Measuring and Testing Instruments

1. Measuring and Testing Instruments Small Engines

2. 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.

3. 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…

4. 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.

5. 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.

6. 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.

7. Micrometer Caliper • Micrometers usually are designed to measure within a 1 inch range. • 0-1 inch • 1-2 • 2-3 • Etc…

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.