I've recently been dealing with spark plugs and they're even worse. The motor mower that I curate last had a new spark plug about 30 years ago. By pretending that it was for an old British outboard motor I was able to buy a spark plug that fits. By correlating the box that it came in with the NGK charts I have been able to identify a plug which is still manufactured, will fit, and should run at the right temperature.
This damned plug has a 18mm by 1.5mm thread on it AND a 13/16 Imperial hex!
A spark plug that is running too cool doesn't burn off the carbon from the insulator. A spark plug that is running too hot cracks the porcelain. Different plugs absorb more or less heat from the combustion gasses and throw away more or less heat to the cylinder head. The nomenclature and numbering system for these thermal properties is ridiculous:
A "hot" plug doesn't have much cooling, is in a "low" heat range and has a low number in the NGK system.
A "cold" plug has lots of cooling, is in a "high" heat range and has a high number.
To clarify:
Lawnmower - hot - 1,2,3
Racecar - cold - 9,10,11
Richard "simplicity itself" B
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