2. 14
Chapter
Engine Bottom End Construction

3. Objectives After studying this chapter, you will be able to:
Compare the construction of different types of cylinder blocks.
Explain how piston construction affects engine operation.
Describe piston ring variations.
Explain the construction of engine bearings.

4. Objectives
Compare design variations of different engine bottom end components.
Explain safe practices when working with engine bottom end components.
Correctly answer ASE certification test questions on engine bottom end construction.

5. Major Parts of an Engine Bottom End
Cylinder block
Crankshaft
Connecting rods
Rod bearings
Main bearings
Piston pins
Piston rings
Pistons

6. Engine Bottom End Assembly
(Honda)

7. Cylinder Block Construction
Cast iron cylinder block
Very heavy and strong
Aluminum cylinder block
Much lighter, almost as strong as cast iron
Dissipates heat better

8. Cylinder Sleeves
Metal, pipe-shaped inserts that fit into cylinder block
Also repair badly damaged cylinder walls in cast iron blocks
Two basic types
Dry sleeve
Wet sleeve

9. Line Boring Machining operation
Cuts series of holes through block for crankshaft main bearings or camshaft bearings
Also can be done to OHC cylinder head

10. Two- and Four-Bolt Mains
Two-bolt
Two cap screws to secure each main bearing cap to engine block
Four-bolt
Four cap screws
Used on high-performance engines
Crossbolted block
Extra screws in sides of block for added strength

11. Block Girdle Large one-piece cap that fits over entire bottom of block
Secures main bearings
Also called main bearing bedplate or unit main cap

12. Piston Construction Cast pistons Forged pistons Relatively soft
Used in slow-speed, low-performance engines
Forged pistons
Used in today’s fuel-injected, turbocharged, and diesel engines
Exposed to much higher stress loads

13. Piston Parts Piston head, or crown Piston ring grooves Ring lands
Top of piston
Piston ring grooves
Three slots machined in piston for piston rings
Ring lands
Areas between and above ring grooves

14. Piston Parts (Cont.) Piston skirt Piston pin hole Piston boss
Side of piston below last ring
Keeps piston from tipping in its cylinder
Piston pin hole
Machined through piston to accept piston pin
Piston boss
Reinforced area around piston pin hole

15. Major Parts of a Piston

16. Piston Dimensions Piston diameter Pin hole diameter Ring groove width
Distance measured across sides of piston
Pin hole diameter
Distance measured across inside of piston pin hole
Ring groove width
Distance measured from top to bottom of ring groove

17. Piston Dimensions (Cont.)
Ring groove depth
Distance measured from ring land to back of ring groove
Skirt length
Distance from bottom of skirt to centerline of piston pin hole
Compression distance
Distance from centerline of piston pin hole to top of piston

18. Basic Dimensions of a Piston

19. Cam-Ground Piston Slightly out of round when viewed from top
Machined a few thousandths of an inch larger in diameter
Compensates for different rates of piston expansion

20. Piston Taper Top of piston machined slightly smaller than bottom
Maintains correct piston-to-cylinder clearance

21. Piston Head Shape
Flat-top piston
Dished piston
Domed piston

22. Variable Compression Piston
Two-piece design controlled by engine oil pressure
Increases combustion pressure
Prevents engine knocking or pinging

23. Piston Ring Construction
Automotive pistons normally use three rings
Two compression and one oil
Compression rings
Prevent pressure leakage into crankcase
Ring seating
Initial ring wear that makes the ring perfectly match cylinder surface

24. Piston Ring Construction (Cont.)
Two basic designs
Rail spacer type
One-piece type
Three-piece oil ring consisting of two rails, or scrapers, and an expander-spacer
Widely used

25. Piston Ring Construction (Cont.)
Ring expander-spacer
Holds two steel oil ring rails apart and helps push them outward
Ring expander
Can be placed behind one-piece oil ring to increase ring tension

26. Piston Ring Construction (Cont.)
(Ford)

27. Piston Ring Construction (Cont.)
(Ford)

28. Piston Ring Dimensions
Piston ring width
Distance from ring’s top to bottom
Ring side clearance
Difference between ring width and width of piston ring groove
Piston ring radial wall thickness
Distance from ring’s face to its inner wall

29. Piston Ring Dimensions (Cont.)
Ring back clearance
Difference between piston ring radial wall thickness and ring groove depth
Piston ring gap
Distance between ring’s ends where it installs in cylinder

30. Piston Ring Coatings Soft ring coatings Hard ring coatings
Porous metal, usually iron
Helps ring wear in quickly, forming a good seal
Hard ring coatings
Chrome or moly
Increase ring life and reduce friction

31. Piston Pin Construction
Normally made of case-hardened steel
Held in piston by one or two means
Press-fit
Forced tightly into connecting rod’s small end
Full-floating
Secured by snap rings

32. Piston Pin and Piston Boss Designs

33. Piston Pin and Piston Boss Designs (Cont.)

34. Piston Pin Offset
Locates piston pin hole slightly to one side of piston centerline
Helps quiet piston during use
Piston notch or marking on head
Indicates offset and front of piston

35. Exploded View of a Piston and Rod Assembly

36. Connecting Rod Construction
Made of steel
Must withstand tons of force as piston moves up and down in cylinder
Normally has an I-beam shape

37. Connecting Rod Construction (Cont.)
Some have oil spurt hole
Added lubrication for cylinder walls, piston pin, and surrounding parts
Drilled connecting rod
Machined hole through its entire length to supply oil to piston pin
(Federal Mogul)

38. Connecting Rod Numbers
Ensure proper location of each connecting rod in engine
Ensure rod cap installs on rod body correctly

39. Connecting Rod Construction (Cont.)
Broken-surface rod
Scribed and broken off to produce rough, irregular mating surface
Cannot be rebuilt

40. Crankshaft Construction
Usually made of cast iron or forged steel
Forged steel crankshafts for heavy-duty applications
Oil passages leading to rod and main bearings
Cast or drilled in crankshaft

41. Crankshaft Construction (Cont.)
Fully counterweighted crankshaft
Weights formed opposite every rod journal
Partially counterweighted crankshaft
Weights formed opposite journals in center portion of shaft only

42. Engine Bearing Construction
Sleeve-type or one-piece bearings
Press fit into block or head
Split-type or two-piece bearings
Consist of two halves
One half fits into block or head and other half fits into removable cap

43. Engine Bearing Construction (Cont.)
Types
Connecting rod bearings
Crankshaft main bearings
Camshaft bearings
(Federal Mogul)

44. Engine Bearing Construction (Cont.)
Any of three alloys can be plated over steel backing
Babbitt (lead-tin alloy)
Copper
Aluminum
May be used in
combination for
different
applications
(Federal Mogul)

45. Engine Bearing Characteristics
Must operate under
Tremendous loads
Severe temperature variations
Abrasive conditions
Corrosive surroundings

46. Engine Bearing Characteristics (Cont.)
Bearing load strength
Ability to withstand pounding and crushing during engine operation
Bearing conformability
Ability to adjust to imperfections in journal surface
Bearing embedability
Ability to absorb dirt, metal, or other hard particles

47. Engine Bearing Characteristics (Cont.)
Bearing corrosion resistance
Ability to withstand being acted on by acids, water, and other impurities in engine oil
Bearing crush
Used to help prevent split-type bearing from spinning inside its bore during engine operation
Bearing spread
Used on split-type engine bearing to hold bearing in place during assembly

48. Standard and Undersize Bearings
Standard bearing
Original dimensions specified by engine manufacturer for new crankshaft
Undersize bearing
Designed for crankshaft journal machined to smaller diameter

49. Bearing Locating Lugs and Dowels
Correctly position split bearings in bores
(Federal Mogul)

50. Bearing Oil Holes and Grooves
Permit bearing lubrication
Holes allow oil flow through block and between bearing and journal
Provide channel
Oil encircles bearing
(Chrysler)

51. Main Thrust Bearings and Thrust Washers
Limits crankshaft end play
Crankshaft end play
Forward and rearward movement of crankshaft
Thrust washers
Used instead of thrust bearings
Limit crank end play
(Chrysler)

52. Main Thrust Bearings and Thrust Washers (Cont.)
(Deere & Co.)

53. Rear Main Bearing Oil Seal Construction
Prevents oil leakage around back of crankshaft
Types of seals
Two-piece neoprene seal
One-piece neoprene seal
Wick or rope seal

54. Select-Fit Parts
Engine parts selected and installed in certain position to improve fit or clearance between parts
Pistons selected to fit precisely into cylinders

55. Balancer Shafts
Cancel vibrating forces produced by crankshaft, piston, and rod movement in some engines