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Reading measuring tools for the practical test

The official scope of the practical part includes handling testers and measuring instruments. It is a skill that improves with practice: once you know how each scale works, you read quickly and correctly. This page explains each tool with an example.

Last updated: 2026-10-12

At a glance

0.05mm, a common caliper resolution
0.5 + 0.01mm micrometer sleeve and thimble
0.01mm per dial-gauge division
DCVrange for battery voltage

Vernier caliper (ノギス)

A caliper measures outside diameter, inside diameter and depth. To read it: (1) find the main-scale line just before the vernier's zero, which gives the whole millimetres; (2) find the vernier line that lines up exactly with a main-scale line; (3) multiply that line number by the vernier resolution and add it.

Example with a 0.05 mm vernier: the zero sits just past 23 mm and vernier line 7 lines up. The reading is 23 + 7 × 0.05 = 23.35 mm. The resolution is printed on the vernier, so check it before you calculate.

Some calipers have a 0.02 mm vernier (50 divisions). Example: the vernier zero sits just past 41 mm and vernier line 13 lines up. The reading is 41 + 13 × 0.02 = 41.26 mm. Many verniers print the value next to the lines (line 13 is marked near 0.26), so you can also read it directly.

Check zero before measuring: wipe the jaws and close them. The vernier zero must line up with the main-scale zero, and the last vernier line must also line up with a main-scale line. If it is off, every reading is off by the same amount. Keep the jaws square to the part, do not squeeze hard, and look straight at the lines to avoid parallax error.

Micrometer (マイクロメータ)

A micrometer is more precise than a caliper. The sleeve (スリーブ) has lines every 0.5 mm on both sides of the reference line. The thimble (シンブル) has 50 divisions of 0.01 mm. Reading = sleeve value + thimble line × 0.01.

Example: the sleeve shows 12.5 mm (the half-millimetre line past 12 is visible) and thimble line 18 sits on the reference line. The reading is 12.5 + 0.18 = 12.68 mm. The most common error is missing the 0.5 mm line. When closing on the part, turn the ratchet stop (ラチェット・ストップ) so the measuring force is always the same.

One more example to show the half-millimetre trap: the sleeve shows 8 and the half-millimetre line after it is not yet visible, and the thimble reads 37. The reading is 8 + 0.37 = 8.37 mm. If the half-millimetre line after 8 is showing, the reading is 8.5 + 0.37 = 8.87 mm. The two cases differ by one small line on the sleeve, so always look below the reference line before adding.

Check zero: on a 0–25 mm micrometer, turn the ratchet until the anvil (アンビル) and spindle (スピンドル) touch; the thimble zero must sit on the reference line. Larger micrometers (25–50 mm and up) are checked with the setting standard (基準棒) supplied with them. Each micrometer covers only the 25 mm range marked on its frame, so pick the one that matches the part.

Dial gauge (ダイヤル・ゲージ)

A dial gauge measures runout, for example of a brake disc or the bend of a shaft. Each division is usually 0.01 mm. Turn the part slowly through one full turn; the runout is the difference between the highest and lowest needle readings.

Setting up a dial gauge: clamp it to a magnetic stand, keep the spindle at right angles to the surface and give it a little preload so the needle has moved off its rest position. Then turn the outer bezel until zero lines up with the long needle. One turn of the long needle (100 divisions × 0.01 mm) is 1 mm; the small needle counts whole millimetres. Runout example: as the part turns, the long needle swings from −3 to +5 divisions, so the runout is 8 × 0.01 = 0.08 mm. Compare that with the limit in the service manual to decide whether the part can be reused.

Feeler gauge (シックネス・ゲージ)

A feeler gauge is a set of steel blades of different thicknesses for small gaps such as valve clearance or spark-plug gap. The gap equals the blade that slides in and out with a slight drag.

Circuit tester (サーキット・テスタ)

Battery voltage: pick a DCV range above the expected value; red lead on positive (+), black lead on negative (−).
Resistance and continuity: measure with the circuit unpowered.
Unknown value: start on the highest range and step down.

Torque wrench (トルク・レンチ)

A torque wrench tightens bolts and nuts to a specified torque (in N·m). It is not a tool for breaking loose a seized bolt.

The tool figures in the app's mock papers are redrawn, not taken from JASPA papers. The examples above are only for practising the reading method.

See also the practical part format and the 30-day plan.

Sources checked: the JASPA specified skills test page (2024 test summary), the MLIT implementation rules and acceptance handbook (MLIT page) and the Prometric booking page. Rules can change, so check the latest version at these sources.

FAQ

How do I read a vernier caliper?

Take the main-scale value at the vernier's zero, then add the vernier line that lines up exactly with a main-scale line multiplied by the vernier resolution (for example 0.05 mm).

How do I read a micrometer?

Read the sleeve (0.5 mm per line), then add the thimble line on the reference line multiplied by 0.01 mm.

What should I watch when measuring battery voltage with a circuit tester?

Select a DC voltage (DCV) range above the expected value, red lead on positive, black lead on negative.

Do I need the power off to measure resistance?

Yes. Measure resistance on a circuit with no power, or the reading is wrong and the tester can be damaged.

Take a mock exam and see right away which items you missed and why.

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