* Spark plug type: The heat range and design of the spark plug itself significantly affect the optimal gap. You need to know the exact spark plug type (e.g., AC Delco R45TS, Champion N-series) to determine the best gap.
* Ignition system characteristics: While you've specified a 1975 HEI distributor, its specific output (coil strength, dwell) can influence the ideal gap. A hotter ignition system might allow a slightly wider gap.
* Engine compression: Higher compression engines often benefit from a slightly smaller gap.
General guideline: A good starting point for a 350 small-block Chevy with a HEI distributor and standard spark plugs would be 0.035" to 0.045". However, this is just a range.
To find the best gap:
1. Identify your spark plugs: Look at the spark plugs themselves or check your owner's manual or a parts catalog to find the correct type.
2. Consult the spark plug manufacturer's specifications: The box the plugs came in, or the manufacturer's website, will usually give a recommended gap range for your specific plug in a typical application.
3. Consider the engine's condition: If you've modified the engine (increased compression, different camshaft), the optimal gap may need to be adjusted slightly.
4. Experiment (with caution): Start with the manufacturer's recommended gap. If you experience misfires, you might try widening the gap slightly (in small increments, such as 0.002"). If the engine runs rough or misses, try narrowing the gap. Excessive widening can lead to arcing and damage to the ignition components.
It's always safest to start with the manufacturer's recommendation and adjust only if necessary based on the engine's performance. A mechanic or experienced engine tuner can help you determine the ideal gap through testing and observation. Don't just rely on a guess; correct spark plug gapping is crucial for optimal engine performance and longevity.
