2Measuring 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.
3Measuring 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…
4Micrometer CaliperOne 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.
5Micrometer CaliperEvery 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.
6Micrometer CaliperThe 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.
7Micrometer CaliperMicrometers usually are designed to measure within a 1 inch range.0-1 inch1-22-3Etc…
8Thickness GaugesA 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.
9Thickness GaugesThe 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.
10Thickness 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.
11Thickness 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.
12Thickness 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.
13Torque WrenchThere 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.
14Compression TestingCompression 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 psi rating.An accepted rule is that compression problems exist if the reading is 20% less than the minimum spec for the engine.
15Compression TestingCompression 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.
16TachometersTachometers are used to check the operating revolutions per minute of the engine.Cylinder TestingA telescoping gage can be used to measure cylinder wear.