Keith & List;
Metal parts such as rods, cranks, pistons, rocker arms, bolts, etc. all have
a limited fatigue life. This is expressed an "N" cycle life, where "N" is
the number of stress cycles that the part has undergone. For most materials
there are "S-N" diagrams that show how many (N) times the material can be
stressed (S) to a particular level.
Rods are a good example. They alternately stretch and compress; this is
considered to be one "cycle", so one crank revolution gives N = 1 on a
connecting rod. The amount of force on the rod is determined by RPM,
cylinder pressure (horsepower), or detonation. The force and the and the
cross-sectional area of the rod determines its stress . Leaving the shape of
the rod aside for this illustration, the rod's stress is the force divided
by the area. If the force is 1,000 pounds and the area is 1 square inch, the
stress is 1,000 pounds per square inch (1ksi).
If the number of cycles it took to cause the rod to fail with a stress of
1ksi were plotted on a curve and then repeated with 10ksi, 20ksi, 50ksi,
etc., the points on the curve would be what's known as an "S-N" diagram. If
we knew how much stress we had, we could predict when the rod would fail.
Different alloys have different curves.
This is very simplified, but the point is that metal parts have a limited
lifetime in very high stress applications and unless we know something of
the history of them, it's a crapshoot. Of course, if the stresses are low,
the parts will last indefinitely. On the other end of the spectrum are
aluminum rods-- they are guaranteed to fail after a short time. That's OK
for drag racing where they are replaced after only a few runs.
If those Carillos are Magnafluxed, they'd probably be my choice, too.
Regards, Neil Tucson, AZ
-----Original Message-----
From: Keith Turk [mailto:kturk@ala.net]
Sent: Thursday, June 05, 2003 9:17 AM
To: Scott Cowle; Land-speed@autox.team.net
Subject: Re: to buy or not to buy??????
Depends on what it is Scott.... I'd rather have a set of Used Carrillo rods
then a brand new set of eagles....( not that they are bad...they just aren't
as good )
Quality lasts the lifetime of the component...
Money saved never outweighs the quality over time... or at least that's the
way it's been in my life... So I'm not afraid of used.... but it's going
to be the best part going...
K
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