The Most Tortured Part In An Engine

From New Mind.

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A modern head gasket is an intricate hybrid mechanical seal engineered to fill the space between a reciprocating engine’s head and block.

A head gasket must seal the passages that carry engine oil between the block and the head. Engine oil can vary dramatically in viscosity and temperature, ranging from the extreme lows of frigid ambient temperature to as high as 135°C or 275°F.

Similar to engine oil, on most water-cooled engines a head gasket must also seal the passages that carry engine coolant between the head and the block. When compared to engine oil, engine coolant has a relatively consistent viscosity, with a lower maximum temperature of around 120°C or about 250°F, with normal operation seldom reaching above 140°C or 220°F. Much like with engine oils, the materials that seal engine coolant, on top of thermal cycling and movement, must deal with the corrosive properties of engine coolant.

Sealing combustion gasses are, by far, the most brutal and critical requirement of a head-gasket. A head gasket forms part of the combustion chamber and If this seal is compromised, the affected cylinder will lose the ability to produce a normal combustion sequence. Depending on the nature of this failure, the cylinder may also consume or cross contaminant other engine fluids.

A head-gasket must be deformable enough to maintain a seal between the imperfections of the head and block surfaces. In addition to these forces, head-gaskets have to function under the dynamics and extreme mechanical stresses of combustion pressure. The head bolts that fasten the head to the block are also typically not symmetrically spaced, creating an unevenly distributed clamping force across the gasket. With each of these bolts exerting a force of up to 4,500kg or about 10,000 lbs. Beyond these expectations, they must also be durable and capable of lasting across a significant portion of the engine’s life with little to no maintenance.

With the introduction of the internal combustion engines in the 1860s, almost every type of elastic material ever used within steam engines was experimented to seal combustion. As the internal combustion engine transitioned from its experimental early days to a mass produced power plant, copper would become a popular material for these early head gaskets. Their relative motion would create inconsistencies in the clamping force along the gasket’s surface. This was such a problem, that In the early days of motorsport head-gasket failure was the most common reason for race cars to not finish a race.

As the automotive industry began to flourish in the 1920s and 30s, less costly, mass-production friendly head-gasket designs were explored. One durable yet relatively inexpensive option was the steel shim head-gasket. Embossments are stamped, raised regions on critical sealing areas off a gasket that created a smaller contact point.

The beater-add process offered a new lower cost gasket material option that would lead to manufacturers eventually introducing the composite head-gasket in the late 1940s. Metal beads, called fire rings, are created within the gasket’s metal structure to seal the combustion chamber and protect the elastomer material from overheating. The non-metallic surface of the gasket is then impregnated with a silicon based agent to seal any pores and prevent the gasket from swelling when it comes in contact with liquids. Some designs may even incorporate seal elements made from a high temperature, chemical-resistant fluorocarbon based elastomer material called Viton.

In 1970, Japanese gasket maker Ishikawa was issued the first patent for a revolutionary, new type of head-gasket, called the multi-layer steel or MLS head gasket. They effectively combine all of the benefits of previous gasket technologies into an extremely durable and adaptable component. The outer surfaces of the gasket are typically coated with a thin, fluorocarbon based Viton layer, in targeted areas, to aid in surface sealing.

The elastomeric head-gasket is an example of a cost-reduction focused design. These gaskets use a single steel shim with a beaded coating of an elastomeric material such as silicone or Viton for fluid sealing. On the opposite end of the performance spectrum, are modern solid copper head-gaskets. These groves carry a stainless steel O-ring that when combined with a solid-copper head-gasket capable of sealing in some of the highest combustion pressures found within reciprocating engines.