There are actions you can take, whether deliberate or inadvertent, that can lead to ruining an internal combustion engine.
Drain the oil from the engine. Underneath the engine there is a small screw called the oil plug which is used to drain the oil, removing this will allow all the oil to drain from the engine. The most important component of an engine's performance is oil. During the combustion process, the metal pistons, metal rings and metal piston chambers move at thousands of revolutions per minute. Small imperfections in the metals cause the components to resist moving across each other, creating friction that causes heat and wear on the metal parts. Motor oil provides lubrication, also known as viscosity, to smooth the interaction between metal parts. Oil also provides a cooling medium for the internal parts of the engine, called thermal protection. When oil is dirty (carbon buildup from months or miles of combustion) or when oil is low, the viscosity of the oil breaks down and its capacity to reduce the heat of the engine to tolerable levels fails, leaving the metal parts vulnerable to catching upon each other and "seizing." Removal of oil from the crankcase would quickly render any engine useless in a matter of minutes.
Pour the wrong type of fuel into the fuel tank. Another important component in the proper functioning of an internal combustion engine is gasoline of the proper type. Most commercially available gasoline is appropriate for any engine, but some mistakes have been made by motorists who put regular gasoline in their diesel engine, or vice-versa. Diesel gasoline is different from gasoline in that it includes oil in its composition. This oil is necessary for the lubrication of diesel engines, as they typically do not have the same cooling and lubrication mechanisms of regular engines (oil reserves and a mechanism to distribute the oil to the chambers). Unlike other internal combustion engines, diesel engines rely upon a "glow plug" and higher compression of the piston to achieve the proper explosion that will generate motion. Introducing unleaded gasoline into a diesel engine can render it useless until the tank and gas lines have been drained and the engine repaired. Similarly, but not nearly as extreme, the introduction of diesel into an unleaded-only engine system will still run, but the exhaust produced will be thick and obscure the road for other drivers. It also may cause the driver to be cited by law enforcement. Diesel gasoline will need to be purged from the system, an expensive task.
Over-use or improperly use nitrous oxide. Performance-enhancing gases, such as nitrous oxide, are sometimes used in racing--often illegal street racing--to provide a temporary boost of speed to the engine, as nitrous oxide is more volatile than air drawn from the atmosphere. Using nitrous oxide, however, requires certain precautions, the most important of which is the use of reinforced (and thus heavier) pistons. Nitrous oxide may be used in limited cases in cars without these reinforced pistons, but there is a risk of severe damage to the piston or piston rods because the explosions caused by nitrous oxide combustion are several times more powerful than specifications for stock pistons can handle. Nitrous oxide can cause the pistons to shatter, sending metal fragments into other chambers and portions of the engine, effectively ruining the engine.
Add lots of water to the fuel tank. Water in the gas tank is not necessarily a death sentence for an engine; occasional rough idling or performance problems can be caused by small amounts of water in the engine and are cured by gasoline additives that remove water from gasoline. Large quantities, however, would render an engine useless until the lines and tank had been drained and dried. Introduction of water can occur during floods, for instance, as it overflows into the engine via the air intake system or can be present in contaminated gasoline supplies. In this instance, the engine is ruined until a mechanic has overhauled the engine.
Loosen the engine block's bolts. Compression is key to proper running of an engine but metal parts, no matter how well engineered, do not create a strong enough seal when bolted together. Therefore, the use of paper seals between the parts allow the gases created by combustion to be directed properly but if they fail, or are removed, they can ruin an engine. The term "blow a gasket" is common because until recently, gaskets would fail because shade-tree or amateur mechanics would not fasten the bolts of the engine properly (head gasket, exhaust gaskets, or even oil filter gaskets). But engines today are much more precise and, unless tampered with, never "blow" their gaskets. Blowing a gasket will render the engine useless until major repairs have been made, and can be potentially dangerous in high-performance engines.