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Measuring and marking

Feeler gauges – what are they, and what are they for

Feeler gauges are unassuming tools. They are small, all steel and unadorned, and probably the last thing you will notice in a tool box. If you have ever picked up a set, you may have wondered what such a collections of metal strips is really useful for?

In this article, we are going to explain what feeler gauges are, how they work, and what they are used for. In summary, feeler gauges are a mechanical instrument for measuring the width of narrow gaps. The gauges are essentially accurately dimensioned thin pieces of metal, which can be used in metalworking, mechanical assembly and maintenance to check gaps and clearances in parts and machinery.

If you interested in learning more, below you will find a more detailed answer to the most common questions around feeler gauges. Whether you are interested in understanding this instrument better or considering getting a set, I hope that you will find this article helpful!

What are feeler gauges?

Feeler gauges are thin strips of metal sheet with precise known thicknesses. Feeler gauges usually come in a set, where each gauge, called a blade or a leaf, has a different thickness, and the gauges together cover some thickness range.

Although sets vary in size, number or blades and thickness range, you can get a rough idea of what we are talking about by considering that a typical pocket-size feeler gauge set contains 32 blades, each measuring around ½’’ x 4’’, with thicknesses running from .0015’’ (0.04 mm) to .035’’ (0.88 mm). Such a set is displayed in the image below.

Standard 32-blade feeler gauge set
A standard 32-blade dual-marking feeler gauge set. The set runs from .0015” (0.04 mm, 2nd blade clock-wise) to .035” (0.88 mm, 1st blade clockwise), has brass and steel .010” blades (0.25 mm, 5 and 6 o’clock) and a main division of .001” (0.025 mm), as seen in the last three exposed blades.

In the standard design, the blades in a feeler gauge set are stacked and rotate around a common hinge at the root. The arrangement is very practical, as it allows the blades to be easily fanned out when in use, and rotated back to a single stack for storage. It also helps to protect the thinnest blades in the middle of the stack, which would otherwise be bent and torn very fast. In addition to the outermost thick .032’’ and .035’’ blades, the set is often further shielded by the U-shaped sheet metal handle which surrounds the blades in storage position.

The tips of the individual blades are always rounded to avoid scratching surfaces they are in contact with. Some versions may also include a taper in the blade tip to allow their use for narrower gaps.

The feeler gauge blades are very elastic and flexible, and can usually bend to a curve without taking a permanent set. You should beware of bending them too much, however, since dents in the blades may make the set difficult or impossible to stack back to storage position.

What are feeler gauges used for?

1 Gaps

The primary function of feeler gauges is measuring the width of narrow gaps. Such measurements are often needed in metalworking, machinery maintenance and some other areas. Typical applications in this category include

  • spark plug gaps
  • bearing clearances
  • valve clearances
  • fit between mating parts
  • guitar adjustment

2 Straightness and flatness

In combination with a straight edge, a feeler gauge can be also used for checking the straightness of edges and surfaces in machine parts and other objects of interest. This important category of application is discussed in more detail below.

3 Tension

Although they are an instrument for length measurement, feeler gauges can also be used for checking bolt tension. This is made possible by special fastening hardware which, through plastic deformation, convert bolt tension into a gap height, measurable with feeler gauges.

4 Length reference

Due to their low cost and excellent stability, feeler gauges can also serve as crude thickness standards in the shop, where you can use them to check the accuracy of your caliper, for example, at the low end of the measurement range.

How to use a feeler gauge?

Feeler gauges used in measuring the jaw gap of a pair of combination pliers
Feeler gauges used in measuring the jaw gap of a pair of combination pliers: the .006” gauge (bottom) is loose in the gap, the .008” gauge (top) does not go in, and the .007” gauge (middle) goes with some interference. The jaw gap is around .007” (0.18 mm).

A feeler gauge is used simply by trying to slide it into the gap to be measured and checking how well it goes:

  1. If the gauge goes into the gap easily, the gap is wider than the thickness of the gauge in question.
  2. If the gauge does not go into the gap, the gap is narrower than the gauge thickness
  3. If the gauge goes, but only with some resistance and friction, the gap width is close to the gauge thickness

A single feeler gauge will only indicate if the gap width is below, at, or above a certain value; it will not tell you how much thinner or wider the gap is. A typical measurement would use a set of feeler gauges successively to find out the width of the gap more accurately. If the first gauge you try will not go into the gap, for example, you will try a thinner one, and a thinner one, until you find the gauge that just goes – this gauge will tell you the gap width. Conversely, if a gauge goes in without any effort, you will move to larger and larger sizes until you cannot slide the gauge in without using force.

In using feeler gauges, you should never have to use but the lightest of efforts. If you try to force a gauge in, you risk bending the gauges or scratching the part surfaces. You also cannot rely on measurement results that were obtained by using force, since the gap faces may actually give in a bit to accommodate a slightly too thick gauge, particularly when working with soft materials.

Gap widths are often measured with a feeler gauge not just at a single point, but over some length of gap. This is most efficiently done by inserting the gauge at one point, and then sliding it sideways in the gap over the whole length of interest. If the gauge can traverse the whole length without resistance, you know the whole gap is wider than the gauge thickness.

What do the numbers on a feeler gauge mean?

Feeler gauge blades with dual markings
Dual-marked feeler gauge blades with thickness markings both in decimal inches and millimeters.

The numbers on feeler gauges indicate the gauge thickness. Imperial sets carry the markings in decimal inches and metric sets in millimeters; ‘dual marked’ have both inch and millimeter markings, and are the most common version today. Even if no units are given, you are usually able to tell whether the markings are in inches or millimeters from the magnitude of the numbers.

The markings may be either etched or stamped onto the blades (see image below). Etching or printing is always used on the thinnest blades (below .010” or 0.25 mm), where a stamped marking would just go through the material and increase the blade thickness.

In some low-quality sets, the gauge markings are sadly prone to wearing off, leaving you guessing which size gauge are you actually holding. Fortunately, the gauges in the set are usually ordered logically, so you can deduce a single missing marking from those on neighboring gauges. If more than one marking is illegible or if you are not sure of their ordering in the set in question, you may have to verify the thickness with a micrometer.

A feeler gauge set with printed and stamped markings
A feeler gauge set with etched markings on the thinnest blades and stamped markings on the thicker blades. This set has metric markings as hundredths of a millimeter.

Feeler gauge and straight edge

In combination with a straight edge, feeler gauges can be used to measure straightness of edges and surfaces.

To do this, you first set the straight edge against the feature to be checked for straightness. If there is any curvature or undulation in the feature, the straight edge will not sit flat in contact with the feature, but will instead leave a gap.

Next, to actually quantify how much the feature deviates from a straight line, you use the feeler gauge set to measure the gap height as described above. Note that the gap height will now vary along the length, so you will get a different reading at different points. Further, the gap will now be completely closed at least at two contact points.

By measuring the gap height at different points along its length, you will get a profile for the surface or edge you are inspecting. This profile is very useful if you are planning to straighten the feature later on, since it tells you where material needs to be removed or added.

You should note that, with this method, your result will be only as accurate as the straight edge you use as reference. Before taking any action based on your measurement, be sure to double-check the straightness of the straight edge or verify the measurement with an another straight edge. It only takes one bump to bend a straight edge, and you would be surprised how many of them have some curvature to them.

Can feeler gauges be combined/stacked?

As most other length references, feeler gauges can be combined. This means that by stacking two or more individual blades, you can create thicknesses that are not included in the set.

For example, you can get the thickness .034’’, not included in most standard sets, by stacking the .032’’ and .002’’ blades. The thinnest .0015’’ blade, on the other hand, allows you to create the in-between dimensions for the .01’’ division of the set, that is .0035’’, .0045’’, .0055’’, and so on, effectively increasing the set resolution.

There are a few precautions for stacking feeler gauges, however. First, getting the dimension right relies on a good flat contact between blades; be sure that the blades are clean and free of any dust or debris. At these accuracy levels, a single tiny swarf is enough to throw the measurement off. Second, to combine blades that are not nearest neighbors in the set, you will have to bend them a bit. Here, you must be careful to bend them enough so that they are flat in contact for the whole length that you are using them, but not so much as to cause a permanent set.

Stacking allows you also to extend the range of a feeler gauge set. By combining the thickest and the second thickest blade in the standard set, for example, you get to .067’’ (1.7 mm); add the third-largest, and you are at .097’’; and so on, up to the total thickness of the set at little under ½’’.

However, as the contacts between the blades are always imperfect, each blade adds a small contact error to the stack height. For this reason, stacking a large number of blades is not recommended.

Feeler gauge versions

Feeler gauges are available in a number of configurations: thickness range, number of blades, markings, head shape and material.

1 Thickness range and number of blades

The standard 32-blade sets run from .0015’’ to .035’’ at .001’’ increments for most of the range. There are many lower blade count sets, which have either a coarser division or a narrower range. Blade counts and ranges of some of the most commonly available sets are listed in the table below.

No. bladesmin
[in]
max
[in]
main
division [in]
Notes
32.0015.035.001steel&brass .010”
32.0010.040.001extended range
32.0008.040.001varying division
12.0080.028.002
16.0050.020.001
22.0015.025.001
Typical feeler gauge set configurations: number of blades, lowest and highest blade thicknesses and main division in the blade thickness.

The widest range sets are naturally the most versatile ones, and the best for general-purpose work. However, if you do not need the thinnest blades, you might actually prefer a 12 or 16 blade set which omits them. The reason is that the thinnest blades are delicate and very easy to bend, and you will have to fold and unfold the whole set more carefully if it contains these blades.

2 Markings

  • Imperial: thickness markings in decimal inches
  • Metric: thickness markings in millimeters
  • Dual: thickness markings both in inches and millimeters

3 Blade shape

  • Standard: The standard feeler gauge blades are of constant width and have a rounded end (image below).
  • Tapered: Tapered sets have the blades tapered towards the tip. They allow measurement of gaps that are too short for the ½” wide standard blades (image below).
  • Angled: Blades have a 45 degree bend at the end for better access from an oblique angle, and are particularly suited for valve adjustment.
Straight and tapered feeler gauge blades
The standard straight feeler gauge blade (above) and tapered version (below) for better access into short gaps.

4 Material

Feeler gauges are typically made of either carbon steel or stainless steel, and some sets may also include one or more brass blades.

Carbon steel: Carbon steel is the most common material for feeler gauge blades. The carbon steel blades are harder and less susceptible to wear or bending. However, they rust and lose their accuracy easily in humid environments if not kept well-oiled at all times.

Stainless steel: Stainless steel feeler gauge sets do not rust, and are therefore better than carbon steel sets in moist or wet environments. You should prefer a stainless steel set if you use or store the set outdoors or in unheated spaces. Stainless steel blades have the downside of being softer and thus more susceptible to wear and bending.

Brass: In some feeler gauge sets, you may find a single gauge which is made of brass instead of steel. This gauge, usually .010” (0.25 mm), is now mostly obsolete, but used to serve the special purpose of setting the air gap in some old automotive ignition systems and had to be non-magnetic.

5 Special versions

Go/no-go gauges: These special gauges have the blade tips ground thinner than the rest of the blade. The stepped design allows the operator to easily check whether a gap meets a certain tolerance, i.e. is between two height values, with a single blade. Go/no-go gauges are particularly advantageous in production and quality control use, where efficiency is vital.

Ramp type gauge: A single-piece feeler gauge consisting of a round disk with a ramp ground around the edge. These gauges are tailored for spark plug gap inspection and are very compact and robust, but do not have the range, accuracy or reach of the standard sets.

Wire gap gauges: These sets use L- or U-shaped wire instead of sheet metal for the blades. They usually offer a fewer thicknesses than the standard sets, but are more robust and allow access from a wider range of angles.

How much do feeler gauges cost?

Feeler gauges are very affordable tools: you can get the standard 32-blade carbon steel set at between $5 and $10. Special versions, such as stainless steel or curved sets are slightly more expensive on average, but rarely go much above $10.

Considering utility and expected lifetime of feeler gauges, if properly used, cost should mostly be no object to getting a set into your tool box.

Feeler gauge alternatives

Feeler gauges allow you to measure gaps which are too thin or too hard to access for most measurement instruments. In many applications, there are few convenient alternatives to using feeler gauges.

However, if you do not have a set of feeler gauges at hand, there are some ways you can try to get around this:

  1. If the gap to be measured is relatively high, around .04’’ (1 mm) or more, you may use the upper jaws of a caliper to measure the gap directly.
  2. A caliper (or a micrometer) also allows you to get a rough measurement of thinner gaps indirectly: you can measure a piece of thin wire or sheet, and use them as gauge substitutes. Metal sheet and paper, credit cards, coins, electrical wire, fishing line and guitar strings make decent gauges.
  3. Even without a caliper or a micrometer, you can use some objects of known thickness as feeler gauge substitutes. Here office paper, credit cards and electrical wire are probably your best choices, since you can find out their thickness roughly with a short online search.

The table below shows you the thicknesses of some commonly available items that you may turn into feeler gauges in a bind.

object[in][mm]Notes
office paper.0050.1220lb. / 80 g/m²
28 AWG wire.0130.32solid, w/o insulation
26 AWG wire.0160.40solid, w/o insulation
24 AWG wire.0200.51solid, w/o insulation
22 AWG wire.0250.65solid, w/o insulation
credit card.0300.78+/- 0.002”
20 AWG wire.0320.81solid, w/o insulation
Thicknesses of some common standard-dimension items that can be used as substitutes for feeler gauges.

Some words of caution are in order here. Most of the feeler gauge alternatives lack the precision, consistency and reach of real feeler gauges. Specifically, the alternatives are

  • inaccurate: you rely on your own measurement or an uncertain product dimension
  • unstable: most alternatives are softer than proper feeler gauges, and suffer from fast wear, scratching, denting or bending
  • poor reach: alternatives are mostly too flexible to be cut and used in slender high-reach gauges
  • non-stackable: many feeler gauge alternatives do not stack reliably

Conclusion

Although feeler gauges may look plain and unexciting, they perform an advanced measurement function that may often be essential in many jobs. As we have seen, feeler gauges are also quite versatile, with functions from gap width measurement to flatness and bolt tension checking and application areas from metalworking to musical instruments.

So when do you need a set of feeler gauges? In my opinion, you should own a set of feeler gauges if one or more of the following applies:

  • your work involves high precision
  • you work with machine tools
  • you service machinery

As the list suggests, feeler gauges are an essential in metalworking and in machinery maintenance. Although they are somewhat of a specialty tool and you will not need them very often, there are tasks where they are pretty much irreplaceable.

When working with wood, your work is somewhat rougher, and consequently you may never need feeler gauges in your main workflow. However, it is good to note that you could still make good use of a set of feeler gauges in adjusting your tools and machinery.

Overall, given the versatility of feeler gauges, you will probably have at least some use for them in the workshop and around the house. When you factor in the low price and long service life, feeler gauges are very likely to earn their keep.