Thickness Gauges

Part of Precision Measurement

Making and using feeler gauges, taper gauges, and fixed gauges to check clearances and thicknesses.

Why This Matters

Many critical clearances in machinery cannot be measured with calipers or micrometers because they are gaps — the space between two parts rather than a solid dimension. The clearance between a valve and its seat, the gap in a piston ring, the space between gear teeth, the bearing clearance in a crankshaft — all are critical gaps that determine whether a machine will function, wear properly, or fail.

Thickness gauges (also called feeler gauges or gap gauges) are simple, robust, and inexpensive instruments for measuring these gaps. A set of steel leaves from 0.05 mm to 1.00 mm covers the vast majority of gaps encountered in mechanical work. They require no calibration beyond being made to the right thickness, and they last indefinitely with basic care.

Making a set of feeler gauges from scratch is one of the simpler precision instrument-making tasks — requiring only flat steel, a micrometer to verify, and care in the manufacturing process.

Types of Thickness Gauges

Feeler gauge set (leaf type):

• Flat steel leaves of precise thickness
• Typically 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.40, 0.50, 0.75, 1.00 mm
• Multiple leaves can be combined to measure intermediate gaps
• Fan out from a rivet at one end

Step-type taper gauge:

• Wedge-shaped with graduations on the side
• Insert into gap until it stops; read the graduation at the edge of the gap
• Good for quick approximate measurements; less accurate than leaf gauges

Wire gauges:

• Round wire of precisely known diameter
• Used where round cross-section fits the gap better (piston ring gaps, etc.)

Drill blank gauges:

• Twist drill shanks are made to very accurate diameters
• Can be used as “go/no-go” gauges for holes or round gaps
• Cheap, readily available, durable

Making Feeler Gauge Leaves

Material:

• Thin hardened spring steel strip is ideal
• Blue-tempered clock spring steel has very consistent thickness and is already hardened
• Cold-rolled steel strip can be used and hardened after cutting

Process:

1. Obtain or make thin steel strip of the desired thickness. Rolling your own is difficult; if spring steel is not available, shim stock (packing shim material from machinery) is often available in standard thicknesses.

2. Verify thickness with a micrometer. Measure in multiple places along the strip. Good feeler gauge stock varies no more than 0.002 mm along its length. Strip varying more than this will make gauges that cannot be trusted.

3. Cut to shape. Typically 100–150 mm long, 10–12 mm wide. Round or angle the end that enters the gap (taper the last 10 mm to make insertion easier).

4. Engrave or stamp the thickness on the face. Use a center punch with an impresso stamp, or scratch-mark clearly.

5. Assemble into set. Drill a common hole at one end, rivet together with a loose-fitting rivet that allows the leaves to fan out.

Combined Thicknesses

With a set of leaves, any gap can be measured to 0.05 mm by combining. A 0.35 mm gap = 0.25 + 0.10 leaf. Always use the minimum number of leaves to avoid stacking errors. Never use more than three leaves in combination.

Using Feeler Gauges

The drag test:

1. Select a leaf slightly thinner than the expected gap
2. Slide into gap — should enter freely
3. Try the next size up — should not enter, or should enter with noticeable drag
4. The correct gap is between these two sizes

“Just grabs” feel: The target for most clearance checks is a leaf that “just passes with a slight drag” — not free, not jamming. This feel, once learned, gives consistent results.

Common Errors

• Forcing a gauge into a tapered gap — the leaf bends and gives a false small reading
• Using worn or bent leaves — they compress or spring, giving false readings
• Checking spark plug gaps at engine temperature — gaps increase as metal heats; always check cold unless checking hot running gap

Critical Applications for Feeler Gauges

Engine work:

Gap	Typical Clearance
Spark plug gap	0.6–0.9 mm
Valve clearance	0.10–0.40 mm (cold)
Piston ring gap	0.25–0.65 mm
Main bearing clearance	0.03–0.08 mm
Big end bearing clearance	0.03–0.08 mm

Machine tools:

Gap	Typical Target
Lathe spindle bearing clearance	0.01–0.03 mm
Slideways clearance (dovetail)	0.01–0.02 mm
Gear tooth backlash	0.05–0.20 mm

Structural:

• Checking flatness with a straightedge (any gap means out-of-flat)
• Checking parallelism of machine surfaces
• Setting up for welding with precise joint gaps

Making a Wire Gauge Set

Round wire can fill gaps differently than flat leaves — useful for piston ring end gaps and similar applications:

Sources of calibrated wire:

• Music wire (piano wire) comes in calibrated sizes
• Resistance wire for electrical heating elements is precisely controlled in diameter
• Quality welding wire

Verification: Always verify wire diameter with a micrometer before using as a gauge. Wire may be labeled incorrectly, or may have been drawn slightly undersized.

Making a gauge from wire: Bend into a U-shape and solder or swage to a handle. Stamp the diameter on the handle. Multiple wires on one handle create a multi-gauge.

Maintaining Feeler Gauges

Feeler gauge leaves are delicate:

• Never bend them by using force to enter a gap that is too tight
• Clean with oil to prevent rust (rust adds effective thickness)
• Check for bent or kinked leaves — these must be discarded
• Store folded flat in the fan assembly, never loose

A set of feeler gauges that is kept straight, clean, and verified occasionally against a micrometer will give accurate, reliable service for decades.