The most common use for a cast piston is in mass-produced engines that are not likely to be modified. A cast piston, while structurally sound, cannot be subjected to mishandling or heavy abuse or it will break. A cast piston's engineering lends itself to stable dimensions. Forged pistons are heavier than their cast-piston counterparts but have less stable dimensions.
Manufacturing and Cost: Cast Pistons
A cast piston cannot be manufactured inexpensively. The die casting used in its construction is costly because the machines needed to do the job are very large and expensive. For this reason, cast pistons are only available in limited custom sizes. They are generally considered to be at the forefront of piston technology but are not necessarily the best choice for every engine.
Manufacturing and Cost: Forged Pistons
The forged piston is much easier for manufacturers to produce. Forged pistons are chosen more often by those seeking a highly customized piston that can be made quickly and is affordable.
The first pistons cast from aluminum were not well-suited to their application. Silicon is much more resistant to heat and, due to its chemical composition, is naturally lubricated. If dropped it will be damaged, however, so great care must be taken in handling these pistons. The first forged pistons were also constructed of inefficient alloys. The addition of nickel to the alloy makes the alloy more resistant to breakage once forged.
A piston has seven parts. The top of the piston is called the crown and is responsible for handling the majority of the heat and force caused by combustion. The second part of the piston is called the ring groove. The ring rotates as the pistons move up and down in the cylinder in a four-stroke engine. The ring lands are the pieces between the grooves. Their main job is to support the shock loads placed on the rings when combustion occurs. The next piece of the piston is called the pin hole. Around the pin hole are two metal supports called the pin bosses. The last piece of the piston is called the piston skirt. The piston skirt accepts the force of the combustion and also compression from the compression stroke.
The main reason to use a cast piston is its efficient mass. It is lightweight and heat flows predictably throughout its construction. Due to its high silicon content it is easily molded and each part of the piston can be shaped to a specific thickness. The forged piston is heavier than a cast piston and its alloy is not as malleable. A forged piston is not very stable dimensionally and the rate at which it expands is not easily controlled. Even if a forged piston is modified to be similar to a cast piston, it will still retain more mass than a cast piston and thus use its mass less efficiently.
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