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             You must preload the
            spring to a minimum of ½".  Manufactures rate springs
            with a higher preload.  The higher the preload the higher the
            spring rate because the end coils start to touch, effectively
            shortening the spring.  Without adequate preload readings may
            be inconsistent.  Coil springs - even the best quality - can
            vary as much as 5% from the stated rate.  This is normal
            manufacturing tolerance. 
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