Grain: Fine

Forged vs cast pistons, what is the difference?

Mod Tune Win Repeat
Mod Tune Win Repeat
Forged vs cast pistons, what is the difference?

Cast vs forged pistons, what is the difference? Both types use the same alloys but it’s the manufacturing process that gives difference results.

Forged vs cast piston manufacturing

The difference in manufacturing process is cast pistons are made from pouring molten alloy into a mould which is allowed to solidify in a controlled way. Forged pistons are manufactured by using force and pressure to mould a heated slug or ingot into a die.

So, how does a different manufacturing process alter mechanical properties of the same material? The microstructure of the cast piston is influenced by the solidification process used and can often be of high quality. So this allows the microstructure of the forged alloy to change and become finer and aligned during the process.

The outcome of simply allowing the microstructure to align is anywhere between 10-20% increase in tensile and yield strength of the alloy over casting. See below figure for caparison between microstructure of cast vs forged pistons.

Microstructure of cast and forged alloys
Cast (a) vs Forged (b) alloy microstructure (note the different scales)

Forged pistons are heavier because of the material used

A common myth is that forged pistons are heavier because of the materials used. This in false and the additional weight of forged pistons over cast is from the limitations on the forging process. The shape of the die is limited by a vertical pressing action and does not give opportunity for efficient pistons shapes. Cast pistons form to the shape of the mould and the shape of the mould can be more complex saving on weight.

Cast piston treatments

While forged pistons exhibit stronger mechanical properties over cast, there are some treatments used to help them out. Most cast pistons are T5 treated which is a process to ‘age’ the piston in a heated environment to relieve stresses then the piston is quenched. Another treatment is to cast in steel control struts near the piston pin to assist in regulating differential expansion if the cylinder block is a different material ie. iron.

Cast and forged piston materials

Now a little bit on materials and the interesting bit. The same piston alloys are used for both cast and forged pistons. The standard alloy is a eutectic alloy. What does that mean? It’s an aluminium alloy which has a little less than 12% silicon as well as 1% each of copper, nickel and magnesium. The other common alloy is a hypereutectic aluminium alloy. Hyper eutectic ‘piston’ alloys are a mix with greater than 12% silicon, normally 18%. Copper, nickel and magnesium are all still 1% each.

The difference between them are:

  • Eutectic has a higher tensile capacity and yield strength than hypereutectic.
  • Hypereutectic alloys are a tiny bit less dense, expands less and conducts a little less heat than eutectic alloys. So each have their own advantages.

Watch the cast on youtube here:

See more podcast episodes here:


Share on facebook
Share on twitter
Share on pinterest
On Key

Related Posts

Datalog recording of induced low load knock

Knock control tuning

Knock control tuning is important for any modified EFI engine. In this post I explain theory and practical verification of knock control tuning.

Ignition coil dwell calibration

Why calibrate ignition coil dwell? I am using a distributor based ignition for my EFI build and ignition coil dwell calibration will help optimise various

Megasquirt battery voltage calibration

These days injector behaviour at different voltages is well understood. The speed of opening and amount of fuel delivered vary with voltage. Corrections are made in fuelling calculations to account different performance for differnt voltages at the injector. If what the Megasquirt ECU understands is the voltage at the injector is different in reality, then it is likely fuelling calculations will be incorrect. This is why the battery voltage calibration needs to be correct in your Megasquirt ECU.

Manual Idle Air Control

The ITB idle air control conundrum Idle air control is generally a challenge with individual throttle bodies.  This is from the sensitivity of throttle position