I've heard from some guys, and also read on numerous online message forums, about claims of excessive piston slap with JE pistons. I use about 15-20 sets of JE pistons per year and I have NEVER experienced piston slap under normal start-up temperatures (over 20 deg. f) with up to .006" piston clearance. However, very slight piston slap is expected for a short duration during start-up in freezing temperatures with "race spec" clearances.

I have had the opportunity to inspect a couple rebuilt engines with piston slap, built by other shops, and both of those engines had double the claimed piston clearance.

I think the reason that this "JE piston slap" rumor had started was due to amateurs, and some so-called professionals, improperly measuring and machining the piston cylinder wall clearance.

The proper way to measure piston cylinder wall clearance is:
Clearance = Bore_ID - Piston_OD

In constrast, the proper way to measure crankshaft bearing oil clearance, with the bearings installed, is:
Clearance = (Bore_ID - Crankshaft_OD) / 2

I think this is the reason for the reported piston slap issues. I bet a lot of these guys have 0.008-0.010" piston clearance, and that much clearance will most definitely be audible.