The cylinder head gasket provide the critical seal between the engine block and the cylinder head. They seal the combustion in the combustion chambers and keep coolant contained to the cooling ports in the heads and block.
Blown head gaskets can be caused by various engine problems, and can be detected with certain techniques. Left unfixed, a blown head gasket could cause severe engine damage. Many symptoms of a bad head gasket are not apparent until the problem is very bad, including the ones listed later in this article.. In my experience the oil pressure gauge acting funny while the car seems to run pretty normal is a good indicator you may have a bad head gasket. The pressure can be high or with a mechanical gauge it bounces around normal at an idle, when things are running right the dial should be completely steady.
Various different types of head gaskets exist today, for different applications. Care must be taken in removal of the old gasket, selection of a new gasket and proper installation of the new gasket.
Head gasket basics
Different types of head gaskets
Copper and brass were originally used as head gaskets, followed by asbestos/steel gaskets in the 1950's, and composite metal and impregnated fiber or graphite composites in the 1980's. In the 1990's the Multi-Layer-Steel gasket (MLS) came into widespread use. Most new engines today are designed with MLS gaskets.
In MLS gaskets, multiple thin layers of cold-rolled steel are coated with a very thin layer of elastomeric material. The elastomer "micro-seals" the surface imperfections in the metal, and resists gases, oils, coolants, and high combustion temperatures.
Copper head gaskets
Copper head gaskets provide the strongest combustion seal which is why they are used in all nitromethane (Top Fuel) and methanol (Blown Alcohol) applications. They're commonly used in high-performance applications where extreme cylinder pressures will be encountered, such as very high static compression ratios on naturally-aspirated motors or where superchargers or turbochargers are used. Copper head gaskets are re-usable, and they do not have to be re-annealed, however since they are made from a flat sheet of copper, they require relatively flat deck surfaces (no more than .002" differential in any direction). Copper head gaskets are quite forgiving of machining imperfections from higher RA (Roughness Average) surfaces or small scratches in the head and deck surfaces since they are made of malleable (soft) copper which conforms readily to surface irregularities under the compressive clamp load of the tightened head bolts.
To accomplish combustion sealing with standard copper head gaskets, grooves are machined into the block or the head outside of the combustion sealing area to a width that will retain a stainless wire by friction resistance (usually .039" to .040" wide), the depth of the groove is determined by subtracting the desired protrusion (height) of the installed o-ring from the wire diameter. Stainless steel wire (most often .041" diameter) is then seated into the groove by tapping with a soft faced hammer or other interface tool such as plastic or wood (hard faced hammers can cause dents which create combustion leakage paths). It is advisable to begin and end the o-ring nearest a bolt location to take full sealing advantage of increased loading near the bolt upon the joint in the o-ring. Although copper is a relatively soft material, there is a limit to which it can be displaced by an o-ring. Generally speaking, this limit is about 25% of the gasket thickness. For instance, with a 0.032" thick gasket, you would want to limit the height of the o-ring to about 0.008" above the head or deck surface. For an .043" thick head gasket which is the most common thickness, set the o-ring protrusion at 0.010", for an .050" thick gasket about .012", for an 0.062" thick gasket about 0.015" and so forth. If the groove is cut into the head for the wire, a "receiver groove" can be machined into the block. If the groove is cut into the block deck for the wire, then a receiver groove can be machined into the head surface. When the head is bolted to the block, the wire pushes some of the copper up into the receiver groove and makes a very effective seal. However most street / strip applications do not require receiver grooves, receiver grooves are only required on the most extreme racing applications. You'll also need a good sealer around the water passages (K&W Copper Coat is easy to use and easy to find). Copper head gaskets can be re-used several times, simply use a solvent such as brake cleaner to remove any sealant and inspect the area of the gaskets around the combustion seal to insure that there is no 'carbon tracking' which will appear as a shadow on the head gaskets, this is evidence of combustion leaking. If combustion leakage has occurred, the gasket(s) must be replaced.
Modern copper head gaskets
SCE Gaskets makes copper head gaskets with coolant and oil passage seals bonded to the sealing surfaces of the head gaskets. These built-in seals eliminate the need for additional sealants, these head gaskets are installed dry and there are two versions available, for engines with machined o-ring combustion seals or for those without o-rings. The copper head gasket offered by SCE for engines without machined o-rings includes both coolant seals and an Integral Combustion Seal which eliminates the need to have o-ring grooves machined into the block or heads. Development of these head gaskets allow users to exploit the benefits of of copper, wide range of thicknesses, superior strength, superior combustion sealing, conformability to deck surfaces and superior heat conductivity, with none of the prior disadvantages of 'old fashioned' copper head gaskets.
Gasket Works makes a stainless steel o-ring that is only 0.004" thick and eliminates the need to groove the block or head. See: http://www.headgasket.com/images/olympic%20rings.jpg .
Steel (shim) gaskets
Steel shim gaskets come in a variety of thicknesses as the name implies and range from .010 up to .080. Originally, the gaskets were of flat steel sheeting and went on to have an embossed surface. The embossing was strategically placed on the gasket surface around cylinder holes, water inlets, and outer surfaces. The embossing provided extra metal to fill in voids over the sealing surface when torqued into place. Shim gaskets could raise or lower compression in an engine without machining. They were the front runners of the now used fiber and non-ferrous metal gaskets. The term, "blowing a gasket" was common for top racers because they were using a very thin 10 or 20 thousandths shim gasket to raise compression, because of its thinness, the gasket would blow between two inner cylinders. They could stay in the race only as long as the gasket didn't disintegrate and expose the coolant passages to the combustion chamber. Engine builders of the day were often seen applying a couple coats of aluminum paint to both sides of a set of head gaskets to make them stick in place and seal the mating surfaces. The aluminum particles in the paint would fill in the voids in surfaces of the sand cast heads and block.
MLS head gaskets
Sealant use may be required when retrofitting MLS(multi-layer steel) head gaskets to engines which were not originally produced with MLS head gaskets, or when using MLS head gaskets on engines that have not been properly prepared. For proper coolant and oil sealing, MLS head gaskets require surface finishes of 30 RA (Roughness Average) or finer, this is because the elastomeric coating on the sealing surfaces of MLS head gaskets is approximately .001" thick which is too thin to seal leak paths in the peaks and valleys of rougher (RA30+) finishes.
Steel shim head gasket
Steel shim head gaskets are exactly what the name implies, simply a thin sheet of embossed steel with no sealers applied. Steel shim gaskets rely on the combination of increased localized pressure from the embossing (stamped ridges) much like MLS head gaskets, with the addition of a user applied sealant to insure liquid tight operation.
Aluminum cylinder heads
Though aluminum is lighter than cast iron, it expands faster. This quicker cycle of expansion and contraction stresses head gaskets. To handle this, a non-stick coating is often applied to the gasket, like Teflon or molybdenum.
With iron heads, a product can be applied for the opposite effect. Silicone, Viton, or other fluoroelastomer sealants are applied to increase the pressure between the iron heads and the iron block.
"Blown" head gaskets
Reasons why head gaskets fail
- Too much outward pressure that overcomes the clamping force of the head to the block, caused my 'over-blowing' the air-fuel mixture.
- Overheating, possibly from a blown hose, water pump, or thermostat.
- Engine pre-ignition or detonation.
- Improperly torqued cylinder head.
- Faulty cylinder head bolts.
- Some blocks are more prone to head gasket failure, because the bolt positioning applies uneven pressure or there are too few bolts surrounding the cylinder.
- Stretched cylinder head bolts.
Symptoms of a blown head gasket
Some of the symptoms below may also indicate a cracked cylinder head.
- Foaming or bubbling coolant. Indicates head gasket has failed between a cylinder and a passage in the water jacket.
- Rapid pressure buildup in cooling system, before engine is warm. Or, cooling system appears to be malfunctioning.
- Radiator frequently gets low on water (it could be being discharged through the tailpipe).
- White smoke in exhaust, or sweet smelling exhaust.
- "Milkshake" oil, caused by coolant in oil.
- Oil in coolant, causing foaming in coolant.
- Oil or coolant leaking from cylinder head.
- Low cylinder pressure.
- Milky-gray ring around the oil cap. It's a coolant deposit.
- Drops of coolant coming out of the tail pipe.
- Spark plugs with coolant deposits. (confirm this, and expand)
- Sound of air rushing or whistling coming from cylinder head. You can also listen with a stethoscope.
- Two adjacent cylinders with low but equal pressure. Indicates head gasket failure between the two cylinders.
- Car misfires or runs rough at low speeds, due to coolant in the combustion chamber. (confirm)
- A test kit indicates combustion by-products in the coolant. (expand)
Compression testing and leak down testing
- If a compression test shows two cylinders with equal but low compression, then that may indicate a blown head gasket, with a leak between those two cylinders.
- If you can, do a leak down test.
- If you can, pressure test the coolant system. If you pressurize the system, and you lose pressure, you may have a blown head gasket.
How to choose a head gasket
Will depend on desired quench, and engine performance characteristics.
Although static compression ratio should be determined by the piston configuration and the volume of the combustion chamber, small compression ratio adjustments are possible by altering gasket thickness as long as they don't throw you out of spec on the squish dimension (piston crown to under side of cylinder head with the piston at TDC). Generally speaking, this figure is 0.035" to 0.045". There will be some flex in the crankshaft, rods and pistons as they whip around at speed and this clearance will be diminished as a result, with the piston coming in close proximity of the underside of the cylinder head. This will "squish" the otherwise dead mixture out of the area and jet it towards the spark plug, thus fully mixing the mixture, contributing to more complete combustion and the elimination of detonation. The best piston to use for this is one which has a dead flat area where it will meet the cylinder head. Stock Chevy pistons, for instance, have only a thin ring around the perimeter of the piston to accomplish squish. Absolutely flat-top pistons will work best, such as the ones offered by Keith Black. Even the dished pistons offered by them have the flat area opposite the dish.
This is probably one of the most important areas of engine building and should be considered carefully before ever buying any parts. You must know what the piston deck height (distance from the crown of the piston to the block deck surface with the piston at top dead center) is before continuing. I'll use a 350 Chevy for this explanation. If the piston deck height is, for instance, 0.025", then a gasket with a thickness of 0.015" (Fel-Pro FPP-1094) would put the squish at 0.040". If the piston deck height is, for instance, 0.012", then a gasket with a thickness of 0.028" (GM 10105117) would put the squish at 0.040". Mix and match the piston deck height with the gasket thickness in your particular application to achieve this ideal 0.035" to 0.045" squish.
Another consideration is the type of engine, naturally aspirated, supercharged and so on. It's been said that the Cometic MLS (multi-layered steel) gaskets, although relatively expensive, negate the need to "O" ring the block to accommodate higher boost numbers.
Head gasket sealants
Current technology OEM and racing head gaskets are designed to be used without additional sealant, however there are cases where head gasket sealants are required or helpful. Head gaskets sealants are specifically designed for the application; due to extremely high pressure within the combustion chamber which must be contained by the combustion seal, generic fluid sealants such as silicone should not be used for head gasket sealing. Unlike RTV silicone, head gasket sealants do not cure, they remain pliable indefinitely thereby allowing the head/gasket/block interface to achieve metal-to-metal contact under the compressive load of the tightened head bolts. Metal-to-metal contact insures a proper combustion seal. Conversely, curing sealants such as RTV silicone can form a 'rubber layer' upon which the head/gasket/block interface are separated allowing combustion pressure to eventually form a leak path to the coolant system or to the outside of the engine. Differential expansion rates of bi-metal engines are also accommodated by use of proper head gasket sealants which will not shear because they do not cure.
Standard copper head gaskets and sealant
Traditional flat copper head gaskets require the use of additional sealant for cooling and/or oil passage sealing. Similar to steel shim gaskets, traditional copper gaskets have no sealants applied.
Specific head gasket sealant products
The following products are commonly used for head gasket sealing:
- K&W Copper Coat
- Dynatex Hi-Tack Gasket Spray
- Hylomar Universal Blue
- Permatex Copper Spray-A-Gasket Hi-Temp Adhesive Sealant
- various Permatex gasket sealants
Replacing blown head gaskets
Damage caused by a blown head gasket
Time is of the essence when replacing a blown head gasket. If the car overheats, it can warp or crack the cylinder head, especially if it's aluminum or a thin-wall iron casting. If it warps too much, it will have to be discarded.
It can also take out the whole bottom end of your engine: hydrolocking, broken or bent pistons and rods, cracked block, etc.
Removing an old head gasket
Use a putty knife or gasket scraper, but be very careful that you do not gouge the material with the tool. Perhaps a better idea is to use a 3M rotary pad that is commercially available for this purpose. Use it in your electric drill to clean off the surfaces perfectly without damage. You can also use acetone thinner, (confirm) or Permatex gasket remover.
Don't use Scotch-Brite pads.
Don't use wire brushes -- the wires can break off, get into the oil pump, and lock it up.
Head gasket repair additives
Engine additives that claim to repair questionable head gaskets should only be considered a temporary fix, at best. Nevertheless, they may occasionally be used.
Common products include Bar's Leaks head gasket repair, Thermagasketand GOSS Chem-i-Weld. Also, sodium silicate (known also as "water glass") can be used, and it's an ingredient in some commercial head gasket and block/head crack repair additives. It can be found at a pharmacy. It is incompatible with antifreeze, thus the system must be emptied before the water glass can be used. Then the system is flushed and the antifreeze replaced.
Any "leak stopper" or head gasket "repair additive" should be avoided. Use of such products leads to further engine damage, such as seized or leaking water pumps, clogged water or oil passages, or clogged radiators and thermostats. If you use these products, be aware of possible problems caused by them.
Head gasket re-torquing
Stock replacement head gaskets do not generally need re-torquing. Steel shim head gaskets should be retorqued. Racing engine head gaskets should be re-torqued regardless of the type of gasket being used. If your engine calls for Torque-to-yield head bolts (TTY) do not re-torque.
The purpose of re-torquing the cylinder heads is to restore the proper stretch to the head bolts after the first heat cycle. Physics dictates that the engine assembly will expand as the engine temperature increases, this expansion will increase the compressive load on the head gaskets causing a seating effect sometimes referred to as creep relaxation in composite head gaskets.
The seating of the gaskets and threads results in a commensurate relaxation of the head bolts when the engine cools. Re-torquing the head bolts/studs restores the proper stretch to the fasteners which will insure proper cold sealing of the gaskets as well as proper combustion sealing under full load.
One re-torque is all that is necessary (unless the manufacturers instructions indicate otherwise), subsequent re-torquing can cause plastic deformation (stretch) of the head fasteners and damage to the head gaskets.
The process for re-torquing is as follows:
Start the engine & run with no load until operating temperature is reached.
Shut down the engine & let cool completely (overnight).
Retracing the original torque pattern, one fastener at a time, loosen slightly to overcome the friction set of the bolt or nut, then re-torque to specified torque setting.
It is suggested by some to retorque cast iron heads/blocks while still warm (not hot). This should NOT be done with aluminum blocks or heads.
When replacing a blown head gasket
- Check to make sure the heads and block are straight. If they're warped, you could easily end up with another blown head gasket. This can be done with a straight edge and a feeler gauge. This is also a good time to check for cracks. This is especially important if using a thin gasket. The thicker gaskets are made to compensate for low spots on the deck.
- Prep the surface. Mating surfaces should be smooth, clean, and dry and can be cleaned with brake cleaner, or denatured alcohol if used in a well-ventilated area. (confirm).
- Chase the bolt and stud holes. Clean with compressed air.
- Pre-fit the head gasket.
- Don't use sealant unless the manufacturer explicitly specifies it. Some engines require NEW Torque-to-yield head bolts and these bolts should come with sealant already applied.
- Follow specific torque settings and bolt-tightening sequence perfectly. Re-torque as specified.
- Replace both at the same time, even if only one has blown.
- Oil the tips of the cylinder head bolts with light oil. (confirm this, and expand). Also, you can use a little bit of oil on the underside of the bolt, to aid getting the proper torque reading. If using aftermarket fasteners, follow the manufacturer's directions for torque and type of lubricant used.
- Sometimes a sealant will need to be used on the head bolts, as they may intersect the water jackets, and be exposed to engine coolant. Use a non-hardening sealer (confirm and expand).
- Change the oil and oil filter too -- they're likely contaminated with coolant.
- Don't be surprised if some of the water ports are "blocked off" on your gasket. Various gaskets are made to differently meter the coolant and direct its flow. Gaskets are also made differently for street and race applications. 400 small block Chevys are a special case. There are steam holes drilled in the block that release steam pockets which are formed in the block as a result of the cylinders being siamezed, with no water passage between the cylinders. You must use 400 gaskets on this motor that have the holes in the gasket which coincide with the holes in the block deck. If using heads other than 400 heads (which are also drilled with corresponding holes), you must drill steam holes into the deck of the heads to allow the steam to escape from the block and up into the heads to be dispersed.
Re-using head gaskets
Some head gaskets are re-usable several times and others should never be re-used. Steel shim head gaskets are designed for one use only. Composite or graphite head gaskets are most often not re-usable because of rust damage to the steel core, disintegration of the surface material or damage or loss of the sealant material. MLS gaskets are most often not re-usable because the elastomeric coating is scubbed off of the combustion and coolant seals by abrasion from temperature induced expansion and contraction. Traditional copper head gaskets are re-usable, Titan and ICS Titan copper head gaskets from SCE are also re-usable. Even if a head gasket is re-usable, many people prefer to use a new one. The labor involved in replacing a head gasket, and the potential for engine damage from a blown gasket are too great to risk.
When reusing gaskets, they can be coated with the head gasket sealants listed above in "Head gasket sealants"; traditional copper gaskets can also be sealed with aluminum spray paint available from VHT. (confirm this product name and description).
Suppliers and manufacturers of head gaskets
- Best Gasket
- Cometic Gaskets, Inc
- Fel-Pro gaskets
- Flatout Gaskets
- Milodon copper head gaskets
- Mopar Performance Gaskets
- Mr. Gasket
- ROL Gaskets
- SCE Gaskets
- Victor Reinz gaskets
Ideally you would install the new gasket the same way as the one your replacing. But if you didn't look closely enough during disassembly then make sure that the coolant, bolt and cylinder holes line up properly.
- Gasket training manual -- ROL Gaskets
- Blown head gasket?, Hotrodders Bulletin Board, October 28, 2002.
- Head gasket installation, Hotrodders Bulletin Board, July 27, 2003.
- Removing head gasket from block, Hotrodders Bulletin Board, March 9, 2004.
- Re-using an un-run head gasket, Hotrodders Bulletin Board, April 12, 2004.
- Oil on head bolts and head gasket, Hotrodders Bulletin Board, May 25, 2004.
- Question about copper head gaskets, Hotrodders Bulletin Board, August 7, 2004.
- What head gaskets do I need?, Hotrodders Bulletin Board, February 25, 2005.
- Which head gaskets provide the best seal?, Hotrodders Bulletin Board, February 13, 2006.
- Bar's Leak head gasket repair -- anyone use this before?, Hotrodders Bulletin Board, December 12, 2006.
- Head gasket, Hotrodders Bulletin Board, February 25, 2007.
- Blown head gasket...how do I know?, Hotrodders Bulletin Board, March 11, 2007.
- Chevy head gaskets, Hotrodders Bulletin Board, March 26, 2007.
- Leaky head gasket, Hotrodders Bulletin Board, March 28, 2007.
- Which thin head gasket is best?, Hotrodders Bulletin Board, March 28, 2007.
- Head gasket question, Hotrodders Bulletin Board, June 15, 2007.
- Head gasket question, Hotrodders Bulletin Board, July 17, 2007.
- Benefits of Copper Head Gaskets -- SCEGaskets.com
- Cylinder Head and Head Gasket Repairs -- Econofix.com
- Head Gasket -- Partstrain.com
- Head gasket -- Wikipedia
- Head Gaskets -- DuPontElastomers.com
- Head Gaskets and Valve Covers Q&A -- Wiki.Answers.com
- Head Gasket Basics -- MobilOil.com
- Replacing Chrysler, Dodge, or Plymouth head gaskets -- Allpar.com