Spark plugs, coils, coil packs, distributors, caps, rotors, wires, ignition modules, control boxes and switches.
Heat range is the specification that matters on plugs. A plug one step too hot for a modified engine can cause pre-ignition, and one too cold will foul. The listings state the heat range and the gap where the manufacturer publishes it.
Coil-on-plug units are frequently position-specific. A coil that fits cylinder one may not fit cylinder four on a coil-near-plug engine, and the position field on the product page says which one the listing covers.
