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Head Gasket 101

The purpose of the head gasket is more than just to create a seal between head and the deck of the block.

It acts as

1) a guide for coolant flow between the block and the head and

2) to promote heat transfer between them.

The head itself is also a conduit for coolant transfer to the radiator. Most older sportscars use gaskets that are made from a sandwich of very thin copper, a thicker fiber layer (typically asbestos laden) and thin steel. This gasket design makes it difficult to personally customize for racing applications and is inherently weak. It is also hazardous to your health if broken and the fibers are aerosolized and inhaled.

Heat is produced by the engine almost entirely at the combustion chamber. Coolant flows from the radiator to the block where it is pre-heated before it goes to the head where the temperatures are the greatest. The coolant then flows through the head to the radiator where it looses heat. Water is the heat sink of choice since it has the highest specific heat capacity.

In order for the coolant to be functionally efficient it needs to move in-mass from the rear of the head forward. Most engines have coolant holes in the gasket spaced from one end to the other. These gaskets allow the coolant to percolate through to the head less optimally. Generally, there are smaller coolant holes in the gasket place forward which are needed primarily for gaseous escape routes and less so for cooling purposes. Since racing engines produce more heat it would seem intuitive that the flow through the gasket be channeled to maximize cooling efficiency by having more of the coolant flow through the rear of the head. It should also be stated that the flow of the coolant has to be such that there is an adequate dwell time within the head to adequetly absorb heat. Additionally, the coolant should be free of any insulating contaminants such as air (in the form of micro-bubbles) that would preclude ideal heat conductance. Engines like to run efficiently at an optimal temperature typically above 180F but less than 220F. Running an engine hard prior to proper temperature is not good.


This can be a problem at times. It is critical that the fasteners you are using are better than "nominal". Use the best you can afford. A key factor that is not considered often is that the fasteners not only hold down the head but also PULL-UP on the block.

The torquing procedure works in two locations

1) on the head side and the

2) block side.

The threads within the block maybe weak. Some spots within the block are weaker than others -depending upon how much metal is around them. When these weaker spots are stressed they "give" more than other areas. This can actually deform the block and dimple the deck surface a bit. This under-appreciated problem can create sealing dillemas. One method used to mitigate against this effect is to study each motor and relieve (chamfer) the holes where the fastener enters the block.

Bolt loading:

When a bolt is tightened a large portion of the tightening torque is used to overcome the resistance of the threads. Only about 20% (at most) of the torque is transmitted in tension to the fastener. An important factor in this is the surface finish of the threads. Some aftermarket fastener manufatures coat the fastener with a laquer based material that requires a high quality moly lubricant to be placed on the threads for proper torquing. If this is not done improper tightening will occurr. If you wish to use an oil on the threads for torquing - you need to remove this coating with a wire brush.

EXCESSIVE bolt loading will cause problems.

Many backyard mecanics think that if 50 Ft. lbs of torque is good then 60 is better. In fact dynomometer testing has shown that less is best in most instances. Indeed extra horsepower maybe gotten by being "torque frugal". Why??? The more torque that is applied onto the block the more chance of distortion. This distortion is usually seen at the weakest places at the narrowes point of the bore and at the top of the cylinder. At the top of the cylinder where the compression pressures are always greatest any excess distortion will nullify any benefit of that extra clamping force. Blow-by of gasses will cause premature gasket burn through and less horsepower. "Less maybe best".


The ideal situation is to have a finish on the flange surfaces (head and block deck) as smooth as possible. In older engines the typical surface finish was make buy using a fly cutter on a mill that created symmetric arcing lines. This finish was good for composit gaskets. The idea being that there was some imperfection in the surface so that it would "grab and hold' the gasket in place. Modern motors have bi-metal engines typically. Cast iron blocks and aluminum heads. These have different heat characteristics and therefore stretch at different rates. If these surfaces had the typical finish of yesteryear the gasket would fail due to shearing effects. Newer motors have very smooth finishes -in fact most machine shops don't have (but will have to eventually) the equipment to produce this finish.

When using copper gaskets in any motor ask your machine shop to get an RA (roughness average in Microinches) of about 60 for cast iron heads and blocks and closer to 40 for aluminum.

Compression Ratio:

Definition; Compression ratio is the volume of the space above the piston at BOTTOM dead center and the volume of the space above the piston at TOP dead center.

In order to figure out the compression ratio several volumes are needed to make the final calculation.

1) Combustion chamber volume: This volume is the space within the head that the piston pushes the charge of air/fuel into. Typically, people refer to this as "cc'ing" the head. It requires an accurate burrette, a piece of clear glass or plexiglass with a hole (to allow you to place colored alcohol or solvent into the space), some grease and a finnished head with the valves installed. Apply the grease to the valve seats and allow the valve springs to seal them into place. Wipe off any extra grease from the combustion chamber. Lay the head so that the combustion chamber is face up. Place some grease upon the surface around the opening and place the clear plastic plate upon it. Slide the plate and press firmly to get a good seal. Add fluid into the chamber through the hole of the plastic plate and fill the chamber. You need to be exact. Get rid of any air bubbles by tapping the head. Make sure no fluid leaks from around the valves. The amount of fluid you add to this space is the COMBUSTION CHAMBER VOLUME. Do this for each cylinder. Use the same type of fluid in each chamber measurement. Do Not use alcohol in one and solvent in another.

2) Dome Volume: under construction.....

3) Piston Deck height:......."

4) Gasket Volume:......"

5) Valve reliefe volume:....... "

Making Head Gaskets Last

Subaru head gaskets are about as hot of a topic as national health care lately. If you own a Subaru you know what I mean.

Since the 2.5 liter Subaru engine was put into production there have been thousands of head gaskets replaced across the country. We’ve done a good portion of those ourselves. In another post I’ll go into further detail about the details of Subaru head gaskets but for today I would like to offer some suggestions on how to help prolong the ones you have.

Subaru head gaskets can fail for numerous reasons. Failure of sealant, improper torque, surface imperfections in the cylinder head or engine block and of course heat or excess heat.

I’m going to focus on excess heat. Within the engine block the coolant is circulating to keep the engine cool due to internal combustion occurring. The coolant remains in contact with the metal and is able to absorb heat, travel to the radiator and release the heat to the atmosphere.

Two important things must occur to for the coolant to do it’s job. It must have adequate flow to move the heat away from the internal areas of the engine and it must remain in contact with the areas it’s trying to cool.

There are 3 very important items that may individually have a negative impact on the coolant’s ability to do it’s job. If all 3 components are bad or inferior, problems could develop even sooner.

Subaru Radiator Caps OLD vs NEW

Radiator cap:

Keeps coolant in a sealed system, allows overflow to exit and return as coolant expands and contracts, but most importantly it raises the boiling point of the coolant in the system by keeping the cooling system pressurized. Most radiator caps for stock vehicles keep the system pressurized between 13-15psi. This can raise the boiling point depending on the mix of coolant/water an additional 35-40 degrees. A 50/50 mix of anti-freeze and water has a boiling point around 265 degrees. Add to that a radiator cap that holds 13psi and now you have coolant that won’t boil until 300 degrees .

There are areas throughout the engine where the coolant circulates that become very hot. So hot that it can boil coolant. Since we must have the coolant in contact with the metal to perform it’s heat transfer duties we now have a problem. Scenario: Radiator cap is weak (which we run into all the time on Subarus). A weak radiator cap not holding pressure may not let all of the coolant get hot enough to boil but there are areas within the engine that are now boiling. Boiling coolant has air bubbles that now keep the coolant from contacting the cylinder walls and other extremely hot areas within the engine. This heat is more than the engine and gasket were designed to withstand on a regular basis and thus a situation that will accelerate the failure of the gasket.

Flow of the coolant is important also. To keep from boiling the hot coolant must be quickly moved away from the hot cylinder walls up to the radiator so it can release it’s heat. Below is a picture of a Subaru water pump and also a quality Japanese aftermarket water pump. Although we for the most part believe in genuine Subaru parts, here’s a case where genuine Subaru part’s may not be the best choice. Note the stamped steel vanes on the Subaru pump vs the quality cast and machined impeller on the Japanese counterpart. The tight clearances and defined impeller vanes are very efficient at moving coolant through your Subaru engine. (an interesting side note that older Subaru water pumps were made nearly identical to the pump on the right).

Another important part of keeping the hot coolant flowing out to the radiator and away from the internal hot spots in the engine is a high quality thermostat. I’ve shown below the comparison between a generic aftermarket brand on the left and a genuine Subaru thermostat on the right.

Note the Subaru version has a much larger spring, larger diameter central area for coolant flow and is made of steel and brass. The generic brand contains copper, a big no no with Subaru. Subaru actually states that copper in a Subaru cooling system is ill advised and may cause excessive electrolysis and corrosion.

Even after trying to be diligent about providing the best possible cooling for your Subaru you still may need to cross the head gasket bridge some day.

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