I may be the expert on piston slap: I bought a Subaru Outback 2.5 liter engine (4 cyl boxer) new in 1998. It's an aluminum block with steel liners, so the pistons run on a steel surface. Soon after I bought it, I heard piston slap when the engine was cold in the morning for a few miles until it warms up. The noise also goes away when there is very little load on the engine, like driving downhill. The piston slap re-appears when there is a lot of load on the engine at low rpm. as in towing a trailer uphill. Now the engine is warm. I made big noises to Subaru. They told me:"we know, but it's not detrimental to the longevity of the engine". But the mechanic tells me they have supplied replacement engines free of charge in cases where the piston slap wouldn't go away after driving it a few miles. Now, 10 years later, I have 47,000 miles on the engine, it still makes the noise, but it doesn't use any oil (yet). So, maybe I'm not the expert, but I sure hear it every day. I think, the piston skirt is too short, the piston has a too much clearance and slaps in the cylinder. The aluminum block transmits the noise more than a cast iron block. It's not pre-ignition: That is much louder and scary and happens on acceleration. It's really noticeable even for a dumb engineer. I just did my Honda Trail 90: On a two inch bore I had the shop bore it 1/2 thousand inch larger than the piston. When you do that, the dry piston, without rings, stays in the hole against gravity. I broke it in, in several short high speed, high load runs. We have just the hills here for that. Runs wonderful now. Uses no oil, no smoke.