Re: AMC Pistons; Forged have downsides too , Cast has advantages ...

["R L Rapp" <masheen@xxxxxxx>]

You got my gears turning, John!
The weight differences you describe between the cast and the forged pistons
may be as you say.  I however, believe that an aluminum forged piston is
denser than a cast piston due to the fact that it is pounded/beat into its
rough shape and then machined to final dimensions.  Denser material equates
to more weight.  During the forging process, the materials' molecules align
themselves more uniformly than if the same material were poured into a mold
and cooled.  As I said before, stress is induced into a part whenever it is
machined.  Stress is also relieved as a part is machined.  This all depends
upon the machining operation and part geometry.
It should also be noted (again) that a good quality set of aftermarket slugs
are gonna be very close to each other with respect to their weights.  It
doesn't matter if they are cast or forged, manufacturers have designed (into
the piston) areas where material may be removed for the sole purpose of
matching the weights on a set of pistons.
The way I see it, there must've been valid reasoning behind AMC engineers'
decisions to use forged cranks, rods, pistons, etc in their engine designs.
Most likely for the same reason their blocks have high nickel content; it's
all about durability.
My belief is that one shouldn't skimp when it comes to reciprocating parts
in any high output engine.  The piston/rod assembly materials are subject to
both tension and compression.  Cast parts are good (depending on
application) to a point and for many of the reasons you stated.  Ideally, I
would prefer to have all of my stuff manufactured by the sintering process.
You have the best of both worlds and no stress in the part when it's made
this way.


Bob Rapp
NN, VA
70 Machine