I was wondering about this myself, and now know mine should last as long as I will.
I was told by the bike ship that I should never, ever, ever try to take them apart for cleaning. He says I would never believe how strong those (SPD) springs are. I do take advice, but you ever look at a red button labeled "Do Not Push Button"? That's how I'm feeling.
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Some people are like a Slinky ... not really good for anything, but you still can't help but smile when you shove them down the stairs.