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Piston slap is due to excessive clearance space between piston skirt and cylinder. Often heard during warmup before the piston warms up. Not a usual sound in a 911 as long as the factory piston-to-cylinder clearance is to spec. More commonly heard with aftermarket pistons because their greater expansion rate when warm requires more clearance when cold.
Excessive clearance not only results in piston slap but allows the piston to rock slightly causing the rings to wear faster. Replacing pistons/cylinders is usually not part of a top end overhaul; usually related to cylinder heads and stuff contained within it, but they're easily replaced with the heads removed. If the rebuilder managed the same piston clearance on all pistons, you'd hear slap from both sides.
However, the noise might be coming from the loose chain flopping around in the chain box. Good thing you pulled the cover off to inspect it. You can adjust the tension using the screw adjuster and the noise might go away which would confirm the loose chain theory. The real fix is to replace them with the factory tensioners that self-adjust for chain wear and temperature. The mechanical adjusters are not recommended just because no single tension setting will be adequate for all engine conditions.
But then again, all rebuilds are not equal. Who knows what the rebuilder did; how careful was he in selecting, measuring and assembling? There are a hundred places to screw it up if given the chance to neglect it.
Perform a close visual inspection, check for metal bits in the oil, mate the gearbox to the engine and perform a compression test. If it looks okay, I'd reinstall it, but with the factory tensioners. You can borrow a couple from the extra engines you have.
Sherwood
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