Every spark plug threads into the head at a random rotational orientation — the ground strap (the L-shaped electrode arching over the center electrode) can end up pointing anywhere on the clock face. Indexing is the practice of adjusting each plug's rotation so the open side of the electrode gap faces a preferred direction, typically toward the intake valve or the combustion chamber's flame-propagation path.

Why does this matter? The ground strap physically shadows the spark kernel. When the arc jumps the gap, it forms a tiny flame kernel (~1mm diameter initially) that must grow into a full flame front. If the ground strap sits between the kernel and the fresh air-fuel mixture, it blocks flame expansion in that direction and acts as a small heat sink, quenching the early burn. Point the open side toward the intake valve (the freshest, most turbulent charge) and the kernel grows unobstructed into the richest mixture.

Real-world example: NASCAR Cup teams have indexed plugs since the 1970s. Dyno data on a 358 c.i. Cup engine typically shows 2-4 hp gain from proper indexing at 9,000+ RPM — small in absolute terms, but at that level teams fight for every tenth. Pro Stock drag teams claim similar gains. On a stock street engine at 3,000 RPM cruise, the effect is essentially unmeasurable because burn duration is long relative to the small kernel-shadowing effect.

How to index:

• Mark the ground strap position on the plug's ceramic with a paint pen before install (12 o'clock reference)
• Install the plug with a torque wrench and note where the strap ends up (say, 4 o'clock)
• Remove and add indexing washers — thin copper crush washers (0.005" to 0.030") — to advance the final position
• Each 0.010" of washer thickness rotates the plug roughly 30° on typical 14mm threads

Rule of thumb: Point the open side of the gap toward the intake valve. On a modern pent-roof chamber with two intake valves, aim between them. On a hemi, point toward the intake side of the chamber.

One catch: tapered-seat plugs (most modern engines) can't be indexed with washers — they seal on a machined taper, not a crushable ring. You'd need to buy a set of plugs and hand-select ones that happen to land in the right orientation at final torque, which is why indexing has largely faded from street tuning as gasketed plugs disappeared.