I picked up a broken diesel generator for free(10hp Chinese yanmar clone 6kw unit), i didn't know the history of it, all i knew is it was faulty in some way. Anyway the initial inspection was good, it appeared to have decent compression (at least i thought it did) and the injector was spraying evenly and making the distinctive chirping sound when it injects. I tried to start it, which was very hard with out the starter motor (Dead battery), but when it ran it could not get up to full speed and made the most awful noise imaginable (at the time it sounded like heavy pinging) and it created a huge cloud of black diesel smoke. This was odd as at first it looked like an injector issue, but they checked fine so i thought i might take the rocker cover off to change the valve clearance to see if that made a difference. They where a bit loose but i have seen a lot worse before! After that i started it up with the rocker cover off, just to see if anything unusual was happening. It turned out that my previously diagnosed pinging sound was actually the noise of the combustion gas escaping in to the crankcase, presumably from blow by . Sounded kind of like a blown head gasket but 10x louder, i suppose when the engine is sealed the noise sounds more like a knocking sound which is what i previously thought. So my next course of action was to remove the head, a 10 minute job on these yanamr / yanmar clone engines. Upon inspection i noticed everything looked normal, but when i turned the engine over there was a huge dent in the cylinder wall at both ends, literally 1/8" of metal was gone on both sides! This is obviously what was causing the excessive blow by. It looks like i will have to replace the block but what could cause this in the first place? No metal had impacted the valves or the cylinder head. [img:left]https://flic.kr/p/oxYjQe[/img] [img:left]https://flic.kr/p/oeHRT9[/img]