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post #2 of (permalink) Old 23rd August 2007
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When an engine is 1st made, they machine out the bores to accept the pistons. This machining makes the bores totally round in diameter.
As the years progress with the miles ticking away, the pistons are whizzing up & down the bores and are wearing the bores away (very slightly). This is friction between the piston rings & the bore walls. This only happens on the front & back of the bores due to the rotation of the crank and the way the conrod pushes the piston up & down the bore. It puts extra pressure on the front & back.

Eventually, the bores become oval and you begin to lose some compression.

At worse, the piston could eventually turn 90 degree in the bore and ceaze the engine.

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