2020 Chevrolet Silverado 3500HD — Spark Plug Type, Gap & Torque

Quick reference

Note: GM supersessions occur; always verify the exact OEM part number by VIN in the GM EPC or on the underhood emission/service label. Never mix heat ranges.

Why the right plug matters

The 2020 Silverado 3500HD’s 6.6L L8T gasoline V8 is engineered for heavy loads and long intervals. Installing the correct spark plug preserves ignition energy, detonation control, and catalyst life.

• Heat range: Too-hot plugs can cause pre-ignition under tow; too-cold can foul at idle. Stick with the factory heat range for the L8T.
• Electrode metal:
  • Iridium: OE on the L8T. Excellent durability and stable gap for ~100k miles. Best choice for stock engines.
  • Platinum: Acceptable in some applications but typically shorter life than iridium. Not recommended if the OE spec is iridium.
  • Copper/nickel: Great conductivity but short service life; not advised for modern HD usage or long intervals.

How to gap a spark plug

Most OE iridium plugs arrive pre-gapped, but shipping can alter gap. Verify before installation:

1. Use a wire-style gap gauge, not a coin-wedge that can stress fine-wire electrodes.
2. Target gap for L8T: 0.040 in (1.02 mm).
3. If adjustment is needed, lightly bend only the ground strap. Do not pry on the iridium center electrode.
4. Recheck gap at several points around the ring for consistency.

Replacement procedure tips (6.6L Gas L8T)

The L8T uses individual coils mounted on brackets with short plug wires to each cylinder (coil-near-plug design). No distributor.

• Tools: 5/8 in spark plug socket with rubber insert or magnet, 3/8 in torque wrench, 6–12 in extensions, swivel, dielectric grease, compressed air.
• Access: Work one cylinder at a time. Release the plug wire boot by twisting to break it free, then pull straight off. Some wire looms/heat shields may need to be unclipped for clearance.
• Cleanliness: Blow out the plug well before removal to prevent debris from entering the cylinder.
• Removal: Turn out the old plug on a cool engine. If resistance is excessive, stop and soak threads with penetrating oil; allow to cool and try again.
• Install dry: Modern ACDelco plugs come with a plated shell. Do not apply anti-seize; it changes torque values and can promote over-tightening. GM service information specifies dry threads.
• Torque: Hand-thread to avoid cross-threading, then torque to 15 lb-ft (20 Nm). Do not overtighten—aluminum heads are easy to damage.
• Boots: Inspect plug wires/boots for heat damage or carbon tracking. Apply a small amount of dielectric grease inside the boot to aid future removal and seal out moisture. Push until you feel/hear a positive click.
• Coils and wiring: Ensure connectors are fully seated and wire routing is returned to factory clips to prevent chafing on heat shields.

Interval: Under normal service, iridium plugs are inspected around 97,500–100,000 miles. Severe-duty (heavy towing, lots of idling, dusty conditions) may warrant earlier inspection.

Known issues

• Two-piece plug breakage (Ford 3-valve 5.4/5.3 Triton-era concern) does not apply to the L8T.
• Broken plug shells from overtightening: Avoid anti-seize and use a torque wrench to stay within 15 lb-ft (20 Nm).
• Stuck plugs in high-mileage, hot-cycle engines: Work only on a cool engine and use penetrating oil if a plug feels seized. Back out in small increments rather than forcing it.

Diesel note: The 6.6L L5P Duramax does not use spark plugs. If you’re servicing starting issues on the diesel, you’ll be looking at glow plugs and the glow plug control module instead.

Need plugs replaced? Find a shop on Repairius.