Ladies here is some good FYI information from headgaskets.com:
Quote:
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.
Torquing:
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".
SURFACE FINISH:
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 the 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.
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