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LO 9-9 Identify and describe the purpose, construction, and applications of cylinder head gaskets

Site: CDX Learning Systems
Course: DE Section 2-1: Engine Blocks and Cylinder Heads
Book: LO 9-9 Identify and describe the purpose, construction, and applications of cylinder head gaskets
Printed by: Guest user
Date: Thursday, 5 November 2026, 7:45 PM

Table of contents

1. Cylinder Head Gaskets

LO 9-09: Identify and describe the purpose, construction, and applications of cylinder head gaskets.

 

Cylinder Head Gaskets

The cylinder head gasket is arguably the most important seal in the diesel engine. The head gasket must maintain the seal around the combustion chamber at peak operating temperatures and pressures as well as seal compression air, coolants, and engine oil at their respective peak temperatures and pressures. The design and materials used must be thermally and chemically resistant to the products of combustion and to the coolants and oils used in the engine.

The most widely used materials include:

  • Steel
  • Stainless steel
  • Fiber-based composition materials
  • Graphite
  • Fluoropolymers, nitriles, neoprenes, polymeric resins, polytetrafluoroethylene (PTFE or Teflon ® ), and others (i.e., chemical formulations containing silicone)

Metal and asbestos gaskets replaced traditional copper and brass during the 1950s. Composite metal fiber or graphite composite gaskets superseded these gaskets in the 1980s. Since the early 1990s, those gasket systems have largely been replaced by the development of the multilayer steel (MLS) gasket. An MLS gasket is the preferred method of sealing between the cylinder head and the engine block. Today an estimated 80% of new engines are designed with MLS gaskets as standard equipment. In the MLS system, multiple thin layers of cold-rolled, spring-grade stainless steel are coated with elastomeric (rubber) material. The resilient elastomer is necessary to the gasket structure to provide micro-sealing of metal surface imperfections while making the gasket chemically resistant to aggressive combustion gases, oils, and coolants Figure 67.

Advantages of an MLS gasket include the following:

  • Uniform loading distribution on sealing beads
  • Constant operation thickness
  • High crush strength
  • Low relaxation coefficient
  • Reduced clamp load requirement, resulting in diminished cylinder bore distortion
  • High strength for increased durability
  • Do not require retorquing after engine run-in
   Fig09_67

FIGURE 09-67: This MLS gasket has elastomeric sealing material between layers of laminated steel. The elastomer seals liquids, such as coolant and oil.

 

Cylinder bores are commonly sealed by a ring of stainless steel positioned over a wire around the perimeter of the combustion chamber. This is known as the fire ring because it is directly exposed to combustion. This ring is critical to sealing the combustion chamber during high-pressure conditions. Graphite or rubber facings are applied to a head gasket for sealing liquids. Graphite lubricant is especially useful on aluminum-headed engines because it allows the head to slide or creep easily. This is especially important when an aluminum head is used over a cast iron block because the materials have different expansion coefficients. Surface finish of the block deck and cylinder head is a critical factor when installing gaskets. If the surface is too smooth, the gasket will be unable to properly grip the cylinder head and block deck surfaces and the gasket will tend to slip, resulting in combustion leaks. If the surface is too rough, the gasket may be unable to fill in the voids between the metal peaks and valleys. These voids can form leak paths for fluids and combustion gases.

 

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1.1. Head Gasket Replacement

LO 9-09: Identify and describe the purpose, construction, and applications of cylinder head gaskets.

 

Head Gasket Replacement

To prevent warping of cylinder heads, damaged bolts and threads, and gasket leakage, consistent and adequate clamping of cylinder heads is necessary. A traditional technique for clamping cylinder heads to the block requires the use of a torque wrench and a spiral pattern sequence for tightening the head bolts down in the correct order. Starting at the center of the cylinder head and working out, this technique prevents pinching of the head gasket by ensuring an even spreading out of the gasket from the gasket center out and along the block deck surface. Any bolt-tightening sequence should tighten the cylinder head down in a minimum of three progressive steps of torque, and five or six steps is not uncommon Figure 68. Reversing this tightening pattern when removing a cylinder head will prevent warping of the head. It is especially important to follow this practice in the case of aluminum cylinder heads.

After removing a cylinder head, it should be checked for warping. Generally, no more than 0.006" (0.015 mm) of warping per foot (30 cm) is allowed on a cylinder head. Manufacturers’ specifications and techniques will vary. Diesel engines are allowed very little removal of material from the cylinder head or block deck during resurfacing because machining will cause valve-to-piston interference Figure 69.

  

Fig09_68

FIGURE 09-68: A spiral pattern of three to five progressive steps of torque, starting from the center and working toward the ends, is used to tighten head bolts.

 

  Fig09_69

FIGURE 09-69: Because there are very small clearances between the valves and the pistons in a diesel engine, very little material can be milled from a cylinder head to correct warpage.

   Fig09_70

FIGURE 09-70: Improper liner protrusion can lead to a cracked or warped head plus a head gasket leak.

  

 

While diesel engine cylinder heads are ruggedly built, they can be warped and cracked by incorrect liner protrusions. A liner flange that protrudes too far above the block deck can allow combustion gases and coolant to leak because the head gasket does not seal properly, and it can also distort the cylinder head. Remember that the clamping forces of 36–42 head bolts can add up to over 1,000,000 lb (453,592 kg) of force applied by the head to the block deck through the gasket. A protruding liner allows those forces to be exerted on a small area Figure 70.

 

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1.2. Cylinder Head Inspection

LO 9-09: Identify and describe the purpose, construction, and applications of cylinder head gaskets.

 

Cylinder Head Inspection

Cylinder heads should also be inspected for cracking. Several techniques are available to the technician:

  • Dye penetrant
  • Magnetic flux
  • Pressure testing

An effective method for checking for head gasket and combustion leakage into coolant from a cracked cylinder head is to check for a continuous release of gas bubbles from the cooling system Figure 71. After warming up the engine, a hose is connected to the coolant overflow pipe at the pressure cap. The other end of the hose is then submerged in a bottle of water and the amount of gases continuously released from the surge tank or radiator overflow is observed. In a normal engine, there will be a small amount of gas bubbles released in a steady stream as some coolant continuously vaporizes. However, a large and steady stream of bubbles indicates combustion leakage in the cooling system, which requires further diagnosis. A defective air compressor head gasket or air-operated device connected to the cooling system can be eliminated as a cause by opening the air tank drain valves while the engine is operating. Because system air pressure cannot build, little or no air will enter the cooling system if a defect exists in one of these accessories. Cavitated liners can also leak combustion gases into the cooling system, but they tend to produce hydrostatic locks after the engine is shut down when hot.

   Fig09_71

FIGURE 09-71: A defective head gasket will allow pressurized combustion gases to pass into the cooling system.

 

A head gasket leaking coolant into the engine can quickly cause catastrophic engine damage because coolant will easily displace oil lubrication film and prevent separation of the bearing journals and engine bearings. To check for an internal head gasket leak, a scheduled oil sampling can detect coolant in the oil. However, to pinpoint the source of a leak, such as a leaking liner O-ring or head gasket, the engine oil is drained and the oil pan is removed. After pressurizing the cooling system with a radiator pressure tester, one can easily inspect the internal parts of an engine from the crankcase and identify where coolant is leaking.

If a cylinder head is suspected of being cracked or leaking, often the over-pressurization of the cooling system is observed only after an engine has run under heavy load for some time and thermal expansion opens a crack. If this is the case, the cylinder head can be removed and pressure tested in a hot tank with special fixtures that direct pressurized air into the intake and exhaust ports where leak paths are formed. In the tank, the head is sealed by the fixtures to prevent leakage out of the valve ports and then it is carefully inspected for any leaking air bubbles. Cracks are easily detected by observing where the bubbles originate from the head.

Cylinder head cracks often develop between the valve seats after an engine has overheated Figure 72. When the head is removed, the valves are also removed and the seat areas visually inspected. Always use an appropriate lifting sling to remove and reinstall cylinder heads Figure 73. Dye penetrant testing, which is a three-step process that allows dye to penetrate cracks to make them more visible, is a convenient way to identify casting cracks. A magnetic flux technique is another method that uses a powerful magnet and iron filings to identify cracks. In this method, a cylinder head is magnetized by either alternating current (AC) using a specialized piece of equipment or a strong magnet placed on either side of a suspected crack. Iron filings are then blown onto the area using a small squeeze bulb filled with filings. Because cracks will develop a north and south polarity, the filings will line up end to end along a crack, which enables the crack to be easily detected.

  Fig09_72

FIGURE 09-72: Cracks often develop between valve seats and around an injector hole when the engine has overheated.

   Fig09_73

FIGURE 09-74: A lifting sling is used to remove and reinstall cylinder heads. The sling is adjustable to keep the head balanced and level.

  

 

  Technicians Tip  
  When removing a cylinder head, the best technique to prevent warping a cylinder head is to remove the head bolts starting at each end of the cylinder head. Alternately removing a bolt from each end while working toward the center in a spiral-like pattern minimizes the likelihood of the cylinder head lifting, twisting, and warping. This sequence is the reverse of the sequence used to tighten head bolts.  

 

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1.3. Measuring Cylinder Head Warpage

LO 9-09: Identify and describe the purpose, construction, and applications of cylinder head gaskets.

 

Measuring Cylinder Head Warpage
Before installing a cylinder head, the casting should be checked for warpage to prevent a head gasket leak Figure 74. A warped cylinder head will not clamp against the block deck properly to allow the gasket to seal correctly. If an engine has overheated, a leak has occurred, or liner protrusions have not been properly established before the head has been installed, it is particularly important to check the deck surface for warpage. To measure cylinder head warpage, follow the steps in Skill Drill 9-1.

  Technicians Tip  
  Loss of engine coolant or excessive cooling system pressure can often be attributed to a cylinder head gasket leaking externally or internally or to a cracked cylinder head. Air bubbles originating from the engine block will appear in the radiator, and pressure will build rapidly in the cooling system, which will cause the engine to lose coolant or even push coolant out of the engine under heavy load. Technicians can check for hydrocarbons or carbon monoxide in the gas bubbles on gasoline engines but not on diesel engines because only air is present in the cylinders during the compression stroke. Test kits that measure carbon dioxide (CO2) gas in coolant are used to evaluate gas bubbles in diesels.  
  Fig09_74

FIGURE 09-74: Inspecting a cylinder head for warpage using a straight edge and a thickness gauge.

 

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1.4. Video: Straight Edge

LO 09-9 Video: Straight Edge

 

 

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1.5. Video: Measure Head Warpage

LO 09-9 Video: Measure Head Warpage

 

 

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1.6. SKILL DRILL 9-1  Measuring Cylinder Head Warpage

SKILL DRILL 9-1  Measuring Cylinder Head Warpage

 

  Fig6_01  

 

STEP 1
Thoroughly clean the cylinder head and block deck surfaces with solvent. Remove any foreign material, gasket, or silicone with a cleaning pad. Choose a cleaning pad that removes soft material but not metal; this will protect the casting surfaces from damage that more aggressive abrasives and tools can cause.

STEP 2
Use a purpose-made straight edge. Lay the straight edge along the flat cylinder head surface. Try to slide a feeler gauge or thickness gauge between the straight edge and the casting. Any gaps between the straight edge and the surface indicate warpage of the head deck. A general specification allows no more than 0.006" (0.152 mm) of warpage in 12" (30 cm) of casting surface when using MLS gaskets. This means a 0.006" (0.152 mm) thickness gauge should generally be used to check for warpage, but the manufacturer’s specifications should be checked to be sure. Much smaller clearances are permissible on smaller heads used in automobile engines.

STEP 3
Check for warpage in the longitudinal and diagonal directions as well as across the head deck. Follow the chart in this box when measuring.

STEP 4
If the feeler blade is slipped beneath the straight edge and there is little resistance when pulling on the blade, it indicates a depression or warpage of the head surface. If the clearance is beyond specifications, make a service recommendation for the cylinder head to be resurfaced by a machine shop. Note, though, that due to the small clearance space between valves and pistons in diesel engines, very little, if any, material can be machined from the cylinder head to correct warpage. Always check specifications for head thickness if a head has been machined.

STEP 5
Check the cylinder block deck using the same procedures as those used when checking the cylinder head.

 

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