Auto Service World
Feature   January 1, 2000   by Auto Service World

Knowledge Building: Learning to Measure Engine Parts

Accurate measurement of engine components is critical to a quality engine build, both in terms of longevity and performance as well as profitability–a part that was machined or ordered improperly due to an inaccurate measurement adds time and cost to the whole process of rebuilding an engine.

There are two types of precision measuring tools: direct and indirect. Due to the realities of working with metal, most measurements are of the plus/minus type. You may already be familiar with this concept, often written 2.125 +/-.0005″ (42.00+/-.005 mm). The first number is the preferred size. The +/- number is the amount of variation between preferred and acceptable. During inspection of a part, we are given ideal or preferred dimension. After we measure a part we want to compare our sizes against the preferred size.

SAE Example:

Specification: 2.1247+/-.0005″

Our part must not be larger than 2.1252″

Nor smaller than 2.1242″

ISO Example:

Specification: 72 mm+/-0.025 mm

Our part must not be larger than 72.025 mm

Nor smaller than 71.975 mm

Occasionally, specifications are referred to as a window. This means that the part must be within an upper and lower limit.

SAE Example:

Specification: 3.3125-3.3133″

Our part must not be larger than 3.3133″

Nor smaller than 3.3125″

Our window would be .0008″

ISO Example:

Specification: 52.13 mm-52.25 mm

Our part must not be larger than 52.25 mm

Nor smaller than 52.13 mm

Our window would be .12 mm

To determine these measurements accurately requires the use of precision measuring tools.

The Direct type includes outside micrometers, inside micrometers, depth micrometers, dial indicators (both plunger and finger types), vernier and ideal calipers, gauges and machine tool dials. Machine tool dials are those dials on machines used to indicate cutter or part movement.

For the purposes here, discussion will be restricted to the micrometer, one of the most important measuring devices in a machinist’s toolbox.

The micrometer (or mike, or mic)–whether outside, inside or depth type–all use the same method to read or indicate the dimension. The most common micrometer is the outside type. As a precision measuring instrument, it should be treated with care and maintained properly.

1) Never drop a micrometer, from any height.

2) Always return the micrometer to its case when you have completed your measurements.

3) Do not lay the micrometer where it will get banged around by parts or tools.

4) Do not lay the micrometer around steel or cast iron chips, or grinding grit.

5) Check calibration periodically. If it’s used every day, check it weekly.

6) Never use the micrometer on moving work.

Micrometers measure only one inch, or 25 millimeters, at a time. The size or range of the part to be measured must fit within that one inch or 25 millimeters. Micrometers are usually purchased in full-inch or 25mm increments, i.e. 0-1″, 1-2″, 2-3″, etc. or 0-25 mm, 25-50 mm, 50-75 mm, etc.

Inch micrometers can be used to measure metric parts, provided that the metric specifications are provided with inch conversions.

SAE Measurements

The spindle is threaded on one end with 40 precision-ground threads per inch. It will travel one inch when the thimble is rotated 40 times. One revolution of the thimble moves the spindle .025″. The thimble is marked with 25 (0-24) divisions. When it is rotated and aligned with any one of the 25 marks, a measurement in thousandths is obtained.

The one inch scale on the barrel is numbered from 0 (zero) to 0 (zero), with the last zero signifying 10/10s of an inch. Each imprinted number represents .100″. Each number is further divided into quarters, i.e., zero, one quarter, two quarters, three quarters and four quarters or 100 thousandths.


The number 3 is visible on the barrel = .300″

Two quarters are visible = .050″

The twenty-third division aligns with the zero line = .023″

Reading = .373″


The number 5 is visible on the barrel = .500″

No quarters are visible = .000″

The second division aligns with the zero line = .002″

Reading = .502″


The number 4 is visible on the barrel = .400″

One quarter is visible = .025″

The twelfth division aligns with the zero line = .012″

The fifth line on the vernier scale aligns with the line above it = .005″

Reading = .4375″

ISO Measurements

The screw pitch of a metric micrometer is 0.5 mm (.50 mm), which means that when the thimble is rotated one turn, the spindle moves 0.50 mm. The thimble is graduated into 50 divisions (0.01 mm each).


The number 12 is visible on the

barrel = 12.00 mm

No halves are visible = 0.00 mm

The thirty-fifth division aligns with the zero line = .35 mm

Reading = 12.35 mm


The number 21 is visible on the

barrel = 21.00 mm

One half is visible = 0.50 mm

The forty-eighth division aligns with the zero line = .48 mm

Reading = 21.98 mm

When you’re just starting out, it is good to get into the habit of writing all the numbers down so you can go back and verify your work. This will also help you learn how to use the micrometer more quickly.

The preceding was excerpted with permission from “Sunnen’s Complete Cylinder Head and Engine Rebuilding Handbook,”by John G. Edwards.

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