Pistons - All

Complete removal and replacement of all eight pistons in the 390ci V8 engine, including connecting rod bearing replacement and cylinder honing if necessary.

Warnings

Tools required

Parts

• Piston set (8 pistons) × 1 — Use OEM specification or aftermarket equivalent for 390ci
• Piston ring set (8 cylinders) × 1 — Match to piston type and bore size
• Connecting rod bearing set × 1 — Standard or appropriate oversize
• Connecting rod bolts × 16 — Replace if torque-to-yield or worn
• Head gasket set × 1 — Use OEM specification
• Intake manifold gasket set × 1 — Use OEM specification
• Exhaust manifold gaskets × 2 — Use OEM specification
• Oil pan gasket × 1 — Use OEM specification
• Oil pump pickup tube O-ring × 1 — Use OEM specification
• Engine oil and filter × 1 — 5W-30 or 10W-30, approximately 5 quarts
• Engine assembly lube × 1 — Moly-based or manufacturer specified
• RTV silicone gasket maker × 1 — High-temp rated
• Anti-seize compound × 1 — For exhaust fasteners

Fluids

• Engine oil (SAE 10W-30 or 10W-40) — 5 qt
• Engine coolant (ethylene glycol) — 4 qt

Preparation

1. Disconnect negative battery cable and remove battery
2. Drain engine coolant completely into appropriate container
3. Drain engine oil and remove oil filter
4. Remove air cleaner assembly and carburetor or intake system
5. Label and disconnect all vacuum lines, electrical connectors, and fuel lines from engine
6. Remove distributor after marking rotor position relative to block
7. Remove valve covers
8. Remove intake manifold with carburetor as assembly
9. Remove exhaust manifolds
10. Remove both cylinder heads after removing all head bolts in reverse of tightening sequence
11. Remove harmonic balancer using proper puller (never hammer on crankshaft)
12. Remove timing chain cover, timing chain, and camshaft sprocket
13. Support engine from above with hoist or support fixture if working with engine in vehicle
14. Remove oil pan and oil pump assembly
15. Clean work area thoroughly and prepare clean surface for component storage

Procedure

1. 1

   Remove cylinder ridge
   Rotate crankshaft to bring piston 1 to bottom dead center. Using a ridge reamer, carefully remove the wear ridge at the top of each cylinder bore. Follow the ridge reamer manufacturer's instructions to avoid cutting into the cylinder wall below the ridge. Clean all debris thoroughly from cylinders after reaming. Repeat for all eight cylinders.
   ⚠Failure to remove ridge will break piston ring lands during removal
2. 2

   Mark and remove connecting rod caps
   Using a stamp or permanent marker, mark each connecting rod and cap with cylinder number (1-8) and orientation (notch forward or mark toward front). Verify rods are numbered from factory; if not, mark clearly. Remove connecting rod cap bolts/nuts and carefully remove each cap, keeping cap with its matching rod. Inspect rod bolts for stretching, thread damage, or galling - replace if any defects found.
   ⚠️Mixing connecting rod caps between rods will cause bearing failure and engine destruction
3. 3

   Remove piston and rod assemblies
   Push piston and rod assembly upward out of cylinder bore from below. Guide connecting rod bolts carefully to avoid scratching crankshaft journals. Use a wooden hammer handle to gently tap the rod if needed. Remove all eight piston assemblies, keeping each with its numbered cap. Install rod caps back on their respective rods finger-tight to prevent mixing.
4. 4

   Disassemble and inspect pistons
   Remove piston rings using ring expander tool or carefully by hand, expanding only enough to slide over piston. Remove piston pins using appropriate press or removal tool depending on pin type (pressed or full-floating). Keep all components organized by cylinder number. Inspect pistons for cracks, scuffing, excessive carbon buildup, and damage to ring lands. Measure piston skirt diameter and check against specifications. Inspect connecting rods for bending, twisting, or damage.
5. 5

   Measure and inspect cylinder bores
   Using a dial bore gauge, measure each cylinder bore at three depths (top, middle, bottom) and two directions (parallel and perpendicular to crankshaft). Record all measurements. Check for out-of-round and taper conditions. Compare measurements to AMC specifications (typically 4.165 inches nominal for 390ci). Determine if cylinders need honing or require machine shop boring to next oversize (0.010, 0.020, 0.030, or 0.040 inch).
   ℹ️If cylinders exceed 0.003-inch taper or 0.002-inch out-of-round, boring is typically required
6. 6

   Hone cylinders or obtain machine work
   If cylinders are within specification for standard or existing oversize pistons, perform plateau honing to deglaze and create proper 45-degree crosshatch pattern for ring seating. Use 220-280 grit stones with proper honing oil. If cylinders require boring, remove block from vehicle and transport to qualified machine shop for boring, honing, and decking if needed. Clean cylinders thoroughly with hot soapy water and oil immediately after honing to prevent rust.
7. 7

   Install piston pins and fit new pistons to rods
   Install new pistons on connecting rods ensuring correct orientation - piston notch or marking faces forward (toward front of engine), oil squirt hole in rod faces toward camshaft side. Install piston pins according to type: press-fit pins require hydraulic press and proper support fixtures; full-floating pins require new retaining clips installed in grooves. Verify pin has no excessive clearance and rotates smoothly. Repeat for all eight assemblies.
   ⚠Incorrect piston orientation will cause interference with valves and catastrophic damage
8. 8

   Install and gap piston rings
   Clean ring grooves thoroughly using ring groove cleaner tool. Install new rings on each piston starting with oil control ring (expander and rails first), then second compression ring, then top compression ring. Ensure rings are installed with markings facing upward. Before installation, insert each ring into its cylinder bore and measure end gap using feeler gauge - typical gap is 0.010-0.020 inches for compression rings, 0.015-0.055 inches for oil rings. File ring ends if gap is too tight. Verify ring-to-groove clearance is within specification.
9. 9

   Check bearing clearances
   Clean crankshaft rod journals thoroughly with solvent. Install new connecting rod bearings in rods and caps (dry, no oil). Place Plastigage strip on each journal parallel to crankshaft. Install piston and rod assembly into its original cylinder, install cap with bearings in correct orientation, and torque rod bolts to specification (typically 45-50 ft-lbs for AMC 390, consult service manual). Do not rotate crankshaft. Remove cap and measure Plastigage width against scale - clearance should be 0.0005-0.0025 inches. Repeat for all eight cylinders.
   ⚠Do not rotate crankshaft with Plastigage installed or measurement will be invalid
10. 10

    Prepare cylinders and install pistons
    Remove all rod caps and bearings. Clean Plastigage residue completely. Coat cylinder walls with clean engine oil. Coat new bearings with assembly lube and install in rods and caps. Coat piston rings and skirts with engine oil. Stagger ring gaps 120 degrees apart around piston (not aligned with pin or thrust faces). Install piston ring compressor tightly around piston, compressing all rings fully into grooves.
11. 11

    Install piston assemblies into block
    Rotate crankshaft to position rod journal for cylinder 1 at bottom dead center. Guide piston and rod assembly into cylinder from top, ensuring notch on piston faces forward. Protect crankshaft journal with rod bolt protectors or rubber hose over bolt threads. Tap piston gently into bore using wooden or plastic hammer handle on piston crown while ensuring ring compressor remains tight. Guide connecting rod onto crankshaft journal. Coat rod bearing and journal with assembly lube. Install rod cap in correct orientation matching marks made during disassembly.
    ⚠Ensure rod bolts do not contact or scratch crankshaft journal during installation
12. 12

    Torque connecting rod bolts
    Install rod bolts or nuts and tighten in two stages: first to 25-30 ft-lbs, then to final torque specification (typically 45-50 ft-lbs for AMC 390 - consult factory service manual for exact specification as none was provided in database). If using torque-to-yield bolts, follow angle-torque procedure instead. After torquing, verify rod side clearance on crankshaft journal is 0.006-0.018 inches using feeler gauge. Verify rod cap is properly seated. Rotate crankshaft to check for binding - should turn smoothly. Repeat installation process for remaining seven cylinders.
13. 13

    Install oil pump and oil pan
    Install oil pump with new pickup tube O-ring. Prime oil pump by filling with assembly lube or petroleum jelly. Clean oil pan and block mating surfaces thoroughly. Install new oil pan gasket or apply thin bead of RTV silicone according to gasket manufacturer instructions. Install oil pan and torque bolts to specification (typically 10-12 ft-lbs) in circular pattern from center outward. Allow RTV to cure per manufacturer recommendation before adding oil.
14. 14

    Install timing components
    Align timing marks on crankshaft and camshaft sprockets per factory specifications. Install timing chain and sprockets ensuring marks are aligned correctly (typically both marks point toward each other with chain slightly slack). Install timing cover with new gasket. Install harmonic balancer using proper installation tool - torque center bolt to specification (typically 100-130 ft-lbs). Never hammer balancer onto crankshaft.
15. 15

    Reassemble cylinder heads and valvetrain
    Install new head gaskets on block ensuring correct orientation (most have TOP marking and front indicator). Carefully lower cylinder heads into position. Install head bolts with light coat of oil on threads and under heads. Torque head bolts in proper sequence and stages per AMC specifications (typically 90-110 ft-lbs final torque in three steps). Install pushrods in original locations, install rocker arms, and adjust valves to proper specification (typically 0.012-0.014 inches cold). Install intake manifold with new gaskets, exhaust manifolds with new gaskets and anti-seize on bolts.

Reassembly

1. Reinstall all remaining components in reverse order of removal
2. Install new distributor O-ring and reinstall distributor aligned to original rotor position mark
3. Reconnect all vacuum lines, electrical connections, and fuel lines per original routing
4. Install new spark plugs gapped to specification (typically 0.035 inches for 1970 AMC 390)
5. Install carburetor with new gaskets and reconnect all linkages and fuel line
6. Install air cleaner assembly
7. Fill crankcase with correct grade and quantity of oil (typically 5 quarts with filter)
8. Fill cooling system with proper 50/50 coolant mixture
9. Reinstall battery and connect cables

Verification

• Before starting, remove spark plugs and crank engine with starter while observing oil pressure gauge - should show pressure within 10-15 seconds indicating proper oil pump prime
• Reinstall spark plugs and start engine - may run rough initially until fuel system primes
• Check for oil, coolant, and fuel leaks immediately upon starting
• Allow engine to reach operating temperature while monitoring temperature gauge and checking for leaks
• Engine will smoke during initial run as assembly lube burns off - this is normal
• Listen for abnormal noises such as knocking, ticking, or rattling which could indicate assembly problems
• Perform initial break-in procedure: vary engine speed between 1500-2500 RPM for first 20 minutes, avoid full throttle or sustained high RPM
• After 30 minutes of running, shut down and recheck all fluid levels and retorque cylinder head bolts if required by gasket manufacturer
• Change oil and filter after 500 miles of break-in driving
• Expect to consume some oil during first 1000-2000 miles as rings seat - check level frequently

More procedures for this vehicle