Rings were checked, yes.

I know a lot of aftermarket pistons slap from cold; a lot of my mate's Zetec turbo engines have piston slap from cold. What's really strange with this is that it never piston slapped even in the slightest before, and it's been started/run a lot of times. It only did it when it got hot, unless that was a coincidence.

The fact that all 4 pistons are doing it the exact same amount. You would think if the heat had done anything it would have been slightly worse on one cylinder than another. It indicates it's a machining error.

I'm going to whip a piston out Saturday and measure it against the bore.