C H A P T E R 17 Cylinder, Crankshaft, and Piston Service.

C H A P T E R 17

Cylinder, Crankshaft, and Piston Service

Permission granted to reproduce for educational use only.© Goodheart-Willcox Co., Inc.

• Describe how to inspect a cylinder for damage and measure a cylinder for wear, taper, and out-of-roundness.

• Explain the difference between boring and honing and identify when each process should be used.

• Summarize the steps in inspecting a crankshaft for damage and measuring it for wear.

• Describe the steps in main bearing service.• Summarize the steps involved in piston, rod, and

ring service.

Learning Objectives


Cylinder Reconditioning

• Inspection• Measurement• Reboring• Honing


Cylinder Inspection

• Scuffing and scoring on cylinder wall• Hot spots

– Discoloration

• Loose valve seats• Nicks and grooves in sealing surfaces• Spark plug and head bolt bore

– Stripped threads


Cylinder Inspection


Cylinder Inspection

• Ring ridge• Out-of-roundness

– Greatest wear 90 from crankshaft centerline

• Cylinder taper– Greatest wear at top of piston travel– Less lubrication, more pressure


Cylinder Measurement

• Cylinder measured at four locations• Two spots at top of ring travel• Two spots at bottom of ring travel• Parallel and perpendicular to crankshaft• Inside micrometer or telescoping gauge


Cylinder Measurement


Measuring Cylinder with an Inside Micrometer


Measuring Cylinder with a Telescoping Gauge


Cylinder Taper

• Diameter at top of cylinder minus diameter at bottom of cylinder

• Corrected by reboring or replacement


Cylinder Out-of-Round

• Largest difference between any two measurements

• Acceptable limit less than .006• Corrected by reboring or replacement


Reboring the Cylinder

• Chrome-plated, aluminum cylinders– Replace

• Replaceable cast-iron sleeve– Removed and replaced

• Cast in sleeve or solid cylinder– Can be rebored


Reboring the Cylinder

• Rebored in .010 increments• Oversized pistons and rings required

– Ensure oversize parts available before boring

• Performed with boring machine• Does not produce satisfactory surface

– Bore cylinder .0025 smaller than desired size– Extra material removed during honing


Honing the Cylinder

• Produces surface that promotes ring sealing– Crosshatch finish

• Cylinder and rings wear smooth together• Honing not recommended in certain cases

– Aluminum cylinders or coated rings– Check manufacturer recommendations


Honing Procedure

• Install hone in electric drill• Insert hone in cylinder

– Stones must contact cylinder walls squarely

• Start drill– Slowly move hone up and down– Do not allow stones to extend out of cylinder

• Wash, dry, and oil cylinder


Proper Cross-Hatched Pattern


Crankshaft and Main Bearing Service

• Parts worn by extreme forces• Common cause of engine failure• Worn or damaged parts

– Must be replaced or properly reconditioned– Can cause overhauled engine to fail quickly


Inspecting the Crankshaft

• Remove ball or tapered roller bearings• Inspect crankshaft journals

– Discoloration or scoring– One surface vs multiple surfaces– Emery cloth may remove light metal transfer– Deep scoring requires crankshaft to be

reconditioned or replaced


Inspecting the Crankshaft

• Inspect crankshaft keyway– Excessive wear, damage, deformation– Replace crankshaft if damage found– Use fine file to remove burrs from keyway

• Reconditioning– Ground and polished– Undersize bearings


Measuring Crankshaft Journals

• Wear, taper, and out-of-roundness• Greatest wear

– Top of journal during upstroke– Bottom of journal during downstroke

• Measured with outside micrometer– At least two points on each journal, 90 apart


Measuring Crankshaft Journals


Inspecting the Main Bearings

• Engine may have different bearing types– Different loads on flywheel side and PTO side

• Some bearing types must be replaced• Other bearing types can be reconditioned


Caged Needle Bearings

• Main bearings on some 2-cycle engines• Held by crankcase cover or retainer• Removed and replaced during overhaul

– Driven or pressed out– Keep old bearing for comparison


Tapered Roller Bearings

• Main bearings on heavy-duty engines• Outer race held by cover or retainer• Bearing cone pressed on crankshaft• Always replaced during overhaul

– Always replace cone and outer race as set– Pressed out or pulled with bearing separator– Keep old bearing for comparison


Roller Bearings

• Main bearings on many engines• Pressed onto crankshaft or into crankcase• Some held in crankcase with retainers• Always replaced during overhaul

– Pressed off or pulled with bearing separator– If held by retainers, remove retainers– Keep old bearing for comparison


Plain and Bushing-Type Bearings

• Inspect for metal transfer and scoring• Measure for wear

• Telescoping gauge

• Recondition if worn or damaged– Plain bearings reamed and bushing installed– Bushings driven out and replaced


Measuring a Plain Bearing


Piston Assembly Service

• Piston and ring problems– Low compression– Blow-by– Fouled plugs

• If not damaged, piston and rod are reused• Rings are always replaced during overhaul

– If cylinder rebored, use oversize piston/rings


Checking Piston Clearance

• Clearance allows for piston expansion– Also allows lubrication– Proper clearance typically .003 to .005

• Measure piston skirt diameter– 90 from piston pin bore, 1/2 from bottom


Checking Piston Clearance

• Subtract skirt diameter from cylinder diameter• Clearance too great

– Replace piston

• Clearance too small– Due to reboring cylinder and oversizing piston– Rehone cylinder


Measuring Piston Skirt Diameter


Ring Side Clearance

• Allows rings to stay in contact with cylinder– Move in and out of groove

• Clean and inspect ring grooves– Remove carbon buildup– Check for clogged oil return holes and wear


Ring Side Clearance

• Insert new compression ring in groove• Slide feeler gauge blade between groove and ring• Repeat with next larger blade size• Clearance equals largest blade size that fits• Replace piston if clearance is too great


Checking Ring Side Clearance


Removing Piston Pins

• Remove piston pin retainers– Wear eye protection while removing retainers

• Remove piston pin– Some pins may slide out easily– Other pins driven or pressed out– Be careful not to lose needle bearings


Measuring Piston Pins and Bosses

• Measure pin diameter with micrometer• Measure bores in rod and piston

– Telescoping or small hole gauge

• Subtract pin diameter from bore diameters• Excessive clearance

– Ream bores and install oversize pin– Replace piston, rod, and pin


Inspecting Connecting Rods

• Inspect saddle and piston pin bore– Scoring, metal transfer,damage– Replace if damaged

• Check for warpage and cracks– Replace if warped or cracked


Measuring Connecting Rod Bores for Wear

• Install cap on connecting rod• Measure large bore with telescoping gauge

– Inline and at 90 to connecting rod centerline– Out-of-round is difference in measurements– Wear size equals largest measurement

• Replace connecting rod if wear size or out-of-round is excessive


Measuring Connecting Rod Bores for Wear

• Measure pin bore– Telescoping or small hole gauge– If excessively worn, ream bore and install oversize

pin– If oversize pin unavailable, replace rod

• Connecting rod can be reused if undamaged


Measuring a Connecting Rod’s Piston Pin Bore


Review

Once a cylinder is measured, how are cylinder wear, taper, and out-of-round calculated?

Wear size is the largest of the readings taken. Taper is the difference between largest reading at the top of the cylinder and the largest reading at the bottom of the cylinder. Out-of-roundness is the largest difference between any two readings.


Review

What is the primary difference between boring and honing?

Boring removes a significant amount of material to resize the cylinder. Honing removes a smaller amount of material to create the proper cylinder wall finish.


Review

What things should be looked for in a visual inspection of a crankshaft?

Metal transfer, scoring, and discoloration on the crankshaft journals. Damage and wear in the crankshaft keyway.


Review

How are the different types of main bearings treated differently during an engine overhaul?

Tapered roller, ball, and caged needle bearings are always replaced. Plain and bushing-type bearings are inspected and replaced or reconditioned as needed.


Review

What conditions would require a connecting rod to be replaced?

The connecting rod is warped or cracked, if the bores are excessively worn or out-of-round. The connecting also needs to be replaced if piston pin clearance is excessive and an oversized pin is unavailable.

C H A P T E R 17 Cylinder, Crankshaft, and Piston Service.

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