Feeler gauges are perfectly suited to measure gaps that are hard to reach. Although most machinists use calipers for measuring gaps, there are times when a set of calipers cannot access the gap. Feeler gauges come in various size ranges and can be useful for measuring gaps and slots in machined parts. In a pinch, you can also use feeler gauges to measure the gap in a spark plug, just make sure they are not rusty and that they are amply oiled to prevent damage from sticking between the metal surfaces in a gap.
Select the correct range of feeler gauges for the gap you are trying to measure. Feeler gauges come attached within a certain range of thicknesses. You can use more than one blade at a time to measure a gap, but since they are often graduated in 0.005 inches, it is often necessary to do so.
Oil the blade you will use to measure the gap to assure that it will not get stuck in the gap you are measuring. If the blade is not oiled and gets stuck in the gap, damage could occur from trying to forcefully remove the blade. Any kind of oil will suffice, simply apply a small amount to a rag or cloth and apply to the blade.
Slide the blade in the gap. You are looking for a tight fit, but do not force the blade in the gap if it is too large, simply use the next one down in size. If that is now too loose, you will know that your measurement lies between those two numbers. Continue trying different sizes until you find the one that fits snugly but can be easily removed.
Use the feeler gauges to gap a spark plug. Although spark plug gap tools are the preferred method for this application, feeler gauges are an ample solution. Spark plugs need a certain sized gap in order to work properly. Refer to the manufacturer's specifications. Slide the blade of that size between the top of the spark plug and the ground electrode. Make the proper adjustments to make the gap the correct size by pressing the ground electrode against a flat surface.
Wipe the feeler gauges off before putting them away. Close them and make sure that they are place securelyd in their box or case. This will prevent damage to them when they are not in use. Keep them amply oiled to prevent any rust from ruining them for future use, as rust buildup can cause incorrect readings.