Spark Plugs - V6

Replace all 8 spark plugs on the 1966 Chevrolet Corvette 327ci V8 engine to restore ignition performance and combustion efficiency.

Warnings

Tools required

Parts

• Spark plugs × 8 — AC Delco R44 or equivalent (check heat range for your application)

Preparation

1. Ensure engine is completely cold to touch.
2. Park vehicle on level surface and engage parking brake.
3. Open hood and secure with hood prop.
4. If equipped with air cleaner assembly or aftermarket air intake, remove to improve access to valve covers.
5. Optionally label each spark plug wire with cylinder number using masking tape to ensure correct reinstallation.

Procedure

1. 1

   Inspect and photograph spark plug wire routing
   Before removing any wires, carefully observe the routing of all 8 spark plug wires from distributor cap to each cylinder. Take a photo if possible. Note which distributor cap terminal connects to which cylinder. The 1966 327 uses a specific firing order (1-8-4-3-6-5-7-2) and incorrect wire placement will prevent proper engine operation.
2. 2

   Remove spark plug wire from cylinder #1
   Starting with cylinder #1 (driver side, front-most cylinder), grasp the spark plug wire boot firmly at the base where it connects to the spark plug. Twist the boot gently back and forth while pulling straight up to break the seal. Never pull on the wire itself. Use a wire puller tool if the boot is stuck. Remove the wire completely and set aside in a clean area.
3. 3

   Clean around spark plug well
   Use compressed air to blow away any dirt, debris, or oil around the exposed spark plug. This prevents contamination from falling into the cylinder when the plug is removed. If compressed air is unavailable, use a clean rag to wipe the area thoroughly.
4. 4

   Remove spark plug from cylinder #1
   Insert the appropriate spark plug socket (13/16" for original 1966 AC Delco gasket-seat plugs, verify against your specific plug) with a 6" extension onto the spark plug. Turn counterclockwise to loosen. Once broken free, unscrew by hand using the ratchet until the plug is completely removed. Inspect the old plug for condition (electrode wear, carbon buildup, oil fouling) as this indicates combustion health.
5. 5

   Inspect cylinder head threads
   Visually inspect the spark plug threads in the cylinder head for damage, carbon buildup, or cross-threading. If threads appear dirty, use a 14mm thread chaser (verify thread size matches your plug) or wire brush to carefully clean them. Do not force any debris into the cylinder.
6. 6

   Gap and prepare new spark plug
   Using a spark plug gap gauge, verify the new spark plug is gapped to specification (typically 0.035" for a stock 1966 327 with points ignition (verify against the factory shop manual for your specific engine variant)). Adjust if necessary by carefully bending the ground electrode. Optionally apply a very small amount of anti-seize compound to the threads (avoid getting any on the electrode or insulator); if anti-seize is used, reduce final torque by approximately 10-20%. Do not over-apply.
7. 7

   Install new spark plug in cylinder #1
   Carefully thread the new spark plug into the cylinder head by hand using the socket and extension. Turn clockwise until finger-tight to ensure proper thread engagement and prevent cross-threading. Once seated, use the torque wrench to tighten to the factory specification (approximately 25-30 ft-lbs for gasket-seat plugs in cast iron heads — verify against the factory shop manual). If no torque specification is available, tighten gasket-seat plugs (as used on this engine) approximately 1/4 to 1/2 turn past finger-tight for a new plug with a fresh gasket.
8. 8

   Reinstall spark plug wire to cylinder #1
   Apply a small amount of dielectric grease inside the spark plug boot to prevent moisture intrusion and ensure good electrical contact. Push the boot firmly onto the spark plug until you feel it seat completely with a slight click or firm stop. The boot should be fully seated against the valve cover.
9. 9

   Repeat process for remaining cylinders
   Repeat steps 2 through 8 for each remaining cylinder in sequence: #3, #5, #7 (driver side), then #2, #4, #6, #8 (passenger side). Work on one spark plug at a time to prevent mixing up the wires. Pay special attention to cylinder #7 (driver side rear), which has very limited access and may require a wobble extension or universal joint socket adapter to reach.
10. 10

    Verify all wire connections
    After all 8 spark plugs are installed, double-check that each spark plug wire is firmly seated on its corresponding plug and that the routing matches your original configuration or photo. Verify each wire is connected to the correct distributor cap terminal according to the 1-8-4-3-6-5-7-2 firing order.
11. 11

    Reinstall removed components
    If the air cleaner assembly or any other components were removed for access, reinstall them now. Ensure all fasteners are properly tightened and all hoses or connections are restored.

Reassembly

1. Double-check all spark plug wire connections are secure and routed correctly.
2. Verify no tools or parts are left in the engine bay.
3. Close hood after final inspection.

Verification

• Start the engine and listen for smooth idle with no misfires or rough running.
• Engine should start easily and run smoothly without hesitation.
• Visually inspect for any spark plug wire arcing or sparking in darkness if concerned about connection quality.
• Test drive vehicle to confirm normal acceleration and performance under load.

More procedures for this vehicle