Billet is a solid piece of metal (aluminum or steel) that is cut and
carved/shaped with a CNC machine (see below) to obtain a given shape. A
billet engine head is obtained from a "brick" of aluminum and cut to
get the fins, valve seats, combustion chamber etc. Billet is an
alternative to cast metal (see below).
Polished and Chromed are often found together. Polishing is the process
of "rubbing" a piece of metal until it becomes as shiny as possible.
This is usually done on aluminum and stainless steel. Other types of
steel will rust over time. Polishing is done by first removing the
surface imperfections of the part with different types of abrasives and
then using buffing wheels and polishing compounds. A polishing compound
is a paste with mild abrasives and other chemicals. Polishing creates a
high luster but is not as shiny as chrome plating. On stainless steel
the metal becomes almost as shiny as chrome but with a slight yellow
shade. Chrome is generally slightly blueish. Polished aluminum is not
nearly as shiny as the chromed variant and requires constant
maintenance since the metal will tarnish over time.
Custom wheels are either cast or forged. Forging means that the metal
is heated at very high temperature making it soft but not melting it.
This soft metal is then pressed into shape with a press able to apply
hundreds or thousands of pounds of pressure. The advantage of forging
is that the metal retains its molecular integrity since it's not
melted. In contrast, cast metal is melted and this can create bubbles
embedded in the final product. In the RoadStar engine, which is cast,
if you remove the rocker arms from the cylinder heads and look inside
the arms' guides, usually you can see several bubbles exposed by
drilling the hole for the rocker arms. That would not happen with
forged or billet parts.
Casting is an economical process where metal is melted and then poured
into a cast. This is a much faster process than machining (billet) or
forging, thus creating parts that cost less. Cast doesn't have the same
sharp lines of billet parts and cannot be used to create certain
shapes. Production engines, both Harley-Davidson and Japanese, are
usually made with cast parts.