DI TECH
ANSWERED: ARE BILLET
OR FORGED PISTONS STRONGER?
Design Elements
Existing forging blanks are often used to make
pistons for different engines with similar dimen-
sions. The position of the ring pack and the pin
can be adjusted within limits to make different
pistons from the same rough blanks. Billets of-
74 | D r a g
I l l u s t r a t e d | DragIllustrated.com
All forged pistons begin as either a forged blank, or
a forged billet. A forged blank (left) is pressed into a
rough shape by hundreds of tons of pressure, whereas
a billet is cut from a piece of forged bar stock.
A forged blank, such as this one, requires a specialized
forging die. This forging die can only produce one
part, making it both an expensive and time consuming
process. Typically, a forging die is only produced to
make parts in significant quantities.
fer much greater customization options because
the machining operations start with a raw metal
slug not constrained by the pre-set shape of a
forging blank.
To accommodate volume requirements, popular
piston selections are reinforced with a good supply
of forging blanks that suit their overall requirements.
Sometimes a suitable blank will not support the de-
sign stipulations of a given piston order and a billet
makes more sense. Once the design elements are
finalized it can proceed straight to the machining
center and the pistons can be made without delay.
Billet pistons are also used for proof of design
concept exercises where a part is developed and
extensively tested and modified before converting
it to a forging via the appropriate tooling. Whether
forged or billet, each type of piston design is com-
puter simulated and evaluated in FEA (finite ele-
ment analysis) first. This occurs regardless of time
constraints, but from that point, billet pistons
can be run almost immediately where a forging
may require new tooling and longer setup time.
Billet slugs are cold sawed from bar stock and
machined directly. Forgings are also cut from bar
stock and then literally punched into shape via
large mechanical or hydraulic presses with thou-
sands of tons of pressure. Both the alloy and the
forging die are heated to about 800 degrees F to
minimize heat transfer and encourage optimum
grain structure within the forging.
Machining Operations
For race applications, billet pistons are ma-
chined from solid 2618 billet aluminum. Billets
Issue 142
W
ITHIN THE BROAD SPEC-
trum of automotive piston de-
signs and materials, the differ-
ence between forged and billet
pistons often blurs. Billet pistons are commonly
thought to be stronger and tougher than forg-
ings when in fact, their strength properties are
surprisingly similar, with forged pistons holding
a slight edge due to the improved grain structure
contributed by the forging process. However, the
difference is such that even in higher-end applica-
tions, strength is not an overriding consideration.
Billet piston demand has risen sharply in re-
cent years primarily because of time requirements
and the expanding need for racers and race teams
to pursue new piston features. When require-
ments and design adjustments call for geom-
etry changes in the strut braces, pin towers and
other areas that cannot be accommodated with
an existing forging blank, custom billets are the
quickest way to meet the emerging need. If the
anticipated power level is determined to exceed
the limits of any available forging option, billet
offers a quicker, cost effective way to implement
specific design requirements without delay. This
is particularly true in low volume applications
where developing a new forging would be costly.