Halderman ch033 lecture

© 2011 Pearson Education, Inc.All Rights Reserved

Automotive Technology, Fourth EditionJames Halderman

PISTONS, RINGS, AND CONNECTING RODS

33

33 PISTONS, RINGS, AND CONNECTING RODS



ObjectivesObjectives

• The student should be able to:– Prepare for Engine Repair (A1) ASE

certification test content area “C” (Engine Block Diagnosis and Repair).

– Describe the purpose and function of pistons, rings, and connecting rods.

– Explain how pistons and rods are constructed and what to look for during an inspection.




ObjectivesObjectives

• The student should be able to:– Discuss connecting rod reconditioning

procedures.– Explain how piston rings operate and how

to install them on a piston.




PISTONSPISTONS




PistonsPistons

• Purpose and Function– Three purposes

• Transfers force• Seals the combustion chamber• Conducts heat




PistonsPistons

• Parts Involved– Piston– Wrist pin– Crank throw/crankpin/connecting rod

bearing journal– Piston rings




Figure 33-1 The piston seals the bottom of the combustion chamber and is attached to a connecting rod.




PistonsPistons

• Piston Operation– Starts, accelerates, and stops twice in each

crankshaft revolution– Piston starts at the top of the cylinder and

ends at the top of the stroke




PistonsPistons

• Piston Operation– Piston head exposed to the hot combustion

gases – Skirt contacts the relatively cool cylinder

wall• Results in a temperature difference of about

275°F (147°C) between the top and bottom of the piston




PistonsPistons

• Piston Operation– NOTE: A typical piston in an engine

operating at 4000 RPM accelerates from 0 to 60 mph (97 km/h) in about 0.004 second (4 ms) as it travels about halfway down the cylinder.




PISTON PISTON CONSTRUCTIONCONSTRUCTION




Piston Construction Piston Construction

• Piston Ring Grooves– Located between the piston head and skirt– Factors that determine minimum piston

height:• Width of grooves





• Piston Ring Grooves– Factors that determine minimum piston

height:• Width of lands between ring grooves• Number of rings





• Piston Ring Grooves– Outside diameter lands is about 0.02 to

0.04 in. (0.5 to 1 mm) smaller than the skirt diameter




Figure 33-2 All pistons share the same parts in common.




Figure 33-3 A piston diameter is measured across the thrust surfaces.




Piston ConstructionPiston Construction

• Cast Pistons– Usually made using gravity die casting– Molten aluminum alloy and about 10%

silicon are poured into a mold





• Cast Pistons– Silicon increases the strength and helps

control the expansion of the piston when it gets hot





• Cast Pistons– Metals used in the aluminum alloy include

copper, nickel, manganese, and magnesium




Figure 33-4 A cast piston showing the sprues which were used to fill the mold with molten aluminum alloy.





• Hypereutectic Pistons– Eutectic pistons contain about 9% to 12%

silicon – Hypereutectic pistons are stronger due to a

16% silicon content





• Hypereutectic Pistons– Advantages:

• Strength• 25% weight reduction • Lower expansion rate





• Hypereutectic Pistons– Disadvantages:

• Higher cost• More difficult to cast and machine





• Forged Pistons– Dense grain structure– Very strong– Often used in turbocharged or

supercharged engines





• Forged Pistons– Less porous than cast pistons– Conduct heat more quickly than cast

pistons– Run about 20% cooler than cast pistons




Figure 33-5 The top of the piston temperature can be 100°F (38°C) lower on a forged piston compared to a cast piston.





• Piston Head Designs– Shape is vital to the combustion process





• Piston Head Designs– Flat-top pistons

• Close to the cylinder head • Recesses are cut in the piston top for valve

clearance– Commonly called: eyebrows, valve reliefs,

valve pockets





• Piston Head Designs– Pistons in high-powered engines may have

raised domes (pop-ups) on the heads





• Piston Head Designs– Pistons in other engines may be provided

with a depression (dish)– Varying dish depths provide different

compression ratios





• Piston Head Designs– NOTE: Newer engines do not use valve

reliefs because this requires that the thickness of the top of the piston be increased to provide the necessary strength. The thicker the top of the piston, the farther down from the top the top piston ring sits.





• Piston Head Designs– NOTE: To reduce unburned hydrocarbon

(HC) exhaust emissions, engineers attempt to place the top piston ring as close to the top of the piston as possible to prevent the unburned fuel from being trapped (and not burned) between the top of the piston and the top of the top piston ring.




Figure 33-6 Valve reliefs are used to provide valve clearance.





• Slipper Skirt Pistons– Shorter on the two sides that are not thrust

surfaces– Advantages include:

• Lighter weight





• Slipper Skirt Pistons– Advantages include:

• Allows for shorter overall engine height

– Most engines today use a slipper skirt piston





• Cam Ground Pistons– Provides method of expansion control – Piston thrust surfaces closely fit the

cylinder– Piston pin boss diameter is fitted loosely





• Cam Ground Pistons– Expands along the piston when heated– Nearly round at its normal operating

temperatures




Figure 33-7 Piston cam shape. The largest diameter is across the thrust surfaces and perpendicular to the piston pin (labeled A).





• Piston Finish– Varies with manufacturer– All finishes reduce scuffing (condition

where the metal of the piston actually contacts the cylinder wall)





• Piston Finish– Reduce scuffing by coating piston skirts

with tin 0.0005 in. (0.0125 mm) thick or a moly graphite coating




Figure 33-8 A moly graphite coating on this piston from a General Motors 3800 V-6 engine helps to prevent piston scuffing.





• Piston Head Size– Smaller in diameter than the rest of piston– Horizontal separation slots act as heat

dams





• Piston Head Size– Slots reduce heat transfer from the piston

head to the lower skirt• Keeps the skirt temperature lower to reduce

skirt expansion• Can be used for oil drainback and expansion

control




Figure 33-9 The head of the piston is smaller in diameter than the skirt of the piston to allow it to expand when the engine is running.





• Piston Strut Inserts– Add strength to the piston in the piston pin

area– Help control thermal expansion





• Piston Strut Inserts– Good piston-to-cylinder wall clearance at

normal temperatures– Cold operating clearance as small as

0.0005 in. (0.0127 mm)• Will prevent cold piston slap and noise




Figure 33-10 Steel struts cast inside the piston help control expansion and add strength to the piston pin area.




PISTON PINSPISTON PINS




Piston PinsPiston Pins

• Terminology– Attaches the piston to the connecting rod– Also known as wrist pins or gudgeon pins





• Terminology– Transfers the force produced by

combustion chamber pressures and piston inertia to the connecting rod





• Terminology– Made from high-quality steel in the shape

of a tube – Interior hole is sometimes tapered




Figure 33-11 Most piston pins are hollow to reduce weight and have a straight bore. Some pins have a tapered bore to reinforce the pin.




Piston Pins Piston Pins

• Piston Pin Offset– Some piston pin holes are not centered in

the piston– Located toward the major thrust surface,

approximately 0.062 in. (1.57 mm) from the piston centerline





• Piston Pin Offset– Designed to reduce piston slap and the

noise• Minor thrust• Major thrust





• Piston Pin Offset– NOTE: Not all piston pins are offset. In fact,

many engines operate without the offset to help reduce friction and improve power and fuel economy.

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Figure 33-12 Piston pin offset toward the major thrust surface.




Figure 33-13 Engine rotation and rod angle during the power stroke cause the piston to press harder against one side of the cylinder, called the major thrust surface.





• Piston Pin Fit– Size is held to tens of thousandths of an

inch– Loose pins will make a sound while engine

is running (double knock)





• Piston Pin Fit– Tight pins will restrict piston expansion

along the pin diameter and lead to piston scuffing

– Normal clearances range from 0.0005 to 0.0007 in. (0.0126 to 0.018 mm)




PISTON PIN RETAINING PISTON PIN RETAINING METHODSMETHODS




Piston Pin Retaining Methods Piston Pin Retaining Methods

• Full Floating– Free to “float” in the connecting rod and

the piston– Retaining device keeps piston from

scraping against the cylinder wall





• Full Floating– Most often used in high-performance

modified engines– Some type of lock ring used to retain the

piston pin





• Full Floating– Two common types of lock rings:

• Internal snap ring• Spiral lock ring




Figure 33-14 Circlips hold full-floating piston pins in place.




Piston Pin Retaining MethodsPiston Pin Retaining Methods

• Interference Fit– Connecting rod hole slightly smaller than

the piston pin– Pin is installed by heating the rod or by

pressing it into the rod




Piston Pin Retaining MethodsPiston Pin Retaining Methods

• Interference Fit– Be careful to have the correct hole sizes

and center the pin – Least expensive method to use




Figure 33-15 A typical interference fit piston pin.




PISTON RINGSPISTON RINGS




Piston RingsPiston Rings

• Purpose and Function– Form a sliding combustion chamber seal to

prevent the high-pressure combustion gases from leaking past the piston

– Keep engine oil from entering the combustion chamber





• Purpose and Function– Transfer some of the piston heat to the

cylinder wall, where it is removed from the engine through the cooling system




Figure 33-16 The rings conduct heat from the piston to the cylinder wall.





• Classifications– Classified in two types:

• Two compression rings• One oil control ring





• Classifications– NOTE: Some engines, such as Honda high-

fuel economy engines, use pistons with only two rings: one compression ring and one oil control ring.





• Compression Rings– Forms seal between the moving piston and

the cylinder wall to get maximum power from the combustion pressure





• Compression Rings– Must keep friction at a minimum– Space in the ring groove above the ring is

called side clearance – Space behind the ring is called the back

clearance




Figure 33-17 Combustion chamber pressure forces the ring against the cylinder wall and the bottom of the ring groove to effectively seal the cylinder.




Figure 33-18 The side and back clearances must be correct for the compression rings to seal properly.





• Oil Control Rings– Scraping action allows oil to return through

the expander and openings in the piston– Spacer expander lies between the top and

bottom rails





• Oil Control Rings– Spacer expander keeps the rails separated

and pushes them out against the cylinder wall




Figure 33-19 This typical three-piece oil control ring uses a hump-type stainless steel spacer-expander. The expander separates the two steel rails and presses them against the cylinder wall.





• Ring Gap– Allow some leakage past the top

compression ring• Leakage provides pressure on the second

ring to create seal





• Ring Gap– Amount of gap is critical

• A gap that is too great will allow excessive blowby (leakage of combustion gases past the rings)





• Ring Gap– Amount of gap is critical

• A gap that is too little will allow the piston ring ends to touch together when the engine is hot, causing excessive wear and possible engine failure





• Ring Gap– Gaps reduce losses of high-pressure

combustion gases




Figure 33-20 Typical piston ring gaps.





• Piston Ring Shapes– Taper face ring– Positive twist ring– Reverse twist ring





• Piston Ring Shapes– Scraper ring– Barrel face ring




Figure 33-21 The taper face ring provides oil control by scraping the cylinder wall. This style of ring must be installed right side up or the ring will not seal and oil will be drawn into the combustion chamber.




Figure 33-22 Torsional twist rings provide better compression sealing and oil control than regular taper rings.




Figure 33-23 Scraper-type rings provide improved oil control.




Figure 33-24 The upper barrel face ring has a line showing contact with the cylinder wall. The second taper face ring shows contact along the lower edge of the ring.




PISTON RING PISTON RING CONSTRUCTIONCONSTRUCTION




Piston Ring ConstructionPiston Ring Construction

• Piston Ring Materials– Plain cast iron– Pearlitic cast iron– Nodular cast iron





• Piston Ring Materials– Steel– Ductile iron





• Chromium Piston Rings– Greatly increases piston ring life– Slightly chamfered at the outer corners– Rings are prelapped or honed before

packaging




Figure 33-25 The chrome facing on this compression ring is about 0.004 in. (0.10 mm) thick.





• Molybdenum Piston Rings– Introduced in the early 1960s– Plasma method is a spray method used to

deposit molybdenum on cast iron to produce a long-wearing and low-friction piston ring





• Molybdenum Piston Rings– Most have a 0.004 to 0.008 in. (0.1 to 0.2

mm) groove filled with molybdenum cut into its face





• Molybdenum Piston Rings– Will survive under high-temperature and

scuffing conditions better than chromium face rings




Figure 33-26 The moly facing on this compression ring is about 0.005 in. (0.13 mm) thick.





• Moly-Chrome-Carbide Rings– Used in some original equipment (OE) and

replacement applications





• Moly-Chrome-Carbide Rings– Coating properties hardness of chrome and

carbide combined with heat resistance of molybdenum





• Ceramic-Coated Rings– Ceramic coating applied through a process

called physical vapor deposition (PVD)– Used when additional heat resistance is

needed




CONNECTING RODSCONNECTING RODS




Connecting RodsConnecting Rods

• Purpose and Function– Transfers the force and reciprocating

motion of the piston to the crankshaft





• Purpose and Function– Small end reciprocates with the piston– Large end rotates with the crankpin





• Purpose and Function– Connecting rods are manufactured by

casting, forging, and powdered (sintered) metal processes




Figure 33-27 The connecting rod is the most highly stressed part of any engine because combustion pressure tries to compress it and piston inertia tries to pull it apart.




Figure 33-28 The I-beam shape (top rod) is the most common, but the H-beam shape is common in high-performance and racing engine applications.





• Connecting Rod Design– Big end must be a perfect circle– Once a rod and cap are initially machined,

they must remain a “matched set”





• Connecting Rod Design– Assembly bolt holes are closely reamed in

both the cap and connecting rod to ensure alignment





• Connecting Rod Design– Bolts have piloting surfaces that closely fit

the reamed holes– Made with balancing bosses (pads) so

weight can be adjusted to specifications





• Connecting Rod Design– Some have a spit hole that bleeds some of

the oil from the connecting rod journal




Figure 33-29 Rod bolts are quickly removed using a press.




Figure 33-30 Some rods have balancing pads on each end of the connecting rod.




Figure 33-31 Some connecting rods have spit holes to help lubricate the cylinder wall or piston pin.




Figure 33-32 Some engines, such as this Ford diesel, are equipped with oil squirters that spray or stream oil toward the underneath side of the piston head to cool the piston.





• Cast Connecting Rods– Can be identified by their narrow parting

line





• Forged Connecting Rods– Generally used in heavy-duty and high-

performance engines– Lighter weight and stronger but more

expensive





• Forged Connecting Rods– Can be identified by their wide parting line– Many high-performance rods use a bronze

bushing in the small end




Figure 33-33 A cast connecting rod is found on many stock engines and can be identified by the thin parting line.




Figure 33-34 This high-performance connecting rod uses a bronze bushing in the small end of the rod and oil hole to allow oil to reach the full-floating piston pin.





• Powdered Metal Connecting Rods– Used by most new production engines– Advantages over convention cast (forged)

rods including precise weight control





• Powdered Metal Connecting Rods– Created with measured amount of material

so rod and engine are balanced without extra weighting and machining operations





• Powdered Metal Connecting Rods– Start as powdered metal (iron, copper,

carbon, other alloying agents)– Powder placed in a die and compacted

(forged)





• Powdered Metal Connecting Rods– Part is heated, without melting, to about

2,000°F. – Ingredients are transformed into

metallurgical bonds





• Powdered Metal Connecting Rods– Machining includes boring and drilling – Big end is fractured to help ensure a

perfect match when assembled




Figure 33-35 Powdered metal connecting rods feature a fractured parting line at the big end of the rod.




CONNECTING ROD CONNECTING ROD SERVICESERVICE




Connecting Rod ServiceConnecting Rod Service

• Removing Pistons From Rods– Removed using a special fixture





• Inspection– Check rod for a twist before reconditioning – Hole at the small end and the hole at the

big end should be parallel





• Inspection– Twists greater than 0.002 in. (0.05 mm) are

not acceptable – Some specialty shops can remove twist by

bending the rod cold





• Inspection– Both cast and forged rods can be

straightened– Many engine builders replace the

connecting rod if it is twisted




Figure 33-36 A press used to remove the connecting rod from the piston.




Figure 33-37 If the rod is twisted, it will cause diagonal-type wear on the piston skirt.




Figure 33-38 A rod alignment fixture is used to check a connecting rod for bends or twists.





• Reconditioning Procedure– STEP 1: Parting surfaces of the rod and cap

are smoothed. A couple thousandths of an inch of metal is removed from the rod cap parting surface. The amount removed from the rod and cap only reduces the bore size 0.003 to 0.006 in. (0.08 to 0.15 mm).





• Reconditioning Procedure– STEP 2: The cap is installed on the rod, and

the nuts or cap screws are properly torqued. The hole is then bored or honed to be perfectly round and of the size and finish required to give the correct connecting rod bearing crush.





• Reconditioning Procedure– NOTE: Powdered metal connecting rods

cannot be reconditioned using this method. Most manufacturers recommend replacing worn powdered metal connecting rods.




Figure 33-39 Rod bearing bores normally stretch from top to bottom, with most wear concentrated on the rod cap.




Figure 33-40 To help ensure that the big ends are honed straight, many experts recommend placing two rods together when performing the honing operation.




PISTON AND ROD PISTON AND ROD ASSEMBLYASSEMBLY




Piston and Rod AssemblyPiston and Rod Assembly

• Interference Fit Rods– Pin is put in one side of the piston– Check small end of connecting rod for

proper size





• Interference Fit Rods– Small eye on connecting rod is heated

before pin is installed causing rod eye to expand

– Pin must be rapidly pushed into the correct center position




Figure 33-41 The small end of the rod is being heated in an electric heater and the piston is positioned properly so the piston pin can be installed as soon as the rod is removed from the heater.





• Full-Floating Rods– Full-floating piston pins operate in a

bushing in the small eye of the connecting rod





• Full-Floating Rods– Bushings and pistons are honed to the

same diameter to allow the pin to slide freely through both





• Full-Floating Rods– Pin is held in place with a lock ring at each

end– Lock rings expand into a small groove in

the pin hole

• NOTE: The original lock rings should always be replaced with new rings.




PISTON RING SERVICEPISTON RING SERVICE




Piston Ring ServicePiston Ring Service

• Steps– Check side clearance– Check ring gap– Install oil control ring




Piston Ring ServicePiston Ring Service

• Steps– Install compression rings– Double-check everything




Figure 33-42 The side clearance of the piston ring is checked with a feeler gauge.




Figure 33-43 The ring gap is measured using a feeler gauge.




Figure 33-44 A hand-operated piston ring end gap grinder being used to increase the end gap of a piston ring so that it is within factory specifications.




Figure 33-45 A typical ring expander being used to install a piston ring on a piston.




Figure 33-46 Identification marks used to indicate the side of the piston ring to be placed toward the head of the piston.

Halderman ch033 lecture

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molten aluminum

major thrust

piston strut

taper face

moly graphite

cam ground

pressure combustion

piston ring