Forged pistons

[Robert Legere]

> Differences in expansion and contraction of cast,
> forged, and hyper..I
> can't spell the rest of that word...pistons?

*****Hypereutectic....most people seem to like to add
an 'n' in there somewhere, but there isn't one.

>I don't know, but I'd like
> to have a more complete answer, too.

*****Typical cast pistons have low expansion rates,
and can run clearances of WELL under .001". A
hypereutectic piston has some advantages...stronger
than cast pistons, but less expansion than forged
pistons. Can usually run .001" to .0015" clearance or
so, depending on bore diameter. Hypereutectics don't
tolerate detonation though, they essentially shatter.
     Forged pistons expand at a greater rate, and are
less consistant. Higher heat forces higher expansion.
A very tight tolerance forged piston such as a J.E. or
Cosworth might have .0025" to .0035" of clearance,
which is still as much as 4 times greater than that of
a cast piston. So the piston will rock within the
bore, creating a rattling sound, especially when the
engine is cold (sounds a little like a bad timing
chain tensioner).
     As they expand, the tolerance decreases, but if
they are overheated, it's not uncommon for a forged
piston to gall the bore and piston skirts, and even
seize in the bore. All that rocking wreaks havoc on
the ring seal (and hp), and in the case of moly-coated
rings, it can strip the moly clean off the face of the
ring in some instances.
   Worst-case scenario, a high expansion forged
piston, can utilize .005"-.007" of clearance for a
street engine, and as much as .010" for a high-boost
turbo engine. So unless you're truly dedicated to
building a racing engine for the street, a cast piston
is probably your best option for most street
performance usage. The sound made by high expansion
forged pistons when they're cold sounds a bit like 4
spun rod bearings......

Bob