There are very few engine failures that any mechanic would consider beyond repair, but first among them is the cracked engine block. While such failures are fairly rare, the end result is generally irreparable damage. Additives like sodium silicate (a.k.a. "liquid glass") can help delay complete failure of a slightly fractured block, but it will never be as strong or reliable as it once was.
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Overheating is, by far, the most common cause of cracked blocks. Engines blocks are cast with many small channels, sharp edges and tight clearances. Coolant can only cool what it touches, so extreme overheating causes localised temperature spikes in the engine block. The metal in those places will try to expand away from the cooler areas, causing the engine block to literally rip itself apart from the inside.
Rapidly cooling hot metal with cold water will cause those metal directly in contact with the water to contract faster than the metal beneath. The metal develops microscopic fractures that quickly propagate into large cracks. Pouring cold water into a dry and overheated cooling system can cause another type of localised heat-related cracking when the portions of the block directly in contact with the water contract away from the hotter portions.
"Core shift" is a manufacturing defect. Engine blocks are "investment-cast" by injecting liquid metal into moulds made of highly compressed sand. The sand can shift slightly during the casting process, leaving the block thinner in some places than in others. Such core shift is fairly uncommon on newer engines, but poses a serious and un-fixable threat to older castings.
Overpowering engine blocks to the point of destruction was once fairly rare, but the explosion in popularity of aftermarket turbocharging/supercharging (a.k.a. "forced induction") and nitrous oxide (N2O) injection makes this a very real possibility for enthusiasts looking for more power. Before these technologies came to dominate the marketplace, engine horsepower was generally limited by the connecting rod's ability to withstand breaking at high RPM. Forced induction and nitrous work by increasing power throughout the engine's entire RPM range, making engine block strength the limiting factor. Stress-related cracking usually occurs in the area surrounding the engine's main crankshaft caps, known as the "webs." The webs act as a mounting point for the main caps, which secure the crankshaft to the block and bear most of the abuse.
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