Chevy 350 Spark Plug Gap by Engine and Ignition
Chevy 350 spark plug gap chart for Chevrolet Performance 350/290 and Ram Jet 350, plus plug selection and gapping guidance.
A Chevy 350 does not have one universal plug gap. Chevrolet specifies 0.045 in (1.14 mm) for the 350/290 with the recommended AC R45TS plug and HEI, while the Ram Jet 350 specification is 0.040 in (1.02 mm) with an AC MR43LTS plug. Match the exact engine and ignition before gapping.
Spark-plug gap affects the voltage required to jump the electrodes and ignite the mixture. Too wide a gap can cause misfire when cylinder pressure rises; too narrow a gap reduces the size of the initial flame kernel.
The familiar 'Chevy 350' name covers decades of cylinder heads and ignition systems. Start with the engine's actual plug part number and ignition package, then use the corresponding manufacturer specification.
Interactive spark-plug gap guide
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Choose the documented engine example to compare the electrode gap. Do not force fine-wire electrodes.
Chevy 350 Spark Plug Gap
| APPLICATION / ITEM | SPECIFICATION | NOTES |
|---|---|---|
| 350/290 + HEI | 0.045 in / 1.14 mm | AC R45TS in Chevrolet guide |
| Ram Jet 350 | 0.040 in / 1.02 mm | AC MR43LTS in Chevrolet guide |
| 350 HO | Verify plug/application | Guide recommends AC R44LTS; confirm gap for ignition |
| Modified engine | Builder/plug-maker spec | Compression, boost, fuel, and ignition matter |
Older original engines, points ignition, emissions calibrations, aftermarket aluminum heads, boosted combinations, and modern electronic ignition systems may require different plug designs and gaps.
How to use this specification correctly
- 01
Identify the engine version, cylinder-head plug seat and reach, ignition system, and recommended plug part number.
- 02
Check each new plug with a wire-style gap gauge; do not rely only on the box label.
- 03
Adjust only the ground electrode and support it with the proper tool. Never pry against the center electrode or insulator.
- 04
Inspect the seat and threads, then install the plug using the plug or cylinder-head manufacturer's torque instructions.
- 05
After running, evaluate misfire data or plug condition only in the context of fuel mixture, timing, heat range, and engine load.
Why Chevy 350 gaps differ
- HEI, points, capacitive-discharge, and ECU-controlled ignition output
- Iron versus aluminum cylinder heads and plug design
- Compression ratio, boost, fuel, and cylinder pressure
- Plug reach, seat style, heat range, and electrode material
Do not re-gap every modern plug aggressively
Fine-wire platinum or iridium electrodes are easier to damage than traditional nickel plugs. Verify the plug manufacturer's adjustment policy and move only the ground strap in small increments.
Gap is only one part of a tune-up
A correct gap cannot compensate for excessive wire resistance, a damaged cap or rotor, weak coil voltage, incorrect timing, poor grounds, rich or lean mixture, or low cylinder compression.
Chevrolet 350 Small-Block (5.7L V8) specifications can change by model year, engine, platform, wheel, and installed component. Confirm the VIN or engine identity, part numbers, fastener condition, and the current manufacturer instructions before service.
Chevy 350 Spark Plug Gap by Engine and Ignition FAQs
Is every Chevy 350 spark plug gap 0.035 inch?+
No. That commonly repeated value does not cover every ignition or engine. Chevrolet documents 0.045 inch for the 350/290 HEI example and 0.040 inch for Ram Jet 350.
What gap does a Chevrolet 350/290 use?+
Chevrolet Performance specifies 0.045 inch with the recommended AC R45TS plug in its 350/290 guide.
What gap does a Ram Jet 350 use?+
The Chevrolet Performance Ram Jet 350 guide specifies 0.040 inch with an AC MR43LTS plug.
Does HEI change the required plug gap?+
Ignition energy is one factor in gap selection, but use the exact engine and plug specification instead of choosing a gap from ignition type alone.
Can I change the gap on iridium plugs?+
Follow the plug manufacturer's directions. Fine-wire center electrodes can be damaged if the tool contacts or loads them.
Technical sources
TorqueSheet favors engine- and component-specific manufacturer instructions over unsourced universal values.
1Chevrolet Performance 350/290 Long Block guide (P/N 19172591)Primary Chevrolet Performance guide for the pre-1986-style 350/290 long block.↗2Chevrolet Performance Ram Jet 350 guide (P/N 19417619)Primary Chevrolet Performance installation and calibration guide for Ram Jet 350 engines.↗3Chevrolet Performance 350 HO Base Engine guide (P/N 12366576)Primary Chevrolet Performance installation and specification manual for the 350 HO crate engine.↗