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Spark Plugs Engine Gapping

Sprak Plugs - How's Your Gap? - Engine

Don't Leave Horsepower On The Table With Poorly Gapped Spark Plugs
By Jeff Huneycutt
Photography by Jeff Huneycutt
Spark Plugs Engine Gapping
All those thousands of dollars... 
   
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Spark Plugs Engine Gapping
All those thousands of dollars of high-end engine parts depend on relatively cheap spark plugs to work. Give your plugs the best chance possible by gapping every one before installation.
In racing, spark plugs are practically a disposable commodity. Use them for a while and throw them away. If the engine starts acting funny, replace the plugs just in case. It's cheap insurance and really not a bad idea at all.

Spark Plugs Engine Gapping Tool
The most dependable way to... 
   
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Spark Plugs Engine Gapping Tool
The most dependable way to gap a set of plugs is with a dedicated gapping tool. It usually is a disk arrayed with several steps or pieces of wire in marked thicknesses. Without having to force it, find the thickest piece of wire that will fit between the spark plug electrode and the ground strap. This is your current gap.
The critical distance between the spark plug's electrode and the ground strap (that piece of steel that curves out over the end of the plug) is called the "plug gap." The act of setting this distance correctly is commonly referred to as "gapping" your plugs. If the gap is too small, there isn't enough room for enough of the air/fuel mixture to get between the electrode and ground strap, so when the plug does fire there is a chance nothing will ignite. On the other hand, if the gap is too large the spark that does appear will be weak and may not be strong enough to ignite the charge.

Spark Plugs Engine Gap Tool
If your plug gap needs to... 
   
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Spark Plugs Engine Gap Tool
If your plug gap needs to be adjusted, don't go the "old school" route of opening up the gap by prying on the end of the ground strap with a screw driver. You are just as likely to chip something as achieving the proper gap. Instead, use the notch in the gap tool to gently open up the gap at the ground strap.
For most racing applications, you usually want the plug gap to be between 0.020 and 0.040-inch. Most engine builders seem to settle around 0.035-inch. Factors such as the type of ignition you run, cylinder heads, fuel, and even timing can affect how much gap will work best for you. Generally, you want as much gap as you can get by with to ensure ignition each time the plug fires. For more specific recommendations, talk to your engine builder and even call the tech help line for your preferred plug manufacturer.

Spark Plugs Engine Ground Strap
While you're at it, take a... 
   
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Spark Plugs Engine Ground Strap
While you're at it, take a moment to inspect the quality of the ground strap. These are welded onto the plug, and the weld isn't always strong. If a weld seems flimsy, it's better to discard a questionable new plug than have the strap fall off inside the engine.
Spark Plugs Engine Opposite Direction
Likewise, by sliding the notch... 
   
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Spark Plugs Engine Opposite Direction
Likewise, by sliding the notch farther back and applying pressure in the opposite direction, you can close down the gap. It's much more accurate than tapping the end of the plug on a table.
Spark Plugs Engine New Plugs
Some fancy new plugs like... 
   
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Spark Plugs Engine New Plugs
Some fancy new plugs like this one from E3 use engineered ground straps to produce multiple spark pathways. You do not need to adjust the gap on these plugs.

Audi A4 Research
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