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Assumption based on miles of use
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Posted by MI-Roger [Email] (#882) [Profile/Gallery] (more from MI-Roger) on Thu, 17 Nov 2011 18:07:12 Share Post by Email
In Reply to: Re: Here's to hoping the problem is gone!, SWEDECAR [Profile/Gallery] , Thu, 17 Nov 2011 08:17:24
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I agree with you that the porosity could be occurring anywhere in the engine, but its sudden appearance in this case after 115K miles of use caused my assumption that some type of surface wear exposed a porous area of the casting. Another jump of reasoning placed this surface wear in the cylinders.

Something changed to cause porosity where it did not exist before. If not surface wear, then another possibility is that the block was originally cast with a porosity problem, repairs in the foundry proved to have only a moderately long life and failed again after 115K miles of use.

The reason the castings look like sintered together balls of aluminum is because the blocks were cast using the "lost foam" process. Tiny beads of poly-styrene are melted together using steam heat, this light weight sacrificial form is then placed in a mold and hot molten aluminum melts the plastic foam and replaces it. After cooling it is machined into an engine block with a strange appearing outer surface. The tiny ball appearance should not go all the way through the block walls since the aluminum replaces both the beads and any space between them, but a failure of the molten aluminum to advance through the foam in a single cohesive flow could leave tiny capillary like voids resulting in porosity.

This process was adopted due to toxic fumes released from the previous generation of resins gluing together the sand particles used to construct the cores necessary for sand casting of engine blocks. Recent advances in core making technology have made lost foam obsolete and manufacturers are again using sand molds with precision cores constructed with different chemicals. Whether they are truly less toxic is yet to be determined.

posted by 68.43.27... 
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