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  • Figure 30-1 The seats and guides for the valves are in the cylinder head as well as the camshaft and the entire valve train if it is an overhead camshaft design.
  • Figure 30-2 A wedge-shaped combustion chamber showing the squish area where the air-fuel mixture is squeezed, causing turbulence that pushes the mixture toward the spark plug.
  • Figure 30-3 Locating the spark plug in the center of the combustion chamber reduces the distance the flame front must travel.
  • Figure 30-4 The combustion chamber of the 5.7 liter Chrysler Hemi cylinder head shows the two spark plugs used to ensure rapid burn for best power and economy with the lowest possible exhaust emissions.
  • Figure 30-5 The shrouded area around the intake valve causes the intake mixture to swirl as it enters the combustion chamber.
  • Figure 30-6 A typical cross flow cylinder head design, where the flow into and out of the combustion chamber is from opposite sides of the cylinder head.
  • Figure 30-7 Method for measuring the valve opening space.
  • Figure 30-8 Comparing the valve opening areas between a twoand three-valve combustion chamber when the valves are open.
  • Figure 30-9 Typical four-valve head. The total area of opening of two small intake valves and two smaller exhaust valves is greater than the area of a two-valve head using much larger valves. The smaller valves also permit the use of smaller intake runners for better low-speed engine response.
  • Figure 30-10 Four valves in a pentroof combustion chamber.
  • Figure 30-11 An Audi five-valve cylinder head, which uses three intake valves and two exhaust valves.
  • Figure 30-12 The intake manifold design and combustion chamber design both work together to cause the air-fuel mixture to swirl as it enters the combustion chamber.
  • Figure 30-13 A port-injected engine showing the straight free-flowing intake and exhaust ports.
  • Figure 30-14 A cutaway head showing the coolant passages in green.
  • Figure 30-15 Coolant flows through the cylinder head, and the passages are sealed by the head gasket.
  • Figure 30-16 Overhead camshafts may be (a) held in place with bearing caps, (b) supported by towers, or (c) fitted into bearing bores machined directly into the head.
  • Figure 30-17 Always follow the specified loosening sequence to prevent valve spring tension from bending the camshaft.
  • Figure 30-18 Pushrods can be kept labeled if stuck through a cardboard box. Individual parts become worn together. Using cardboard is a crude but effective material to keep all valve train parts together and labeled exactly as they came from the engine.
  • Figure 30-19 Cylinder heads should be checked in five planes for warpage, distortion, bend, and twist.
  • Figure 30-20 A precision ground straightedge and a feeler gauge are used to check the cylinder head for flatness.
  • Figure 30-21 Warped overhead camshaft cylinder head. If the gasket surface is machined to be flat, the camshaft bearings will still not be in proper alignment. The solution is to straighten the cylinder head or to align bore the cam tunnel.
  • Figure 30-22 A cast-iron cylinder head being resurfaced using a surface grinder.
  • Figure 30-23 A graph showing a typical rough surface as would be viewed through a magnifying glass. RA is an abbreviation indicating the average height of all peaks and valleys.
  • Figure 30-24 The material that must be removed for a good manifold fit.
  • Figure 30-25 Using an intake manifold template to check for the proper angles after the cylinder heads have been machined.
  • Figure 30-26 An integral valve guide is simply a guide that has been drilled into the cast-iron cylinder head.
  • Figure 30-27 All aluminum cylinder heads use valve guide inserts.
  • Figure 30-28 Valve guides often wear to a bell-mouth shape to both ends due to the forces exerted on the valve by the valve train components.
  • Figure 30-29 A small-hole gauge and a micrometer are being used to measure the valve guide. The guide should be measured in three places: at the top, middle, and bottom.
  • Figure 30-30 The diameter of the valve stem is being measured using a micrometer. The difference between the inside diameter of the valve guide and the diameter of the valve stem is the valve guide-to-stem clearance.
  • Figure 30-31 Measuring valve guide-to-stem clearance with a dial indicator while rocking the stem in the direction of normal thrust. The reading on the dial indicator should be compared to specifications because it does not give the guide-to-stem clearance directly. The valve is usually held open to its maximum operating lift.
  • Figure 30-32 Sectional view of a knurled valve guide.
  • Figure 30-33 Valve guide replacement procedure.
  • Figure 30-34 A type of fixture required to bore the valve guide to accept a thin-walled insert sleeve.
  • Figure 30-35 Trimming the top of the thin-walled insert.

Halderman ch030 lecture Halderman ch030 lecture Presentation Transcript

  • CYLINDER HEAD AND VALVE GUIDE SERVICE 30
  • Objectives
    • The student should be able to:
      • Prepare for ASE Engine Repair (A1) certification test content area “B” (Cylinder Head and Valve Train Diagnosis and Repair).
      • Identify combustion chamber types.
  • Objectives
    • The student should be able to:
      • List the steps necessary to recondition a cylinder head.
      • Describe how to inspect and measure valve guides.
      • Discuss valve guide repair options.
  • INTRODUCTION
  • Introduction
    • Repair of cylinder heads is the most frequent engine repair
    • Highest temperatures and pressures in the engine are in the combustion chamber of the cylinder head
    • Valves open and close thousands of times during engine operation
  • CYLINDER HEADS
  • Cylinder Heads
    • Construction
      • Supports the valves and valve train
      • Handles flow of intake, exhaust gases, coolant, and sometimes oil
  • Figure 30-1 The seats and guides for the valves are in the cylinder head as well as the camshaft and the entire valve train if it is an overhead camshaft design.
  • Cylinder Heads
    • Design Features
      • Squish area
      • Quench area
      • Spark plug placement
  • Cylinder Heads
    • Design Features
      • Surface-to-volume ratio
      • Valve shrouding
      • Crossflow valve placement
  • Figure 30-2 A wedge-shaped combustion chamber showing the squish area where the air-fuel mixture is squeezed, causing turbulence that pushes the mixture toward the spark plug.
  • Figure 30-3 Locating the spark plug in the center of the combustion chamber reduces the distance the flame front must travel.
  • Figure 30-4 The combustion chamber of the 5.7 liter Chrysler Hemi cylinder head shows the two spark plugs used to ensure rapid burn for best power and economy with the lowest possible exhaust emissions.
  • Figure 30-5 The shrouded area around the intake valve causes the intake mixture to swirl as it enters the combustion chamber.
  • Figure 30-6 A typical cross flow cylinder head design, where the flow into and out of the combustion chamber is from opposite sides of the cylinder head.
  • Cylinder Heads
    • Combustion Chamber Designs
      • Created as two parts
      • Upper part: cylinder head and walls
      • Lower part: top of the piston
  • Cylinder Heads
    • Combustion Chamber Designs
      • Most common shapes include:
        • Wedge
        • Pentroof
        • Hemi
    ?
  • Cylinder Heads
    • Four-Valve Cylinder Heads
      • More than two valves allow more gas to flow through the engine
      • Valve duration: number of degrees by which the crankshaft rotates when the valve is off the valve seat
  • Cylinder Heads
    • Four-Valve Cylinder Heads
      • Maximum amount of gas moving through the opening of a valve depends on the distance around the valve and the degree to which it lifts open
  • Cylinder Heads
    • Four-Valve Cylinder Heads
      • The distance around a valve is calculated by the equation:
        • pi × D or 3.1416 × valve diameter
  • Cylinder Heads
    • Four-Valve Cylinder Heads
      • Four valves on the pentroof design are operated with dual overhead camshafts or with single overhead camshafts and rocker arms
  • Cylinder Heads
    • Four-Valve Cylinder Heads
      • Four valves make it is possible to place the sparkplug at the center of the combustion chamber
  • Figure 30-7 Method for measuring the valve opening space.
  • Figure 30-8 Comparing the valve opening areas between a twoand three-valve combustion chamber when the valves are open.
  • Figure 30-9 Typical four-valve head. The total area of opening of two small intake valves and two smaller exhaust valves is greater than the area of a two-valve head using much larger valves. The smaller valves also permit the use of smaller intake runners for better low-speed engine response.
  • Figure 30-10 Four valves in a pentroof combustion chamber.
  • Figure 30-11 An Audi five-valve cylinder head, which uses three intake valves and two exhaust valves.
  • INTAKE AND EXHAUST PORTS
  • Intake and Exhaust Ports
    • Purpose and Function
      • Port: part of the intake or exhaust system passage cast in the cylinder head
  • Intake and Exhaust Ports
    • Purpose and Function
      • Ports lead from the manifolds to the valves
  • Intake and Exhaust Ports
    • Purpose and Function
      • Inline engines may have both on the same side of the engine
      • Cylinders on older engines could share a port due to restricted space called Siamese ports
  • Intake and Exhaust Ports
    • Intake Ports
      • Larger ports and better breathing are possible in engines when the intake and exhaust ports are on opposite sides
  • Intake and Exhaust Ports
    • Intake Ports
      • A restricting hump within a port could increase its airflow capacity
  • Intake and Exhaust Ports
    • Intake Ports
      • Modifications in the field, such as porting or relieving, restrict the flow of a carefully designed port
  • Intake and Exhaust Ports
    • Intake Ports
      • Intake manifold and combustion chamber design work together to cause the air-fuel mixture to swirl in the combustion chamber
  • Figure 30-12 The intake manifold design and combustion chamber design both work together to cause the air-fuel mixture to swirl as it enters the combustion chamber.
  • Intake and Exhaust Ports
    • Exhaust Ports
      • Designed to allow free flow of exhaust gases from the engine
      • Shorter than the intake ports to help reduce the amount of heat transferred to the coolant
  • Figure 30-13 A port-injected engine showing the straight free-flowing intake and exhaust ports.
  • CYLINDER HEAD PASSAGES
  • Cylinder Head Passages
    • Coolant Flow Passages
      • From the coolest to the warmest portions of the engine
      • Water pump takes the coolant from the radiator
  • Cylinder Head Passages
    • Coolant Flow Passages
      • Circulated through the block and directed around the cylinders
      • Flows upward through the gasket to the cooling passages in the cylinder head
  • Cylinder Head Passages
    • Coolant Flow Passages
      • Heated coolant is collected and returned to the radiator to be cooled and recycled
  • Cylinder Head Passages
    • NOTE: Reversed-flow cooling systems, such as that used on the Chevrolet LT1 V-8, send the coolant from the radiator to the cylinder heads first. This results in a cooler cylinder head and allows for more spark advance without engine-damaging detonation.
  • Figure 30-14 A cutaway head showing the coolant passages in green.
  • Cylinder Head Passages
    • Head Gasket Holes
      • Gasket surface of head leading to cooling passages has large holes
  • Cylinder Head Passages
    • Head Gasket Holes
      • Head gasket performs an important coolant flow function when openings are too large
  • Cylinder Head Passages
    • Head Gasket Holes
      • Special-size and smaller holes correct the coolant flow rate at each opening
  • Cylinder Head Passages
    • Head Gasket Holes
      • Correct head gasket installation is necessary for proper engine cooling
  • Figure 30-15 Coolant flows through the cylinder head, and the passages are sealed by the head gasket.
  • Cylinder Head Passages
    • Lubricating Oil Passages
      • Lubricating oil is delivered to overhead valve mechanism
      • Special openings in head gasket allow oil to pass between the block and head without leaking
  • Cylinder Head Passages
    • Lubricating Oil Passages
      • Oil returns to the oil pan through oil return passages
  • Cylinder Head Passages
    • NOTE: Many aluminum cylinder heads have smaller-than-normal drainback holes. If an engine has excessive oil consumption, check the drain holes before removing the engine.
  • CYLINDER HEAD SERVICING
  • Cylinder Head Servicing
    • Cylinder Head Servicing Sequence
      • Disassemble and thoroughly clean the heads
      • Check for cracks and repair as necessary
  • Cylinder Head Servicing
    • Cylinder Head Servicing Sequence
      • Check the surface that contacts the engine block and machine, if necessary
  • Cylinder Head Servicing
    • Cylinder Head Servicing Sequence
      • Check valve guides and replace or service, as necessary
      • Grind valves and reinstall them in the cylinder head with new valve stem seals
  • Cylinder Head Servicing
    • Disassembling Overhead Camshaft Head
      • When one-piece bearings are used, the valve springs have to be compressed with a fixture or the finger follower has to be removed
  • Cylinder Head Servicing
    • Disassembling Overhead Camshaft Head
      • When bearing caps are used, loosen valve springs alternately so that bending loads are not placed on either the cam or bearing caps
  • Figure 30-16 Overhead camshafts may be (a) held in place with bearing caps, (b) supported by towers, or (c) fitted into bearing bores machined directly into the head.
  • Figure 30-17 Always follow the specified loosening sequence to prevent valve spring tension from bending the camshaft.
  • Cylinder Head Servicing
    • Valve Train Disassembly
      • Disassemble cylinder head
      • Keep the top part of the pushrod at the top
  • Cylinder Head Servicing
    • Valve Train Disassembly
      • Keep the rocker arms with the same pushrods
      • Intake and exhaust valve springs must be kept with the correct valve
  • Figure 30-18 Pushrods can be kept labeled if stuck through a cardboard box. Individual parts become worn together. Using cardboard is a crude but effective material to keep all valve train parts together and labeled exactly as they came from the engine.
  • Cylinder Head Servicing
    • Cylinder Head Inspection
      • Clean surface and inspect as follows:
        • Remove old gasket material and draw a file across the surface of the head to remove any small burrs
  • Cylinder Head Servicing
    • Cylinder Head Inspection
      • Clean surface and inspect as follows:
        • Check the head in five planes for warpage, distortion, bends, and twists (slide a 0.004 in. (0.1 mm) feeler gauge under a precision straightedge held against the head surface; clearance between the two should not vary by more than 0.004 in. overall)
  • Cylinder Head Servicing
    • Cylinder Head Inspection
      • NOTE: The cylinder head surface that mates with the top deck of the block is often called the fire deck.
  • Cylinder Head Servicing
    • Cylinder Head Inspection
      • NOTE: Always check the cylinder head thickness and specifications to be sure that material can be safely removed from the surface. Some manufacturers do not recommend any machining, but rather require cylinder head replacement if cylinder head surface flatness is not within specifications.
  • Figure 30-19 Cylinder heads should be checked in five planes for warpage, distortion, bend, and twist.
  • Figure 30-20 A precision ground straightedge and a feeler gauge are used to check the cylinder head for flatness.
  • ALUMINUM CYLINDER HEAD STRAIGHTENING
  • Aluminum Cylinder Head Straightening
    • Purpose and Function
      • STEP 1: Determine the amount of warpage with a straightedge and thickness (feeler) gauge. Cut shim stock (thin strips of metal) to one-half of the amount of the warpage. Place shims of this thickness under each end of the head.
  • Aluminum Cylinder Head Straightening
    • Purpose and Function
      • STEP 2: Tighten the center of the cylinder head down on a strong, flat base. A 2 in. thick piece of steel that is 8 in. wide by 20 in. long makes a good support for the gasket surface of the cylinder head (use antiseize compound on the bolt threads to help in bolt removal).
  • Aluminum Cylinder Head Straightening
    • Purpose and Function
      • STEP 3: Place the head and base in an oven for five hours at 500°F (260°C). Turn the oven off and leave the assembly in the oven.
  • Aluminum Cylinder Head Straightening
    • Purpose and Function
      • NOTE: If the temperature is too high, the valve seat inserts may fall out of the head! At 500°F, a typical valve seat will still be held into the aluminum head with a 0.002 in. interference fit based on calculations of thermal expansion of the aluminum head and steel insert.
  • Figure 30-21 Warped overhead camshaft cylinder head. If the gasket surface is machined to be flat, the camshaft bearings will still not be in proper alignment. The solution is to straighten the cylinder head or to align bore the cam tunnel.
  • Aluminum Cylinder Head Straightening
    • Purpose and Function
      • Cool head in oven at least four to five hours
      • Heating/cooling process can be repeated if necessary
  • Aluminum Cylinder Head Straightening
    • Purpose and Function
      • Gasket surface (fire deck) can be machined in the usual manner after straightening
  • CYLINDER HEAD RESURFACING
  • Cylinder Head Resurfacing
    • Refinishing Methods
      • Two common methods:
        • Milling or broaching uses metal-cutting tool bits fastened in a disc
  • Cylinder Head Resurfacing
    • Refinishing Methods
      • Two common methods:
        • Grinding uses a large-diameter abrasive wheel
      • Both can be done with table-type or precision-type resurfacers
  • Cylinder Head Resurfacing
    • Refinishing Methods
      • NOTE: Resurfacing the cylinder head changes the compression ratio of the engine by about 1/10 point per 0.01 in. of removed material. For example, the compression ratio would be increased from 9.0:1 to 9.2:1 if 0.02 in. were removed from a typical cylinder head.
  • Figure 30-22 A cast-iron cylinder head being resurfaced using a surface grinder.
  • Cylinder Head Resurfacing
    • Surface Finish
      • Measured in microinches (abbreviated “μ in.”)
      • Arithmetic average roughness height (RA) is used to express surface finish
  • Cylinder Head Resurfacing
    • Surface Finish
      • Root-mean-square (RMS) classification is becoming obsolete
  • Cylinder Head Resurfacing
    • Surface Finish
      • Surface finish roughness recommendations for cylinder heads and blocks include the following:
        • Cast iron
          • Maximum: 110 RA (125 RMS)
  • Cylinder Head Resurfacing
    • Surface Finish
      • Surface finish roughness recommendations for cylinder heads and blocks include the following:
        • Cast iron
          • Minimum: 30 RA (33 RMS)
  • Cylinder Head Resurfacing
    • Surface Finish
      • Surface finish roughness recommendations for cylinder heads and blocks include the following:
        • Cast iron
          • Recommended range: 60 to 100 RA (65 to 110 RMS)
  • Cylinder Head Resurfacing
    • Surface Finish
      • Surface finish roughness recommendations for cylinder heads and blocks include the following:
        • Aluminum
          • Maximum: 60 RA (65 RMS)
  • Cylinder Head Resurfacing
    • Surface Finish
      • Surface finish roughness recommendations for cylinder heads and blocks include the following:
          • Minimum: 30 RA (33 RMS)
  • Cylinder Head Resurfacing
    • Surface Finish
      • Surface finish roughness recommendations for cylinder heads and blocks include the following:
          • Recommended range: 50 to 60 RA (55 to 65 RMS)
  • Figure 30-23 A graph showing a typical rough surface as would be viewed through a magnifying glass. RA is an abbreviation indicating the average height of all peaks and valleys.
  • INTAKE MANIFOLD ALIGNMENT
  • Intake Manifold Alignment
    • Purpose
      • Grinding gasket surfaces of the heads may cause intake manifold to not fit correctly
      • Remove enough metal to rematch the ports and bolt holes
  • Figure 30-24 The material that must be removed for a good manifold fit.
  • Intake Manifold Alignment
    • Procedure
      • Machine shops have tables that specify the exact amount of metal to be removed
      • Remove some metal from the front and back gasket surfaces of closed-type intake manifolds
  • Intake Manifold Alignment
    • CAUTION: Do not remove any more material than is necessary to restore a flat cylinder head-to-block surface. Some manufacturers limit total material that can be removed from the block deck and cylinder head to 0.008 in. (0.2 mm).
  • Intake Manifold Alignment
    • CAUTION: Removal of material from the cylinder head of an overhead camshaft engine shortens the distance between the camshaft and the crankshaft. This causes the valve timing to be retarded unless a special copper spacer shim is placed between the block deck and the gasket to restore proper crankshaft-to-camshaft centerline dimension.
  • Figure 30-25 Using an intake manifold template to check for the proper angles after the cylinder heads have been machined.
  • VALVE GUIDES
  • Valve Guides
    • Types
      • Integral with the head casting in cast-iron heads
      • Removable or pressed-in valve guides and valve seat inserts are always used in aluminum heads
  • Figure 30-26 An integral valve guide is simply a guide that has been drilled into the cast-iron cylinder head.
  • Figure 30-27 All aluminum cylinder heads use valve guide inserts.
  • Figure 30-28 Valve guides often wear to a bell-mouth shape to both ends due to the forces exerted on the valve by the valve train components.
  • Valve Guides
    • Valve Stem-to-Guide Clearance
      • Intake valve: 0.001 to 0.003 in. (0.025 to 0.076 mm)
      • Exhaust valve: 0.002 to 0.004 in. (0.05 to 0.1 mm)
  • Valve Guides
    • HINT: A human hair is about 0.002 in. (0.05 mm) in diameter. Therefore, the typical clearance between a valve stem and the valve guide is only the thickness of a human hair.
  • Valve Guides
    • Measuring Valve Guides
      • Measure valves for stem wear before measuring valve guides
      • Valve guides are measured in the middle with a small-hole gauge
  • Valve Guides
    • Measuring Valve Guides
      • Gauge size is checked with a micrometer
      • Guide is then checked at each end
  • Valve Guides
    • Measuring Valve Guides
      • Place expanded part of the ball crosswise to the engine where the greatest amount of wear exists
  • Valve Guides
    • Measuring Valve Guides
      • Valve stem diameter is subtracted from the valve guide diameter
      • Valve stem-to-guide clearance can also be checked using a dial indicator (gauge)
    ?
  • Figure 30-29 A small-hole gauge and a micrometer are being used to measure the valve guide. The guide should be measured in three places: at the top, middle, and bottom.
  • Figure 30-30 The diameter of the valve stem is being measured using a micrometer. The difference between the inside diameter of the valve guide and the diameter of the valve stem is the valve guide-to-stem clearance.
  • Figure 30-31 Measuring valve guide-to-stem clearance with a dial indicator while rocking the stem in the direction of normal thrust. The reading on the dial indicator should be compared to specifications because it does not give the guide-to-stem clearance directly. The valve is usually held open to its maximum operating lift.
  • Figure 30-32 Sectional view of a knurled valve guide.
  • Valve Guides
    • Oversize Stem Valves
      • Some domestic manufacturers recommend replacing worn valve guides with oversize (OS) stems
      • Sizes include 0.003, 0.005, 0.015, and 0.03 in.
  • Valve Guides
    • Oversize Stem Valves
      • Valve guide is reamed or honed to fit the oversize stem
      • Clearance of the valve stem in the guide is the same as the original
  • Valve Guides
    • Oversize Stem Valves
      • Oil clearance and heat transfer properties do not change
    • NOTE: Many remanufacturers of cylinder heads use oversize valve stems to simplify production.
  • VALVE GUIDE REPLACEMENT
  • Valve Guide Replacement
    • Purpose
      • Replacement always recommended when valve assembly is reconditioned
  • Valve Guide Replacement
    • Purpose
      • Replacement valve guides can be installed to repair worn integral guides
  • Valve Guide Replacement
    • Valve Guide Sizes
      • Three common sizes:
        • 5/16 or 0.313 in.
        • 11/32 or 0.343 in.
        • 3/8 or 0.375 in.
  • Figure 30-33 Valve guide replacement procedure.
  • Valve Guide Replacement
    • Valve Guide Inserts
      • Integral valve guides can be reconditioned with valve guide inserts
      • Two types commonly used:
        • Thin-walled bronze alloy sleeve bushing
  • Valve Guide Replacement
    • Valve Guide Inserts
      • Two types commonly used
        • Spiral bronze alloy bushing
  • Valve Guide Replacement
    • Valve Guide Inserts
      • Installation kit includes all necessary reamers, installing sleeves, broaches, burnishing tools, and cutoff tools
      • Valve guide must be bored to a large enough size to accept the thin-walled insert sleeve
  • Figure 30-34 A type of fixture required to bore the valve guide to accept a thin-walled insert sleeve.
  • Figure 30-35 Trimming the top of the thin-walled insert.
  • Valve Guide Replacement
    • Spiral Bronze Insert Bushings
      • Screwed into a thread that is put in the valve guide
      • Tightened on an inserting tool and screwed into the spring end of the guide until the bottom is flush with the seat end
  • Valve Guide Replacement
    • Spiral Bronze Insert Bushings
      • Bushing is trimmed to one coil above the spring end of the guide
      • Bushing is temporarily secured with a plastic serrated retainer and a worm gear clamp
  • Valve Guide Replacement
    • Spiral Bronze Insert Bushings
      • Broach is driven through the bushing to firmly seat it in the threads
      • Bushing is reamed or honed to size before the retainer is removed
  • Valve Guide Replacement
    • Spiral Bronze Insert Bushings
      • Final step is to trim the end with a special cutoff tool
  • FREQUENTLY ASKED QUESTION
    • What Is Carbon Knock?
      • Carbon knock was a common occurrence in older engines that were equipped with carburetors and high compression ratios. As carburetors aged, the mixture would tend to be richer than normal, due to a leaking needle and seat, as well as a fuel-saturated float. This richer mixture would often cause carbon deposits to form in the combustion chamber.
    ? BACK TO PRESENTATION During light load conditions when the spark advance was greatest, a spark knock would occur, caused by a higher compression ratio due to the carbon deposits. This knocking was often very loud, sounding like a rod bearing noise, because in some cases the carbon deposits actually caused physical contact between the piston and the carbon. Many engines were disassembled in the belief that the cause of the knocking sound was a bearing, only to discover that the bearings were okay. Carbon knock can still occur in newer engines, especially if there is a fault in the fuel system that would allow a much richer-than-normal air-fuel mixture, causing excessive carbon deposits to form in the combustion chamber. Often a decarburization using chemicals will correct the knocking.
  • TECH TIP
    • Horsepower Is Airflow
      • To get more power from an engine, more air needs to be drawn into the combustion chamber. One way to achieve more airflow is to increase the valve and port size of the cylinder heads along with a change in camshaft lift and duration to match the cylinder heads.
    BACK TO PRESENTATION
    • One popular, but expensive, method is to replace the stock cylinder heads with high-performance cast-iron or aluminum cylinder heads.
    • Some vehicle manufacturers, such as Audi, go to great expense to design high-flow rate cylinder heads by installing five-valve cylinder heads on some of their highperformance engines. SEE FIGURE 30–11.
      • Figure 30-11 An Audi five-valve cylinder head, which uses three intake valves and two exhaust valves.
  • TECH TIP
    • Unshroud the Intake Valve for More Power
      • If an engine is being rebuilt for high performance, most experts recommend that the shrouded section around the intake valve be removed, thereby increasing the airflow and, therefore, the power that the engine can achieve, especially at higher engine speeds.
    BACK TO PRESENTATION
      • This process is often called unshrouding .
  • TECH TIP
    • The Potato Chip Problem
      • Most cylinder heads are warped or twisted in the shape of a typical potato chip (high at the ends and dipped in the center). After a cylinder head is ground, the surface should be perfectly flat. A common problem involves grinding the cylinder head in both directions while it is being held on the table that moves to the left and right.
    BACK TO PRESENTATION Most grinders are angled by about 4 degrees. The lower part of the stone should be the cutting edge. If grinding occurs along the angled part of the stone, then too much heat is generated. This heat warps the head (or block) upward in the middle. The stone then removes this material, and the end result is a slight (about 0.0015 in.) depression in the center of the finished surface. To help prevent this from happening, always feed the grinder in the forward direction only (especially during removal of the last 0.003 in. of material).
  • FREQUENTLY ASKED QUESTION
    • What Is Valve Guide Knurling?
      • In an old and now outdated process known as valve guide knurling , a tool is rotated as it is driven into the guide. The tool displaces the metal to reduce the hole diameter of the guide. Knurling is ideally suited to engines with integral valve guides (guides that are part of the cylinder head and are nonremovable).
    BACK TO PRESENTATION
      • It is recommended that knurling not be used to correct wear exceeding 0.006 in. (0.15 mm). In the displacing process, the knurling tool pushes a small tapered wheel or dull threading tool into the wall of the guide hole. This makes a groove in the wall of the guide, similar to a threading operation without removing any metal.
    ? The metal piles up along the edge of the groove just as dirt would pile up along the edge of a tire track as the tire rolled through soft dirt. (The dirt would be displaced from under the wheel to form a small ridge alongside the tire track.) SEE FIGURE 30–32. The knurling tool is driven by an electric drill and an attached speed reducer that slows the rotating speed of the knurling tool. The reamers that accompany the knurling set will ream just enough to provide the correct valve stem clearance for commercial reconditioning standards. The valve guides are honed to size in the precision shop when precise fits are desired. Clearances of knurled valve guides are usually one-half of the new valve guide clearances. Such small clearance can be used because knurling leaves so many small oil rings down the length of the guide for lubrication.
      • Figure 30-32 Sectional view of a knurled valve guide.
  • TECH TIP
    • Tight Is Not Always Right
      • Many engine manufacturers specify a valve stem-to-guide clearance of 0.001 to 0.003 in. (0.025 to 0.076 mm). However, some vehicles, especially those equipped with aluminum cylinder heads, may specify a much greater clearance.
    BACK TO PRESENTATION For example, many Chrysler 2.2 liter and 2.5 liter engines have a specified valve stem-to-guide clearance of 0.003 to 0.005 in. (0.076 to 0.127 mm). This amount of clearance feels loose to those technicians accustomed to normal valve stem clearance specifications. Although this large amount of clearance may seem excessive, remember that the valve stem increases in diameter as the engine warms up. Therefore, the operating clearance is smaller than the clearance measured at room temperature. Always double-check factory specifications before replacing a valve guide for excessive wear.
  • TECH TIP
    • Right Side Up
      • When replacing valve guides, it is important that the recommended procedures be followed. Most manufacturers specify that replaceable guides be driven from the combustion chamber side toward the rocker arm side.
    BACK TO PRESENTATION For example, big block Chevrolet V-8 heads (396, 402, 427, and 454 cu. 3 ) have a 0.004 in. (0.05 mm) taper (small end toward the combustion chamber). Other manufacturers, however, may recommend driving the old guide from the rocker arm side to prevent any carbon buildup on the guide from damaging the guide bore. Always check the manufacturer’s recommended procedures before attempting to replace a valve guide.