Spark Plug Gap cross reference chart – metric to standard (mm to inch)
| mm | 0.4 | 0.5 | 0.6 | 0.7 | 0.8 | 0.9 | 1.0 | 1.4 | 1.5 | 1.6 | 1.7 | 1.8 | 1.9 | 2.0 |
| inch | .016″ | .020″ | .024″ | .028″ | .032″ | .035″ | .039″ | .055″ | .059″ | .063″ | .067″ | .071″ | .075″ | .079″ |
1) Higher cylinder compression requires higher voltage for the spark to occur.
2)Engine ignition timing – the closer to the engine TDC (Top Dead Center) the spark occurs, the higher is the A/F mixture pressure it has to fire, and the harder for spark it is.
3)when engine is under load, the pressures in the combustion chamber is higher, making it harder for spark to occur.
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.
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