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Noneck,
You are correct about the splined bolt, but it is only on one of the shackles. The opposing shackle is unsplined and the nut tightens against a sholder bolt. The shackles do not begin to wear until the nylon bushing has failed and begins to place undue forces on the shackle bolt (like metal against like metal).
By the time I changed out my shackles and bushings, several (but not all) of the bolts on each side of the coach were at a point that the bolt needed to have a back wrench on the splined side to loosen the nut because the spline also had failed. Look at the pictures and you will see worn bolts, worn out bushings, and elongated shackle holes.
I agree that this may not "supposed to happen", but it does. The early shackle bolts were also splined. Your 2 cents worth is always welcome as is any observation and discussion. I am neither engineer nor metalergist. As a one time mechanical designer long ago and far away, I may be applying ancient technology to a current situation. I wish one of those types would chime in with definitive answers based on first hand, independent observation and not just speculation.
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