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The retaining bolts that hold the piston pins to the pistons are threaded up near the head, with a smooth shank on the bottom half. The bolt threads into the boss in the piston, and the smooth shank into the piston pin.
I was wondering why they did it that way, especially as the piston is cast iron and the pin is steel.
Then I realized that if the piston pin had been threaded, the bolt would have drawn the pin radially towards the piston on one side and would have put a load on the boss on that side. It might have even tended to load the piston pin to one side of the connecting rod.
All those issues are avoided with the bolt threaded to the piston rather than the pin.
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