17 Cylinder, Crankshaft, and Piston Service

Learning Objectives 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.

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 Cylinder taper 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 Remove piston pin 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

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.

What is the primary difference between boring and honing 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.

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.

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.

What conditions would require a connecting rod to be replaced 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.

Glossary Boring machine Cylinder taper Inside micrometer A machine used to enlarge a cylinder bore. Cylinder taper A cylinder wear pattern in which the cylinder diameter is greater at the top than at the bottom. Inside micrometer A micrometer designed to measure the inside diameter of a bore.

Glossary Out-of-roundness Reboring Telescoping gauge The oval cylinder shape created by excessive cylinder wear 90° from the crankshaft centerline. Reboring Using a machine tool to precisely enlarge the diameter of a cylinder bore. Telescoping gauge Transfer-type measuring instruments designed to measure the internal diameter of a bore.