I long ago switched out all infrequently removed clevis pins with stainless pan head screws with ESN lock nuts. I did it because they are way less 'snaggy' than clevis pins and cotter rings. But it turns out they NEVER loosen, either.

On the shackles, NEVER buy the cheap no-name shackles for use on your boat. Just don't do it! Buy those nice pricey French made shackles with the max load stamped on them, I forget the brand. Not only are they made of stronger metal, but the slip side (the non-threaded opening for the pin) Has four corners instead of being just a plain round hole. So as you tighten the pin, the corners engage the sides of the pin, and do so progressivley more strongly as you tighten, so that these shackles are 'self saftied' and never loosen on their own. I have noticed that after a hard day of sailing with a cheap no-name shackle on the mainsheet blocks that the shackle pin will always bend a little. I found this a bit alarming and is the reson I swore off the cheap stuff. The French made shackles do not bend on the mainsheet.

Jimbo