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Difference between revisions of "How to read and select Spark Plugs"
How to read and select Spark Plugs (edit)
Revision as of 02:54, 13 December 2010
, 02:54, 13 December 2010no edit summary
(Created page with "Stay with motorcycling long enough to swat a few gnats with your nose and you will at least begin to realize how much there is to know about spark plugs. Bikers like to t...") |
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Because differences in thread diameters are so large, few people get into trouble through trying to stuff a l4mm plug into a 12mm hole -or vice versa. The same isn't true of plugs' threaded lengths, or "reach." Setting aside for the moment the small variations created by the use of an inch-based standard in a mostly-metric world, there are just four nominal reach dimensions: 3/8-inch, 1/2-inch, 7/16-inch and 3/4-inch. These dimensions are followed by engine manufacturers in the depths they give plug holes, and the idea is that the lower end of the plug's threaded shank should come up flush in the combustion chamber. | Because differences in thread diameters are so large, few people get into trouble through trying to stuff a l4mm plug into a 12mm hole -or vice versa. The same isn't true of plugs' threaded lengths, or "reach." Setting aside for the moment the small variations created by the use of an inch-based standard in a mostly-metric world, there are just four nominal reach dimensions: 3/8-inch, 1/2-inch, 7/16-inch and 3/4-inch. These dimensions are followed by engine manufacturers in the depths they give plug holes, and the idea is that the lower end of the plug's threaded shank should come up flush in the combustion chamber. | ||
We know from personal observation that people do make plug-reach mistakes; using 3/4-inch plugs in 1/2-inch holes is the most common error, and one fraught with unpleasant consequences. One of the disasters you can have from using a long-reach plug in a short-reach hole is purely mechanical in nature. In time the plug threads exposed inside the combustion chamber may become filled with hard-baked deposits. If that happens you'll find it almost impossible to remove the plug without also removing the plug hole threads. Reversing this kind of mistake, using a plug reach too short for the hole, lets deposits fill the plug hole's exposed threads and may cause difficulties when you try to install a plug having the correct reach. | We know from personal observation that people do make plug-reach mistakes; using 3/4-inch plugs in 1/2-inch holes is the most common error, and one fraught with unpleasant consequences. One of the disasters you can have from using a long-reach plug in a short-reach hole is purely mechanical in nature. In time the plug threads exposed inside the [[combustion chamber]] may become filled with hard-baked deposits. If that happens you'll find it almost impossible to remove the plug without also removing the plug hole threads. Reversing this kind of mistake, using a plug reach too short for the hole, lets deposits fill the plug hole's exposed threads and may cause difficulties when you try to install a plug having the correct reach. | ||
The worst and most immediate problem created by an overly-long plug in an engine is that the exposed threads absorb a terrific amount of heat from the combustion process. This raises the plug-nose temperatures, and may take them up high enough to make the side electrode function as a glow plug. And when that happens you have the white-hot electrode firing the mixture far too early, like an over-advanced spark timing but worse because the early ignition causes yet higher combustion chamber temperatures, which causes even earlier ignition. This condition is known as "runaway [[pre-ignition]]," and if it is allowed to proceed it will wreck your engine. | The worst and most immediate problem created by an overly-long plug in an engine is that the exposed threads absorb a terrific amount of heat from the combustion process. This raises the plug-nose temperatures, and may take them up high enough to make the side electrode function as a glow plug. And when that happens you have the white-hot electrode firing the mixture far too early, like an over-advanced spark timing but worse because the early ignition causes yet higher combustion chamber temperatures, which causes even earlier ignition. This condition is known as "runaway [[pre-ignition]]," and if it is allowed to proceed it will wreck your engine. | ||