), water pump, and thermostat housing with fresh gaskets and coolant.
15. 15

    Reinstall engine and connect systems
    Using engine hoist, carefully lower engine into engine bay, aligning with motor mounts and transmission bellhousing. Install motor mount bolts and torque to specification. Align engine to transmission and install bellhousing bolts, ensuring proper torque converter engagement on automatic transmission. Reconnect all electrical connections per tags and photos. Reconnect fuel lines with new clamps. Install exhaust manifold connections. Reinstall all accessories including alternator, power steering pump, and A/C compressor. Install radiator, fan shroud, and reconnect all cooling system hoses. Reinstall hood using marked positions.

Reassembly

1. Refill cooling system with proper 50/50 coolant mixture through radiator and allow air to bleed
2. Install new oil filter and fill engine with proper grade and quantity of oil (5 quarts for 390ci)
3. Reconnect battery cables (positive first, then negative)
4. Double-check all fluid levels before starting engine
5. Verify all electrical connections, vacuum lines, and fuel lines are properly secured
6. Check that all accessory drive belts are properly tensioned

Verification

• Prime oil system by cranking engine with ignition disabled until oil pressure registers on gauge
• Perform initial engine start and check immediately for oil pressure (should register within 10 seconds)
• Check for any fuel, oil, or coolant leaks while engine is running
• Allow engine to reach operating temperature and verify thermostat operation
• Check and adjust ignition timing to factory specification using timing light
• Adjust idle speed and mixture as necessary
• Road test vehicle under light load and verify proper engine operation
• Re-check all fluid levels after test drive and top off as needed
• Inspect for leaks after cool-down period
• Re-torque cylinder head bolts after 500 miles if using composition gaskets (not required for modern MLS gaskets)

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