I will address your question in parts:
1) Gap: on older engines, especially those with mags, gap is critical to prevent coil failure. Most older engines want about .025 to .030 gap, but there are some out there than need .020, no more. (Mayteg 82 and 92 for instance) Use a wider gap and you risk a chance of burning out the secondary windings in the coil. In the 72 twins, setting the gap wider than .025 invites coil failure in the 72-D and DA models - again weak coils. Most small engines, used on mowers and small equipment use a .030 gap, but it is best to use manufacturers recomendations. If you don't have the specs, then, have a look on the internet, and try to find it. If you can't find them, try .020" and see if the engine will run on it. I have seen engines run on as little as .010" gap, but fouling will be problematic. You might get a 1/4" spark out of the engine, but you may not get a spark under compression, if the voltage and current are weak.
If you have a weak coil, or one that is failing, closing the gap may keep the engine running a little longer. Most older coils are going to have degradation of the high tension insulation. This will cause spark to jump between the windings, causing corrosion and carbonization . This in turn spreads between the winding layers, shorting the coil and causing a drop in power out put. Having a too wide a gap increases the voltage and current needed to jump the gap under compression, inviting the spark to jump at the weakest point. This is what causes coil failure, especially if the insulation is weak to begin with.
On modern automobiles, gap also helps ignite leaner charges now used. With todays lean burn engines, you might not even get ignition at a .035 gap. GM, with their HEI electronic ignition, used a .060 gap! Put that large a gap on a mower, and you probably will burn out a coil in short order, if you get spark at all!
2) Heat range: The heat range on a spark plug is determined by the engine manufacturer. It is determined by the compression ratio, combustion pressure , cooling method, combustion chamber design, and other factors. Running too hot a plug will cause the electrodes to burn off, make the insulater crack and fail, create excessive heat in the combustion chamber, and probably may create detonation which may be inaudibal (you won't hear it), but will be destructive to internal engine componants. In aluminum head engines, I have seen several air cooled engines that have actually melted the plug mount threads out, ejecting the hot plug at a good velocity!
Running a plug that is too cool, and you will get carbon build up and an incomplete burn of fuel in the combustion chamber. This will foul the plug. Fouling will decrease heat and will short out the plug under compression, as well as create a loss of power in the long run.
Running a plug that is not designned for the engine may also be problematic. Engines have different reaches - the length needed to get the electrodes and the working end of the plug to the point needed for best ignition charactoristics. Putting a short thread plug in a long reach head invites head damage - carbon build up in the exposed threads and or burning of the head material itself. Trying to install the proper plug at a later date will cause galling in the head threads, and possibly a busted or fouled plug. Putting too long a plug in a short reach head may cause valve/piston damage thru direct contact, or may cause plug threads to be exposed within the combustion chamber. The engine might run with too long a plug, but when you try to remove it, carbon, ash and combustion debris left on the threads will tear up the head threads. I have seen several examples where a too long plug actually broke off in the head - jammed bu the combustion debris left on the exposed threads. Installing plugs with the wrong electrode reach will probably cause ignition problems - the engine power produced will be compromised.
Older small engines, for the most part, are gap critical. You do not want too wide a gap due to the age / condition of the coil used. They are more tolerant of heat range, but it is still important. Larger, multi cylinder engines, like those used in autos, especially those built into the 1960s and later, are heat range critical as well. Too hot a plug will cause damage as noted above. Racing engines have their own set of damands, where gap, heat range, engine heat build up and application, and fuel used are all important.
Heat range is determined in the plugs, by several factors: The thickness and extension of the insulator on the center electrode, the length of the electrodes into the combustion chamber, and the length of the plug itself. Fuel quality to be used in the engine is also taken into consideration. Today's fuel has alcohol in it for the most part. Manufacturers in their infinite wisdom, have decided to cut costs by eliminating the glaze on the insulator. The problem is if these new plugs are used in older engines, with a rich fuel environment, they tend to foul easily. They perform poorly in the old engines. Also, the new fuel tends to short under compression. That is why you cannot prime an auto that runs out of gas. If you put liquid gas into the air inlet, you probably will short the plugs. Even new plugs will short out under compression, if they are wet with fuel. In the old engines, this type of fouling is becoming problematic, as wetting the plugs with the choke on, is shorting them. They will fire out of the engine, but not under compression. In today's throwaway society, nobody bothers to check why a plug isn't working, they just buy another one. In the 'old days', mechaincs used to put the plugs into a tester and apply compression pressure to see if a plug will work. No more, just toss it and buy a new one.
Hope this helps explain things a bit. This is s simplified answer, if you need more specific answers, please ask. There are no such thing as stupid questiions - everyone has something to learn, if they don't know the answer.
Andrew
Originally Posted by exodus0784
I know our plugs are fired by electronic switches and coils, however it is a fascinating read:
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