The thermal grade of the spark plug is of prime importance for tuning kits: if the wrong grade is used, the spark plug will foul or the cylinder may overheat.
A cold spark plug allows a faster transfer of heat to the cylinder head; the temperature necessary to burn off carbon and oil deposits may not be reached, which may lead to cold fouling.
A hotter spark plug, on the other hand, transfers less heat to the cylinder head, which may lead to overheating and piston seizures.
Colder spark plugs (e.g. Bosch W2…) are recommended for racing cylinders, while standard and sport cylinders require hotter spark plugs (e.g. Bosch W3… to W4…).
Many rev counters need a resistor type spark plug in order to work properly; otherwise extreme jumps or absurd rev readings may be the result.
We always give the Bosch grades in brackets as reference.
Short reach = 12.7mm thread length
Long reach = 19.0mm thread length
NGK short-threaded BR9HS (WR2AC) - resistor type