Austarion

if the piston shrinks and expands more than others, then would that not mean that an engine built with these pistons will have a shorter lifetime between rebuilds than an engine built with pistons that have less of a fluctuation in size at different temperatures?

Yes and no.

The rattle is when they're cold before they expand to fill the bore perfectly. They will all expand to fill perfectly, the question is how much do they contract when they get cold. The rattle is literally the pistons whacking against the bores.

yeah i get you there - that doesn't explain the "no" though? when the piston is cold and rattles, wouldn't that mean firstly that it won't be sealing all that well while cold, and secondly that it is damaging the bores more until it's warmed up?

Think of the rattle as more of a light tap rather than a scrape or bash.

If you tap on your plaster wall with a claw hammer does it put a hole in it? You have to give it a bit of a whack to even mark it.

Also, we have cast iron blocks so even less issue. The alloy simply isn't hard enough to cause an issue with the bore. It just makes a shitload of noise and sounds like it's going to explode for a few minutes until it warms up.