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Hello all, Three years ago my engine got hydrolocked. After that I replaced piston on cylinder no. 6, because connecting rod was bent a bit. Even after that, there is some knock while engine is cold, that lasts about 2 minutes when engine is cold. After warm up knock is gone. In three years things were not getting worse, even knock is still there. I can't detect knock origin for sure, it sounds like its on the side of the engine where the distributor is, but I can't be sure if it is in the upper or lower part of the engine. After 1300-1400 rpm even when its cold its very hard to detect (but I'm not sure if it is not masked by the engine or fan noise). Last time I checked compression it was in upper factory range. Rod bearings and crankshaft were replaced year before hydrolock. I always thought that this is valve related because lower part of the engine appeared fine, but I've recently heard about piston slap in 4.0 engines. I guess that piston skirt is not broken since noise disappears after warm up. My questions are: 1. How can I detect what is causing this noise without disassembling engine? I've read that if I remove spark plug wire from cylinder and listen for knock, if it gets quieter then its piston slap, if not then rod bearings or valves 2. If it is piston slap, will it do any damage to cylinder walls? Do I need to replace anything else than entire piston?