Unit 5 Bottom End

ENGINE BOTTOM END

1

ENGINE BOTTOM END

• THE MAJOR PARTS OF AN ENGINE
BOTTOM END ARE:
• CYLINDER BLOCK
• CRANKSHAFT
• CONNECTING RODS
• ROD BEARINGS
• MAIN BEARINGS
• PISTON PINS
• PISTON RINGS
• PISTONS

2

CAST IRON ENGINE BLOCK

• A CAST IRON CYLINDER BLOCK IS VERY
HEAVY AND STRONG.

• NICKEL IS SOMETIMES ADDED TO
IMPROVE STRENGTH AND WEAR
RESISTANCE.

• THEY CAN WITHSTAND HIGH LOADS
FOUND IN TURBO-BOOST APPLICATIONS
IN GAS AND DIESEL ENGINES DESIGNED
FOR HIGHER HORSEPOWER.

3

ALUMINUM ENGINE BLOCK

• ALUMINUN ENGINE BLOCKS ARE MUCH
LIGHTER THAN CAST IRON, BUT
ALMOST AS STRONG.

• ALUMINUM BLOCKS DISSIPATE HEAT
BETTER THAN A CAST IRON BLOCK.

• MOST LATE MODEL VEHICLES USE
ALUMINUM BLOCKS TO REDUCE
WEIGHT AND INCREASE FUEL
ECONOMY.

4

CYLINDER SLEEVES

• CYLINDER SLEEVES ARE METAL
INSERTS THAT FIT INTO THE
CYLINDER BLOCK.
• THEY ACT AS CYLINDER WALLS FOR
THE PISTON TO SLIDE UP AND DOWN
WITHOUT BLOCK WEAR.
• TWO TYPES OF CYLINDER SLEEVES:
• DRY SLEEVES
• WET SLEEVES

5

CYLINDER SLEEVES

• DRY SLEEVES ARE PRESSED INTO A
CYLINDER THAT HAS BEEN BORED
OVERSIZE.
• DRY SLEEVES ARE THIN AND NOT
EXPOSED TO COOLANT.
• THE SLEEVE TOUCHES THE CYLINDER
WALLS OF THE BLOCK.
• DRY SLEEVES ARE USED IN SEVERELY
WORN OR DAMAGED CYLINDERS. THEY
ARE PRESSED IN AND THEN BORED OUT
TO THE ORIGINAL CYLINDER DIAMETER.

6

CYLINDER SLEEVES

• WET SLEEVES ARE EXPOSED TO ENGINE
COOLANT.
• THEY HAVE TO WITHSTAND COMBUSTION
PRESSURE AND HEAT WITHOUT THE
ADDED SUPPORT OF THE CYLINDER
BLOCK.
• CYLINDER HEAD GASKET KEEPS THE TOP
OF THE SLEEVE FROM LEAKING COOLANT.
• A SEAL IS USED AT THE BOTTOM TO
PREVENT COOLANT LEAKING INTO THE
CRANKCASE.

7

LINE BORING

• LINE BORING IS A PROCESS WHERE A
MACHINE SHOP CUTS THE CAM
BORES AND THE MAIN BORES
PERFECTLY STRAIGHT AND IN
ALIGNMENT WITH EACH OTHER.
• THIS PROCESS ALLOWS THE
CAMSHAFT AND CRANKSHAFT TO
TURN FREELY.

8

ENGINE BLOCK MAINS

•ENGINE MAINS ARE DESCRIBED BY THE NUMBER OF BOLTS HOLDING THE MAIN CAPS TO THE BLOCK.
•A TWO-BOLT MAIN USES TWO CAP BOLTS
•A FOUR-BOLT MAIN USES FOUR CAP BOLTS. FOUR-BOLT MAINS ARE USED ON HIGH PERFORMANCE ENGINES.

9

ENGINE BLOCK MAINS

• SIX-BOLT MAIN CAN WITHSTAND
MORE DOWNWARD CRANKSHAFT
PRESSURE WITHOUT PART FAILURE.
• ALSO REFERRED TO AS A
CROSSBOLTED BLOCK, THEY HAVE
EXTRA BOLTS THROUGH THE SIDES
OF THE BLOCK AND MAIN CAPS FOR
ADDED STRENGTH.

10

LOWER CRANKCASE

• A BLOCK GIRDLE, OR MAIN BEARING
BEDPLATE, IS A LARGE ONE-PIECE
CAP THAT FITS OVER THE ENTIRE
BOTTOM PART OF THE BLOCK.
• ALL THE MAIN CAPS ARE FORMED
AS ONE PIECE TO INCREASE
STRENGTH AND BLOCK STIFFNESS.

11

LOWER CRANKCASE

12

CRANKSHAFT

• USUALLY MADE OF CAST IRON
OR FORGED STEEL.
• CHANGES UP-AND-DOWN
MOTION OF THE PISTONS INTO
ROTATING MOTION.

13

PISTONS

• ENGINE PISTONS ARE CAST OR FORGED
FROM AN ALUMINUM ALLOY.
• CAST PISTONS ARE RELATIVELY SOFT AND
USED IN SLOW-SPEED, LOW-
PERFORMANCE ENGINES.
• FORGED PISTONS ARE USED IN HIGH-
PERFORMANCE OR FORCED INDUCTION
VEHICLES. THESE ARE EXPOSED TO
HIGHER TEMPERATURES AND STRESS
LOADS.
• PISTONS ARE DESIGNED FOR MAXIMUM
STRENGTH AND MINIMUM WEIGHT. THE
PISTON IS THICKER AT THE STRESS POINT,
THE TOP, WHERE IT CAN REACH
TEMPERATURES UP TO 650°F.

14

PISTON PARTS

• PISTON HEAD: TOP OF THE PISTON.
EXPOSED TO HEAT AND PRESSURE.
• RING GROOVES: THREE SLOTS
MACHINED IN THE PISTON. UPPER
TWO HOLD THE COMPRESSION
RINGS, LOWER GROOVE HOLDS THE
OIL CONTROL RINGS.
• RING LANDS: AREA BETWEEN AND
ABOVE THE RING GROOVES. THEY
SEPARATE AND SUPPORT THE RINGS
AS THE PISTON MOVES UP AND
DOWN.

15

PISTON PARTS

• PISTON SKIRT: THE SIDE OF THE
PISTON BELOW THE LAST RING. KEEPS
THE PISTON FROM TIPPING AND
JAMMING IN THE CYLINDER.
• PISTON PIN HOLE: MACHINED
THROUGH THE PISTON TO ACCEPT
THE PIN.
• PISTON BOSS: REINFORCED AREA
AROUND THE PIN HOLE. STRONG
ENOUGH TO SUPPORT THE PIN
UNDER SEVERE LOADS.

16

FLAT-TOP PISTON

• HAS A FLAT HEAD THAT IS PARALLEL
TO THE BLOCK SURFACE.
• VALVE RELIEFS ARE MACHINED INTO
THE PISTON TO PROVIDE EXTRA
PISTON-TO-VALVE CLEARANCE.

17

DISHED PISTON

• HEAD IS SUNKEN, OR RECESSED.
• THIS TYPE OF PISTON CAN BE/IS
USED TO LOWER COMPRESSION
PRESSURE IN A TURBOCHARGED OR
SUPERCHARGED ENGINE

18

DOMED PISTON

• HAS A HEAD THAT IS CONVEX,
OR CURVED UPWARD.
• THE PISTON HEAD FILLS THE
DOMED COMBUSTION
CHAMBER AND PRODUCES A
HIGHER COMBUSTION
PRESSURE.

19

DIESEL ENGINE PISTON

20

PISTON RINGS

• PISTONS NORMALLY USE THREE RINGS,
TWO COMPRESSION RINGS AND ONE
OIL RING.
• THE COMPRESSION RINGS PREVENT
PRESSURE LEAKAGE INTO THE
CRANKCASE. THEY ARE MADE OF CAST
IRON WITH A LAYER OF CHROME OR
OTHER METAL TO INCREASE WEAR
RESISTANCE.
• THE PRIMARY FUNCTION OF THE OIL
RINGS IS TO KEEP CRANKCASE OIL OUT
OF THE COMBUSTION CHAMBERS.

21

PISTON PIN

• PISTON PINS ARE HELD IN THE PISTON
BY ONE OF TWO MEANS:PRESS-FIT OR
SNAP RINGS.
• A PRESS-FIT PIN IS PRESSED TIGHTLY
INTO THE CONNECTING ROD’S SMALL
END; IT CAN ROTATE FREELY IN THE
PISTON PIN HOLE.
• A FULL-FLOATING PIN IS SECURED BY
SNAP RINGS. FULL-FLOATING PINS ARE
BETTER THAN PRESS-FIT BECAUSE THEY
REDUCE FRICTION AND WEAR.

22

CONNECTING RODS

• CONNECTING RODS ARE MADE OF
STEEL AND HAVE AN I-BEAM SHAPE.
THIS SHAPE HAS A HIGH STRENGTH-
TO-WEIGHT RATIO.
• SOME RODS HAVE AN OIL SPURT
HOLE OR ARE DRILLED THROUGH ITS
ENTIRE LENGTH TO SUPPLY OIL TO
THE PISTON PIN AND OTHER
SURROUNDING SURFACES.

23

CONNECTING RODS

24

CONNECTING RODS

• CONNECTING ROD CAPS CANNOT BE
MIXED UP OR TURNED AROUND.
• BROKEN–SURFACE OR FRACTURED
CONNECTING RODS ARE SCRIBED AND
BROKEN OFF WHEN MANUFACTURED,
PRODUCING A ROUGH AND IRREGULAR
SURFACE BETWEEN THE BODY AND THE
CAP.
• THIS PROCESS HELPS LOCK THE ROD
AND CAP INTO ALIGNMENT. THE
IRREGULAR SURFACES MATCH
PERFECTLY TO PREVENT SHIFTING
DURING ENGINE OPERATION.

25