What is the plug gap on a 350 Chevy engine?
The spark plug gap for a Chevrolet 350 small-block engine is around 0.035 inches (0.9 mm) in many setups, but the exact specification can vary by year, ignition system, and plug type. Always verify against the factory manual or the plug manufacturer’s spec for your exact configuration.
In this article, we explain the typical gaps you might encounter, the factors that influence the setting, and how to check and adjust the gap safely to ensure reliable starting, smooth idle, and optimal combustion.
Typical gap ranges for a 350 Chevy
These ranges cover common configurations found on SBC 350 engines in street use.
- Stock carbureted or HEI ignition setups: typically 0.035 inches (0.9 mm).
- Modern fuel-injected or late-model 5.7L SBC (Vortec) engines: commonly 0.040 inches (1.0 mm).
- High-performance or forced-induction setups: sometimes 0.030–0.036 inches (0.76–0.91 mm) to prevent spark blowout at high RPM.
In most street configurations, starting at the lower end of the typical range and testing performance is prudent, with adjustments made based on engine response.
How to check and set the correct gap
Follow these steps to verify and adjust the gap on your spark plugs. Always ensure the engine is cool before handling plugs.
- Identify the correct gap specification for your engine and plug type from the owner's manual or spark plug packaging.
- Carefully remove a plug and inspect its condition; replace if fouled or worn.
- Measure the gap using a precise feeler gauge or a dedicated gap tool. Compare to the recommended specification.
- If the gap is too wide, gently bend the ground electrode to narrow it by small increments; if too narrow, carefully widen by the same method. Only bend the ground electrode; do not bend the center electrode.
- Reinstall the plug and torque to the manufacturer's spec; reinstall the spark plug wire or coil boot.
- Repeat the process for all plugs, then start the engine and listen for smooth idle. Re-check gaps after a few heat cycles if you’re pursuing precise settings.
Routine re-checks after heat cycles help ensure the gap remains within specification under operating conditions.
Plug type considerations
Copper, platinum, and iridium plugs
Different plug materials can affect how the electrode wears and how easy it is to maintain a precise gap. Iridium and platinum plugs are typically more durable and may tolerate slight variations, but always follow the plug manufacturer’s recommended gap for your engine. In some cases, performance guidance for high-output tunes may require a slightly different gap than stock.
Summary
The Chevrolet 350 small-block engine typically uses a spark plug gap around 0.035 inches (0.9 mm), but variations by year, ignition system, and plug type mean a range of about 0.030–0.040 inches is common. Always consult the specific service manual, spark plug packaging, or OEM recommendations for your exact build, and verify gaps after installation and initial heat cycles to ensure reliable starts and combustion performance.
