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Measuring and Testing InstrumentsSmall Engines

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Measuring and Testing Instruments Precision measuring instruments are used to

determine if parts are to be rejected resulting in replacement.

The repair of any gasoline engine should be based upon the manufactures recommendations in the service manual.

The listing of clearance or tolerance measurements is expressed in .001 of an inch.

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Measuring and Testing Instruments For example:

The spark plug gap might be listed as .030 meaning for normal operation the gap should be gapped at .030 inch.

Specifications are normally listed for: Armature air gap, valve tappet clearance, piston

to cylinder wall clearance, ring-groove clearance, etc…

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Micrometer Caliper One of the more common measuring devices is the

outside micrometer caliper. It is designed to measure machined components

where tolerances are critical as on a crankshaft journal.

The marks on the thimble represent .001 inch each. One complete revolution of the thimble equals .025.

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Micrometer Caliper Every fourth line on the sleeve is longer than the

others to help identify the whole numbers which represent tenths.

A number of different types of micrometers are manufactured.

The outside micrometer is the most common and can to measure the outside diameter of round objects, and the width and thickness of flat pieces.

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Micrometer Caliper The inside micrometer is used to measure the

diameter of a hole such as the inside diameter of an engine cylinder.

The depth micrometer is used to measure the depth of holes, grooves, and slots.

Micrometers are all read in the same manner.

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Micrometer Caliper Micrometers usually are designed to measure

within a 1 inch range. 0-1 inch 1-2 2-3 Etc…

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Thickness Gauges A variety of different gauges are manufactured for

measuring the clearance or gap between two parts. The most common gauges are the flat feeler gauge,

round wire gauge, reject gauge, and plastigage. Some uses of thickness gauges are:

Spark plug gap, breaker-point gap, connecting rod to crankpin clearance, piston ring end gap.

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Thickness Gauges The most commonly recognized thickness

gauge is the flat feeler gage. Each blade is a different thickness. Each blade is stamped with its thickness. Feeler gauges can be used to measure:

Valve tappet clearance, air gap between the armature legs and the fly wheel magnet, crankshaft end play.

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Thickness Gauges Round-Wire feeler

gauge It has several

applications for working with small engines.

The most common of these is measuring the electrode gap of new or used spark plugs.

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Thickness Gauges Plug Gauge

Commonly used in small engine repair.

Is used to determine if a valve guide bushing needs replacing.

Other names given to a plug gauge are go, no-go, and reject gauges.

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Thickness Gauges Plastigage

Can be used to measure clearance between the bearing journal on a crankshaft and its bearing or rod cap.

The rod cap is tightened to a recommended torque measurement the flatten plastic is measured.

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Torque Wrench There are specs for

the torque to which bolts should be tightened depending upon size and grade.

Torque equals force times distance.

Torque wrenches are usually calibrated in feet or pound inches.

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Compression Testing Compression testing can help determine if

the small engine being studied has a problem with either the cylinder, piston rings, valves and or gaskets.

Normal engines one cylinder engines will have a 60-150 psi rating.

An accepted rule is that compression problems exist if the reading is 20% less than the minimum spec for the engine.

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Compression Testing Compression testers

can either have a rubber tip which is held to the spark plug hole or a threaded end for screwing into the spark plug hole.

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Tachometers Tachometers are used

to check the operating revolutions per minute of the engine.

Cylinder Testing A telescoping gage can

be used to measure cylinder wear.