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The thing in the image above might look like a Swiss Army knife, but it isn’t. It’s a “feeler gauge.”  In this Tool Tutorial, you’ll learn:  
  • What a feeler gauge is
  • Its common uses 
  • How to use one to measure a spark plug gap

What is a “Feeler Gauge?”

A feeler gauge is a tool used to measure the gap between two spaces closed together where a regular tape measure won’t fit.  The tool contains a series of small blades inside a metal container that can be bought individually or as a kit. However, these blades aren’t for cutting – they’re for measuring.  Each blade has a stamp of its thickness on the side. For example, this blade has a thickness of 0.40 mm (or approximately 0,02 inches):
image of blade thickness

Uses of a Feeler Gauge

In the context of motorcycle mechanics, some common uses include:
  • Measuring spark plug gaps
  • Measuring valve play in an overhead camshaft engine
Using this tool is fairly simple – on the tool difficulty scale, it ranks at a difficulty rating of 1 (very easy to use). If you can use a rule or a tape measure, you can learn to use this tool within less than a minute.

How to use a feeler gauge – step-by-step

  1. Bring out the object whose gap you want to measure (in my case, I’ll use a spark plug)
2. Fold out one of the blades and stick it into the gap you want to measure:
The blade is supposed to go into the slot with little to no wiggle room. If you’ve got too much wiggle room, you need to choose a thicker blade. If you can’t get the blade through the gap, you need a thinner blade.
too much wiggle room
not enough space
perfect fit
Once you’ve found one that is thick enough to fit in the slot but small enough so that they’re very little wiggle room, you’ve found the correct measurement. Look at the specs on the side of the blade and note the measurement. In the image above, the blade had a with of 0.50mm – therefore, the spark plug gap is 0.50mm wide.

And there you go – you now have your measurement!