Small Engine CDE

Small Gasoline Engines

Engine

Measuring

&

Testing

Instruments

Measuring

• Service and repair

manuals present
exacting measurements
& tolerances for engine
adjustments and parts,
to determine if they are
in good standing or are
rejected resulting in
replacement

Measuring

• In service or repair manuals, the clearance

or tolerance measurements is usually
expressed in one-thousandths of an inch.

• i.e.
➢ 1000 = 0.001
➢ 1/8 = 0.125
➢ 1/2 = 0.500

Measuring

• Competency in

engine repair &
maintenance
requires
proficiency in the
use of a variety of
measuring
devices.

Measuring Devices:

• Micrometers
• Thickness Gage
• Reject Gage
• Plastigage
• Torque Wrenches
• Compression

Testers

• Telescoping Gage
• Tachometer

Micrometer

• One of the most common

measuring devices.

• Designed to measure

machined components
where tolerance is critical

• i.e.

• Crankshaft journal
• Piston pin

• The micrometer is easy to

use once its major parts
and few principles are
understood.

Parts to a Micrometer

thimble
sleeve
anvil spindle

frame

Parts of a Micrometer

0 1 2 3 4 5 6 7 8 9 0

0

1
2
3

24

22
23

0 1 2 3 4 5 6 7 8 9 0

1/10 inch (0.100)

25/1000 inch (0.025)

1/1000 inch (0.001)

How to Read a Micrometer

0 1 2 3 4 5 6 7 8 9

15

16
17
18

14
13

19

15

17
16

14
13

18
15

0 1 2 3

Smallest Measurement

0.000

Largest sleeve reading

0.300

One mark

past

largest
sleeve
reading

0.025

Thimble
Reading

0.016

0.341
inches

Thickness Gages

• Manufactured for measuring the

clearance or gap between two parts.

• Used to measure:

• Spark plug gap
• Breaker Point
• Engine end play
• Connecting Rod
• Crankpin clearance
• Piston ring

end-clearance

• Main bearings

• Thickness Gages:

➢ Flat Feeler Gage

➢ Round-wire Gage

➢ Dial Indicator

Flat Feeler Gage

• Most commonly used

gage

• Consists of a # of thin,

steel blades

• Each blade is a different

thickness

• Range from 0.001 inch to

0.025 in increments of
0.001, 0.003 and 0.005

• Used to measure valve

clearance, armature
air-gap, breaker-point gap,
and crankshaft end play.

Round-wire Gage

• Has various

applications

• Main application is

measuring spark
plug gaps

• Come in a variety of

designs

Dial Indicator

• Tool used to

determine the
crankshaft end play

• Made of movable

parts that actuates a
dial needle to show
differences in
dimensions

• As the name implies,

“Rejects”
Reject gages are used
to determine if specific
parts should be
replaced, such as
bushings and bearings.

• There are two types:

➢ Plug gage

➢ Reject gage

Reject Gages

Plastigage

• Used to measure

clearance between
bearing journal on
crankshaft and its rod
cap.

• Thin, plastic, thread-like

material

• Placed between the

crank pin and rod cap
and tightened to
recommend torque.

Torque

• Used to tighten bolts at manufactured specifications

• Proper torquing should be done in “Stages,” by

dividing the torque values by three or four

➢ 100 lb/in2 ~ Stage at 25 lb/in2 increments

• Torque = force x distance

• Calibrated in pound-feet or pound-inches

Compression Tester

• Used to determine if a

small engine has a
cylinder, piston rings,
valves and/or gasket
problem.

• A one cylinder engine

will have 60–150 psi

Telescoping Gages

• Measures cylinder

wear

• Place in center of

chamber and
measure at 6 points.