Re: Metallography of Cracked Viggen Pistons..... - Saab NG900 & OG9-3 Bulletin Board - Saabnet.com
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Re: Metallography of Cracked Viggen Pistons.....
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Posted by Dean [Email] (more from Dean) on Mon, 4 Apr 2005 21:01:27 Share Post by Email
In Reply to: Re: Metallography of Cracked Viggen Pistons....., Steven, Mon, 4 Apr 2005 19:46:28
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These are probably high silicon content to reduce expansion effects. See the link for a brief bit of info. "4032 low-expansion aluminum alloy"

elsewhere:"4032 is a high strength aluminum alloy with 12%
silicon content as the primary alloying agent.
These silicon particles are extremely hard
and durable and provide a wear resistant
surface for the rings, pins and skirt
contact areas. Additionally, with the
high percentage of silicon, this
alloy has a low coefficient of
expansion. With 4032, much
tighter piston to cylinder wall
clearances can be achieved. The
end result is a much quieter
running engine on cold start-ups’ as
well as a more stable piston with less
rock over in the cylinder bore. These
properties make 4032 an ideal choice for
street and many mild racing applications."

Again, no piston will stand up to heavy/sustained knocking. Knocking can overheat the piston that then hot cracks. Or the piston can be damaged by high overstessing from knocking and cold crack.

With a high silicon piston, cast or forged, the silicon particles should be very easy to see in a micrographic exam. The particles should be evenly dispursed. Certainly one does not want these interupting the aluminum matrix which would create weak spots and stress risers. So clumps of these would be bad. I remember a study of a 2 stroke cycle piston that had too much silicon to the point of weakening the aluminum matrix... that was over thirty years ago.

If the engine has been run with fire blowing through the crack, I don't know what happens to the metal thermally, short of a crankcase explosion.

http://www.jepistons.com/pdf/2002-sportcompact-intro.pdf

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