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Hyper pistons are cast, the main difference between hyperuetectic pistons
and conventional cast pistons is the silicon content of the alloy. Hyperuetectic
alloys contain as much as 16% silicon. The higher silicon content alloys have
lower thermal expansion rates and as such can be installed with tighter clearances than pistons made with more conventional aluminum alloys. This
is why they have become popular in the car industry because emission control
becomes easier with "tight" fitting pistons. If there is a problem with "piston
slap" or rattle it is most likely due to the manufacturer fitting the pistons
with excessive clearances. Since they "grow" less with heat working into them,
they may not quiet down once the engine is warm because they can't fully compensate for the cold installed clearance. In application terms, hyper pistons can generally be used in applications where cast pistons are appropriate,
but they should never be considered as an alternative in an application that
calls for forged pistons. If your application calls for forged pistons, then that
is what you should use.