[edward]

more on piston slap:

Excessive piston slap occurs when the clearance between the piston and the cylinder bore is too great. The piston to cylinder bore clearance becomes too great either through wear, mismatched pistons and cylinder bores at manufacturing or, a combination of both. The audible noise associated with excessive piston slap is due to the perpendicular impact of the piston against the wall of the cylinder bore. Audible piston slap is typically loudest when the engine is first started up. The pistons then expand with heat reducing the piston to cylinder bore clearance thus, reducing the perpendicular impact of the piston against the cylinder wall and its resulting noise.

In the case of the famous GM piston slap engine defect, the piston design with hypereutectic (high silicon content aluminum alloy) pistons, reduced or eliminated piston skirts (to reduce reciprocating mass), and a higher ring pack to reduce unburned fuel mixture on the sides of the piston crown have made piston to cylinder bore fit much more critical. The amount of tolerance (variation or margin) in allowable clearance between the piston and cylinder bore to prevent audible piston slap has been reduced by a factor of at least 50%. Consistently hitting the narrower margin for piston to cylinder bore tolerance has not happened for GM during mass production. Thus, some engines have no audible piston slap and some have piston slap on only one or two cylinders. What might have looked really good in testing of hand built engines in the lab hasn't transferred to the production line of this corporate giant.
Make no mistake about it, while a lot of these engines don't appear to be driving rods through the blocks, the ones with louder and longer duration piston slap will wear out before the ones that are basically quiet. The perpendicular heavy impact of the piston against the cylinder wall over time will not come without a price. This is also why GM has released a recent TSB saying that opening 4 quarts of oil to add to your crankcase between a 7,500 mile recommended oil change interval (1 qt per 2K miles on an engine with 36K miles or less is "NORMAL". After 36K miles, all bets are off (there is no abnormal oil usage rate). This is why the now common offer of an engine component letter extending your warranty to 5 years or 100K miles is basically worthless. If the piston isn't laying in the oil pan in pieces, the engine will be operating "NORMAL" according to GM