14 Chapter Engine Bottom End Construction

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.

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.

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

Engine Bottom End Assembly (Honda)

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

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

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

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

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

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

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

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

Major Parts of a Piston

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

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

Basic Dimensions of a Piston

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

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

Piston Head Shape Flat-top piston Dished piston Domed piston

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

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

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

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

Piston Ring Construction (Cont.) (Ford)

Piston Ring Construction (Cont.) (Ford)

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

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

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

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

Piston Pin and Piston Boss Designs

Piston Pin and Piston Boss Designs (Cont.)

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

Exploded View of a Piston and Rod Assembly

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

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)

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

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

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

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

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

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

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)

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

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

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

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

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

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)

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)

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

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

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

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