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spark plug questions

Guest longman

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Guest longman


I just got given a NOS box of 12 Firestone SH-40 spark plugs. They are all individually boxed and still wrapped in the cellophane, so no rust on them, they are for all intents and purposes, new.

My questions are:

1) Would these be suitable for my 96A whippet 4 cyl? They are the same thread size, 7/8".

2) The box has a list of plug numbers stating some are hotter, some cooler. The SH-40 is at the cooler end of the chart. What does this mean and what effect does it have on the running of a car?


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The tip of the spark plug has to stay hot enough to burn off the carbon but not so hot as to burn up the spark plug itself.

Too far one way and the plug will foul with deposits and stop working. Too far the other way it will burn out and stop working. Or, if the tip of the plug gets red hot it can cause damaging spark knock.

This is not such an issue today. In the early days it was more critical to have just the right plugs.

Hard working full throttle engines, as in trucks and race cars needed cooler plugs. Cars that putt putt around on short errands need hotter plugs.

If they fit your engine give them a try. Worst that can happen is they will foul up with carbon in a few months and start to misfire. But they will probably be fine.

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