NGK R2556G-8 (91975) Iridium-Platinum Spark Plug
|Gap||0.7 mm (.028")||Heat Range||8|
|Thread||12 mm x 1.25 mm||Reach||26.5 mm (1.04")|
|Longevity||Torque specificatons||Cast Iron: 10.8-18 lb-ft; Aluminum: 10.8-14.5 lb-ft|
|Projection||Recessed||Resistor||5 000 Ω|
|Seat Type||Gasket||Hex Size||5/8" (16mm)5/8" (16mm)|
|Center Electrode||Ground Electrode|
|Name||Reference type||Thread||Reach||Seat Type||Materials||Gap||Heat range||Resistor|
GapGap is a distance between center and ground electrodes. Bigger gap creates bigger spark, which ignite more fuel. On the other hand, bigger spark is harder to occur and has more chances to misfire. That’s why it’s important to stick to OE gap sizes.
ThreadThread diameter and pitch are fundamental dimensions. You physically aren’t able to fit spark plug with another thread, so check it in a first place.
Heat rangeThis parameter indicates how well spark plug transfer heat from the engine. Some engines produce more heat (i.e. race engines), some less, but all spark plugs should operate in same temperatures: from 500°C and below 950°C. Since all manufacturers have their own heat ranges, we’ve unified all of them to NGK’s chart: heat range “14 (4)” means 14 is the brand’s heat range and 4 is NGK’s equivalent. Note: E3 and Autolite cannot be referenced by heat range.
ReachReach is a distance between spark plug seat and the end of a thread. If it will be too short, spark will occur outside the combustion chamber; too long – it’ll collide with the piston. So it’s crucial to follow the OE dimentions.
ResistorOccurring of a spark plug is followed with another event – radio frequency interference (RFI). RFI creates a static electricity, that can accumulate and discharge on car’s electronics and destroy it. Resistor surpress RFI and saves your electronics. If your car from factory has spark plugs with resistor, use only resisted spark plugs. If you have an older car with little of electronics, you can use both resistor and non-resistor spark plugs.
MaterialsAll materials works the same. They differ by the durability in following order (first the strongest):
DocumentationFor this relationship we use official documentation from manufacturers, like this one:
Using it we get a match-list and form tier 1 relations (A = B, B = C). Then we form tier 2 relations: if A is a replacement for B, and B equals to C, then A and C are also can be referenced (A = B = C). That way we reference all plugs from several manufacturers’ charts to make a most comprehensive cross-reference table for spark plugs.