The ________ testing determines where the electrode gap should face in the combustion chamber.

• A. Hot plug
• B. Cold plug
• C. Indexing
• D. Dyno

Answer: (D)

Indexing is the practice of orienting the spark plug so the electrode gap faces a specific direction inside the combustion chamber. This orientation helps control how the flame kernel starts and propagates, so directing the gap toward the center of the chamber or toward the incoming charge can give a more even and complete burn, improving consistency and performance. To find the best orientation, a technician may test plugs in different rotational positions and evaluate engine response or cylinder pressure characteristics. Hot plug and cold plug refer to heat range—how quickly the plug dissipates heat from the tip—not to which way the gap faces. Dyno testing measures overall power and torque, not spark-gap direction. So, the concept being tested is indexing.