Episode 43 – Cast Vs Forged

Today I talk about cast vs forged pistons, crankshafts and connecting rods.

Cast pistons are pretty simple, the aluminum mixture is heated until it melts, then poured into a mold that is the shape of a piston.  After a little clean up, it is ready to go.

Forged pistons are made by heating the metal until it is soft, then hammering it into shape, like a higher precision version of what a blacksmith does.

There is a third type of piston, hypereutectic.  What that means is that it has a higher level of silicon that a standard cast piston.  It is still a cast piston, but it is stronger, and it has less thermal expansion.  That allows it to fit into the cylinder tighter and have less ring gap since it doesn’t expand as much.  That is why most new cars have them, it allows a better seal in the cylinder and less emissions.

Starting with cast cranks, there are three options in increasing order of strength:

Cast iron

Nodular cast iron

Cast steel

The difference between cast iron and nodular cast iron is the shape of the graphite particles in the iron. Cast iron is not just iron, pure iron is too soft to be used as a crankshaft. In regular cast iron, the graphite particles are flakes. In nodular cast iron, they are spherical nodules. This gives the cast iron more strength and flexibility.

Cast steel is stronger due to the fact that steel is stronger than iron. To make a cast crankshaft, they heat up iron or steel until it is melted then pour it into a mold, the same as with cast pistons. Like cast pistons, there is less finish machining to do so they cost less.

With forged steel cranks, a metal bar is heated until it is soft then pounded into the rough shape of the crankshaft. From there it is machined into the final shape. There is more machine work involved, along with the cost of the forging equipment it makes for a higher cost of the final piece. There are different alloys of steel used, that give a crank different levels of strength.

There is one more option for manufacturing a crankshaft, and that is a billet steel crankshaft. To make them, a much larger bar of steel is forged into a cylinder shape as large as the total diameter of the finished crank, then it is machined to the final shape. As you can imagine that is a lot more machining and the final cost reflects this.

It could be cheaper if you need something unique, since it would be cheaper to machine a one off crankshaft than to set up to forge one.

For connecting rods, it is the same thing, with the addition of powdered metal rods.  What that is, is the metal to make the rods is in a powder form with a binder, and it is pressed into the shape it needs to be.  Then it is heated below the melting point, but enough to make the powder bond together.

Since they don’t have to heat it as much, it is less expensive to manufacture.

The drawback to cast parts is that when they fail, they tend to do so by breaking apart.  Forged parts tend to bend so the failures are generally less violent.

What to use, I would say for a mild street buildup, cast parts are fine.  For an engine that will be used harder, running higher horsepower or using nitrous/forced induction go with forged.

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