2. 17
Cylinder, Crankshaft, and Piston Service

3. 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.

4. Cylinder Reconditioning
Inspection
Measurement
Reboring
Honing

5. 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

6. Cylinder Inspection

7. 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

8. 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

9. Cylinder Measurement

10. Measuring Cylinder with an Inside Micrometer

11. Measuring Cylinder with a Telescoping Gauge

12. Cylinder Taper
Diameter at top of cylinder minus diameter at bottom of cylinder
Corrected by reboring or replacement

13. Cylinder Out-of-Round
Largest difference between any two measurements
Acceptable limit less than .006
Corrected by reboring or replacement

14. Reboring the Cylinder Chrome-plated, aluminum cylinders
Replace
Replaceable cast-iron sleeve
Removed and replaced
Cast in sleeve or solid cylinder
Can be rebored

15. 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

16. 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

17. 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

18. Proper Cross-Hatched Pattern

19. 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

20. 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

21. 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

22. 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

23. Measuring Crankshaft Journals

24. 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

25. 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

26. 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

27. 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

28. 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

29. Measuring a Plain Bearing

30. 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

31. 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

32. 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

33. Measuring Piston Skirt Diameter

34. 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

35. 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

36. Checking Ring Side Clearance

37. 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

38. 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

39. 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

40. 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

41. 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

42. Measuring a Connecting Rod’s Piston Pin Bore

43. 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.

44. 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.

45. 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.

46. 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.

47. 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.

48. 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.

49. 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.