Before I started working on my first turbo system for my Fiat Spider back in 1998, in preparation, I installed this high dollar knock-retard system called a J&S Safeguard. The owner told me he actually designed this system for a Fiat 131 which uses the exact same engine as my Spider.
Before I even installed the turbo, the system said the knock sensor was picking up engine knock. I couldn't hear any detonation or any other unusual noises otherwise. The odd thing was I filled up with Sunoco's racing fuel just to see if this had any effect and it actually seemed to effect it in a positive way.
I tried moving the knock sensor to different locations on the engine but eventually gave up on the system, sold it on Ebay and went with a Jacobbs Boostmaster which allowed an automatic of retard under boost and now I run a simple 5 pin GM HEI control module with a fixed 5 degree retard when the turbo spools up.
I had posted this on one of the Fiat forums and the designer was doing a Google search to see what people were saying about his product when he saw my post and emailed me to tell me why he said it didn't work on my car. He said the knock sensor (made by GM) was prone to confusing engine knock with piston slap. He said in my case, the system would not work.
When I replaced a headgasket a few years ago I was cleaning some of the carbon buildup off the pistons and noticed at TDC, I could actually move the pistons by hand like the guy in the video was doing.
But the thing is, this has never caused me any problems. The reason for the headgasket replacement is Fiat twin cams are prone to leaking on the exhaust side below the distributor as well as the gasket between the exhaust cambox and head gasket in that same location. The head gaskets usually blow at 60K miles anyway so adding a turbo probably shortened it a bit. So I wanted to catch it before it actually blew.
Otherwise this is the same engine I had rebuilt back in 1992. I have no idea how many miles is on this block.
Every now and then when it's really cold outside, I will here what sounds like piston slap on my Subaru then it goes away when the engine heats up. I understand Boxer engines are supposedly more susceptible to this for some reason. Otherwise in the summer, my Subaru does not generate any piston slap. Just at subfreezing temperatures.