- Driving style and maintenance
- engine damage
- Engine damage due to lubrication problems
- Engine damage due to cooling problems
- crankshaft damage
- Crankshaft and connecting rod bearings and bearing journal damage
- camshaft damage
- Forming camshaft wear
- valve damage
- Piston damage
- curved connecting rod
- Broken piston pin
Every vehicle needs maintenance. Prescribed parts are periodically replaced during maintenance and the wear condition of other parts is checked at every inspection. If you suspect that the part will not make it to the next service, it is recommended to replace it. In addition to regular maintenance with an inspection, it can happen that a part breaks down. The quality of the material has a lot of influence on this, as does the extent to which maintenance is carried out in a timely manner. A vehicle where maintenance periods are exceeded, repairs are postponed or an unskilled person overlooks wearing parts, has the greatest chance of defects. It becomes annoying, especially when one is on the road with the vehicle and ends up at the side of the road with a defect that could have been prevented.
This page on engine damage originates from the chapter “Diagnostic technology, mechanical measurement” where measurements on engine parts are described. Such measurements are made on disassembled engine parts (eg comparing piston diameter and cylinder diameter, determining cam height, checking clearances) and are based on a diagnosis where the cause of a malfunction was sought. A car can be brought into the workshop, whereby the customer states as a complaint:
“The engine light is on and the engine has noticeably less power than before.”
- The mechanic or diagnostic technician connects the EOBD tester to the vehicle and reads out the trouble codes:
- Error Code P0172 - Fuel Mix Control Bank 1: System Too Rich
- The fuel trims are read via the live data. From this follows the value: -15.
It can be deduced from the error code and the long term fuel trim that the lambda probe is measuring too rich a mixture in the exhaust gases. The mechanic or diagnostic technician does a number of electronic tests and looks for mechanical causes. During his diagnosis, he or she disassembles the valve cover and discovers that the cams on the camshaft above cylinder 4 show signs of wear. This raises the questions:
- Is camshaft wear the cause of the malfunction? Worn cams can cause less oxygen to flow into the cylinder, resulting in a too rich mixture (resulting in excess fuel);
- In case of wear: what was the cause? How can the problem be prevented from recurring in the future?
On the pages “Mechanical diagnosis" and "Measuring engine parts” we will discuss various parts of the measurement techniques such as measuring the camshaft. On this page we focus on the actual damage and its cause. If we can determine the cause, we can also prevent the customer from reporting the same problem again in the foreseeable future.
Driving style and maintenance:
Sooner or later, any combustion engine can be damaged. Some engines are known for their sensitivities and weaknesses, in other cases the vehicle owner has been negligent in maintenance or driving style has played a role in a wear process. Old age also sometimes throws a spanner in the works: no engine has eternal life.
A good driving style benefits every motorcycle:
- Do not let a cold engine idle for too long: the engine stays cold for too long;
- Drive slowly with the engine cold and give the oil time to warm up properly;
- Don't drive too many short trips. Even the occasional long ride does a motorcycle good;
- Always calm driving at a low speed, especially with modern engines, has an increased risk of internal pollution. Think of: a clogged inlet duct (intake manifold), extremely polluted inlet valves, clogged EGR, carbon deposits between the piston rings, resulting in oil consumption. In extreme situations, stuck piston rings cause scratches in the cylinder wall.
- Do not overload the engine at low revs: with fifty in fifth gear, the engine has a low revs. Bearings are heavily loaded. When one also accelerates, enormous forces are applied to the crank / connecting rod mechanism;
- Don't look up the high rev range too often. It doesn't hurt to pull up once in a while, but don't overdo it.
The following image shows a dirty intake path. The sucked in air is less able to pass through the valve, so that less oxygen is available for combustion during the intake stroke. This can be determined, for example, when looking with a endoscope.
In addition to a good driving style, every engine must be preventively maintained:
- Outdated motor oil increasingly loses its dirt-absorbing and lubricating effect. Parts that slide past each other are provided with a - less lubricating - contaminated oil film. The result is that an oil changes into a solid substance (black sludge) and adheres to all (especially cold) engine parts. Oil channels get clogged with all the consequences that entails;
- Poor quality oil: Adding oil with incorrect specifications or viscosity can have a short-term adverse effect on fouling, oil consumption and engine damage;
- Mechanical work such as: checking valve clearance (if applicable), replacing spark plugs, air filter, timing belt, etc. should be checked periodically. A car that often makes highway kilometers can often drive more kilometers with the same spark plugs as a car that drives a lot of city traffic. Therefore, in most cases, in addition to a distance-dependent, also a time-dependent interval is attached to it;
- Defects in parts can often be detected early. Do not continue to drive for too long with malfunction lights or noise. Have the vehicle checked periodically by an experienced auto mechanic.
The photo below shows two situations: the same type of engine with good preventive maintenance (left) and an extremely dirty engine that has run 100.000 km with the same oil (right). In addition to the black deposits on the camshaft (black sludge), the engine parts are also red-brown. This is often a result of old engine oil and too high a temperature due to an oil level that is too low.
Engine damage is not always a direct result of pollution. When we discover extreme contamination of the inlet valves, this has led to complaints such as: reduced power, a burning engine management light where a negative fuel trim appears during the reading, but this does not immediately cause permanent damage. A professional cleaning (carbon / wallnut blasting) can make the complaints disappear. If you nevertheless continue to drive with the extremely polluted valves, damage can eventually occur because the valves can no longer seal properly on the valve seats in the cylinder head.
Internal contamination from outdated engine oil, poor driving style, or any other cause of accelerated wear can cause parts to fail prematurely. This wear and tear process can be stopped when the complaints that have arisen are investigated and detected in the right way. If the indications that something is wrong are ignored, the car may stall en route, or the consequential damage will be greater than if the problem was addressed immediately.
Engine damage due to lubrication problems:
Maintenance intervals have been extended more and more in recent years. In the 70s it was not uncommon to change the oil after 7.500 km. Today we see regulations where the oil only needs to be changed at 30.000 or even towards 40.000 km. With the extended maintenance intervals, there is a risk of driving with too little oil if the oil level is not checked enough. As a result, the (too small) amount of oil becomes a lot warmer, evaporates faster, causes more pollution and has a less and less good lubricating effect. For this reason, vehicles equipped with extended service intervals are fitted with an oil level and quality sensor. On many short journeys, the oil is loaded three times more than when driving the same distance on the highway. The level sensor naturally monitors the level and displays a message to the driver if the level is too low. The quality sensor (usually in the same housing) monitors the quality. With thickened, aged oil, the change period is considerably shorter. We speak of "variable maintenance intervals" when a standard interval of 30.000 km and 2 years is specified, but a maintenance message already appears after 20.000 and one year: the quality of the oil has become so low due to the driving conditions that the oil should be refreshed sooner.
If the oil is not replaced in time, the oil will evaporate and thicken faster, as already described. The sludge that remains ends up in the entire engine. The first place where this substance collects is in the suction oil in the oil sump. The oil pump draws in the oil from the crankcase through this oil strainer. Then the oil pump forces the oil through the filter. The coarse particles are held back by the sieve.
The following image shows an example of an oil screen in good and dirty condition. You can already guess that less oil can pass through the heavily polluted sieve: the oil pump is less able to pump the oil through the engine due to this blockage.
The problems start with the clogged strainer: the oil pump turns too slowly at low engine speeds to get good oil pressure. This can lead to a lack of oil pressure at idle speed. Parts such as the crankshaft and connecting rod bearings, the camshafts in the cylinder head, the pistons in the cylinders and the turbo shaft threaten to move a lubricating oil film that is too small, resulting in more heat development and the risk of friction between metals.
In addition to thickened oil and black sludge, other materials and components can also cause the oil screen to clog. Think of: plastic parts of a broken timing chain guide, remnants of (too much applied) liquid gasket from, for example, the valve cover or sump, dirt particles that ended up in the oil when loosening the oil filler cap and pulling out the oil dipstick, etc.
If you suspect that the engine is heavily polluted internally, you can “flush” the engine by mixing an additive with the old oil. This flushing agent serves as a cleaning agent and ensures that dirt particles come off the engine parts. In extreme situations, the dirt particles collect in the oil screen and remain in it, even when the oil has been drained. It is therefore wise to disassemble the sump and the sieve after flushing in the event of such contamination and to clean both thoroughly before the engine is filled with fresh engine oil.
Engine damage due to cooling problems:
Cooling problems can be a direct result of lubrication problems. In the previous section, examples are given of causes that can lead to a shortage of lubricating oil. If the lubricating oil film between moving parts is too small, a lot of heat is generated, with a high risk of direct engine damage.
A defect in the cooling system can also lead to a lack of engine cooling:
- Insufficient flow through the radiator due to a blockage;
- A malfunctioning cooling fan due to a defect in the control unit;
- Restriction in a coolant hose or channel: eg due to a kink or a softened hose or a clogged radiator;
- Air in the cooling system because the system has not been vented properly after a repair and topping up;
- Insufficient circulation of the coolant due to a defective coolant pump (broken vanes) or slippage between the pulley and the v-ribbed belt (if not driven by the timing belt);
- A faulty thermostat;
- A defective head gasket: combustion gases reach the cooling system and vice versa.
Overheating of the engine can cause warping and cracking of the cylinder head. Therefore, after removing the cylinder head, the flatness should be checked and the head should be checked for cracks. The cracks will usually arise in the surfaces with the least material: here the heat transfer is the least.
Examples of this are: cracks between the valve seats, or between the valve seat and the spark plug hole (petrol engine) or the pre-chamber (old diesel engine). Specialized overhaul companies have the knowledge and tools to weld the cracked cast iron cylinder head in most cases.
The following image shows a crack between the valve seat and the spark plug hole.
Overheating can cause wear on pistons and cylinders. In that case, the temperature has caused too much expansion of the parts, which can cause the piston to jam in the cylinder.
In an earlier section, damage to the crankshaft and connecting rod bearing journals was discussed. Such damage is the result of a lack of lubricating oil.
A crankshaft is subjected to a lot of forces and vibrations. In extreme cases, the crankshaft may break. In almost none of the cases this is a material problem, but a result of a defect in another part of the engine or event while driving:
- Mechanical overload due to abnormal combustion or water hammer;
- Sudden seizing due to a fault in the final drive (gearbox or differential);
- Excessive vibrations due to a defective dual mass flywheel, play in the vibration damper or attached equipment such as a PTO in commercial vehicles where a vibration occurs with too high a frequency in a certain speed range;
- Material weakening due to previous bearing damage;
- Improper mounting of connecting rod and main bearing journals;
- Mechanical damage to the crankshaft before mounting.
Crankshaft and connecting rod bearings and bearing journal damage:
The crankshaft and connecting rod bearings are located at the bottom of the engine block. The lubrication is done by the oil that passes through the oil channels of the crankshaft, through the holes in the crankshaft bearing journals between the bearing journal and the plain bearing. Very large forces are exerted on the plain bearings, so a lubricating oil film between the moving parts is essential.
One of the biggest causes of connecting rod bearing damage is a lack of oil. This can occur in the following situations, among others:
- The engine loses its oil due to a leak. This can be: due to a defective turbo, an incorrect seal between two parts due to a cracked gasket;
- The driver does not check the oil level often enough, even though the engine uses a lot of oil;
- The oil pump has an output that is too low due to a defect in the pump or a restriction in the suction section;
- The engine block is tilted too far:
– in a car, this can lead to lubrication problems, especially in combination with an oil level that is too low.
– On motorcycles, bearing damage occurs after the motorcycle falls over and the engine is not switched off in time. Switch off the engine as quickly as possible via the ignition or the kill switch.
When there is a shortage of oil, the oil pressure light comes on. The pressure has then dropped to 1 bar. This indicator light alerts the operator to stop the engine to prevent further damage. In many cases it is already too late: when the oil pressure light is lit due to an oil level that is too low, the temperature has already risen high and the pressure has been too low for some time. The oil temperature between the crank pin and the plain bearing has also risen. Also, the back pressure through the oil channels will have dropped, creating more clearance. The oil film normally absorbs this slack. Without an oil film, the parts make contact with each other and mechanical friction occurs.
Modern cars are often equipped with an oil level and temperature gauge. Both will give a warning before the oil pressure light is activated due to low oil pressure. When the oil pressure light has come on, it is always a good idea to check the connecting rod bearings for damage. The two images below show the damage caused by oil deficiency.
Damage to crankshaft and connecting rod bearings and pins is not only caused by a lack of lubricating oil. Other factors also contribute to possible damage:
- Low Speed Pre-Iginition: An uncontrolled combustion that occurs while the piston moves from ODP to TDC. Especially downsized engines with direct injection, which are often equipped with a turbo. The combustion takes place at the wrong time, creating enormous forces on the piston. This can damage the piston, distribution and bearings.
- Driving style: with a cold engine the oil is still thick and the lubrication between the bearings and the pins is not yet optimal. With a heavy engine load with a cold engine, there is a good chance that bearing damage will occur.
- high load at low rpm: the upper connecting rod bearings are subjected to extremely high forces at the point where the connecting rod is positioned (almost) perpendicularly above the crankshaft;
- low load at high revs: when moving the piston upwards, an enormous amount of forces are released that are absorbed by the lower connecting rod bearings.
In addition to bearing and bearing journal damage, other engine parts such as the pistons also wear out faster with this driving style. Obviously, the above can be prevented by accelerating with a cold engine, so with little load and not above 3000 rpm.
The image below shows the forces in the motor while rotating in five different positions. The decomposition of the piston forces is shown on the page: dissolving the piston force further explained. In these images we see the force Fh return several times. Fh indicates the force on the main bearing. This force is different in every motor position. Also, when moving the piston from TDC to ODP, the upper main bearing and from ODP to TDC the lower main bearing is loaded. The list below the figure explains the force on the main bearing of the following five figures.
- The connecting rod is perpendicular above the crank pin. The force on the upper main bearing (Fh) is the same as the force on the piston (Fz) due to the combustion pressure (p). The upper connecting rod bearing is also loaded with an equal force.
- The crankshaft twists and the force Fh has dropped;
- The force on the main bearing is 0 because an angle of 90 degrees has been created between the crank pin and connecting rod;
- The lower main bearing and upper connecting rod bearing are loaded;
- The force on the lower main bearing and the connecting rod bearing above increases again here.
By means of an optical check on the bearing shells and bearing journals and by means of the measuring ovality and taper of the main bearing journals and the connecting rod bearing journals with the micrometer the wear can be determined.
When installing connecting rod bearings, it must be ensured that the bearings must absolutely not be interchanged. The bearings are worn on the bearing journals. Exchanging will always cause increased wear on the bearing and possibly also on the bearing journal. When installing new bearings, it is necessary to plastigage check the clearance between the bearing and the pin. Bearings that are too thick make it more difficult to form an oil film between them, causing friction.
The camshafts are located at the top of the engine. The lube oil will arrive last at the camshafts for an engine that has just been started. Damage to the camshafts can occur as follows:
- The cylinder head, in addition to the turbo and the connecting rod bearings, will suffer the most damage due to an oil pressure that is too low;
- If the engine runs at high revs immediately after starting, the oil has not yet reached the cylinder head (enough);
- Moisture in the oil or cylinder head can have a devastating effect on the camshaft. This is further elaborated in the following example.
Frequent short trips can lead to sludge formation. In winter, the sludge (consisting of water vapor and oil residues) can freeze, which can block the oil supply and drainage.
Moisture can also have a destructive effect, as can be seen in the following image. The cams are corroded and contain pitting corrosion. What is striking is that the upper camshaft is more heavily affected than the lower one. This probably has to do with the temperature: the intake camshaft heats up less quickly than the exhaust camshaft, so moisture does more damage.
A lack of oil or changing the bearing caps can cause damage to the camshaft as shown in the following figure. Deep scratches have occurred because material has disappeared.
Such damage can lead to a loss of oil pressure: because significantly more space is created between the bearing cap and the camshaft due to the reduction of the diameter of the camshaft, the oil can also flow out more easily.
The damage has consequences for the camshaft bearings after this bearing. Example: the oil channel runs from cylinder 1 to cylinder 4. The camshaft damage is at cylinder 3. Because at cylinder 3 too much oil “leaks away” past the bearing, the bearing at cylinder 4 gets too little oil.
Due to a lack of lubrication, the camshaft will not only wear at the level of the bearing caps, but wear can also occur on the cams. The ridge height may decrease as material wears away. The two images below show an extremely worn cam (left) and blunt cams (right).
A blunt, so less pointed (high) cam has a negative effect on the valve timing. Not only will the valve open later and close earlier, it will also open less far. The degree of filling decreases. This will be noticed by lower torque and less power at (mainly) higher revs.
Sometimes a camshaft breaks. The reason cannot always be found. On certain vehicles, including Opels (with engine codes Z12XEP and Z14XEP), it is a common issue that is subject to a recall.
Incorrect disassembly and assembly work also runs the risk of breaking the camshaft. During key work, a correct sequence must be followed:
- To install: start at the inner camshaft bearing and cross over towards the outsides (see the picture from 1 to 10);
- Disassembly: Always start with the outer camshaft bearings when disassembling. First unscrew the two bolts of the camshaft bearing A or E, remove the bearing cap, before dismantling the camshaft bearing E. Finally, disassemble bearing cap C.
If the wrong disassembly and assembly sequence is followed, the forces released by the valve springs that press against the camshaft and by “locking up” the camshaft, causing it to bulge, can cause the camshaft to break.
Forms of camshaft wear:
The wear that occurs on camshafts can be divided into three groups:
- adhesive wear;
- abrasive wear.
When we find small pits and cracks in the material of the cams, we are dealing with so-called “pitting”.
Pitting creates small cracks under the hardened surface of the material as a result of fatigue. This phenomenon mainly occurs with sliding contact, as in this case, where the camshaft slides over the rocker arm or hydraulic valve lifter.
When pitting, material disappears, so the only remedy is to replace the relevant camshaft.
This occurs when the surfaces come into contact with each other, for example due to a lubricating oil film that is too thin. This contact can break pieces of metal from the camshaft. If the pieces are small enough, there is no immediate need for engine damage: the particles are discharged to the oil filter. The moment surfaces slide against each other with great force, there is a chance that the metal parts will weld against each other (micro welding). Over time, the material next to these welds breaks through and grooves are formed in the parts that fit together exactly. This is the so-called "cheating" of the camshaft.
This form of wear occurs when particles of another material inadvertently get between the moving parts. This can be the case with adhesive wear, where loosened metal particles end up somewhere in between, or dirt particles that have entered through the oil filler cap, for example. The dirt particles scrape the material from the surfaces of the parts.
A petrol or diesel engine can have to deal with valve damage. In practice we encounter the following damages:
- burnt valves and valve seats;
- corrosion, erosion and dirt deposits on valves and valve seats.
- deformation due to a defect in the distribution;
- damage to valve stem.
The following image shows a burnt exhaust valve. The valve disc shows deformations with discolouration. A burnt valve causes loss of compression: when closed, the valve must hold back the air during the compression stroke, but in this case will not seal properly. During the compression stroke, some of the air escapes past the valve to the exhaust. Although an exhaust valve gets a lot hotter than an inlet valve, an inlet valve can also burn.
A valve can burn if it overheats. The valve will deform, which can cause material breakage. Causes of overheating can include:
- insufficient possibility to dissipate heat via the valve disc to the valve seat, e.g. due to dirt deposits between the sealing parts and too wide valve guide clearance;
- an exhaust gas temperature that is too high;
- too little valve clearance, which may cause the valve to remain open.
The two images below show the result of a broken timing belt. All twelve valves are bent and you can clearly see the imprint of the piston on the valves. In addition to a broken timing belt, one can also get this damage with a broken or stretched timing chain.
Food marks on the piston skirt:
Severely aged and contaminated oil, an oil with the wrong viscosity index, or a lack of oil cause lubrication problems. This can be a cause of eating marks on the piston skirt. When the lubricating oil film breaks through, dark-colored eating marks are created. Usually this surface is not shiny and the piston damage is mainly on one side (guideway force side) in front of.
If, as a result of a too rich mixture or a failed ignition system, there is prolonged incomplete combustion where the injected fuel does not ignite, the fuel precipitates on the cylinder wall and weakens the lubricating oil film.
Food marks on the piston crown and piston skirt:
Overheating may reduce the clearance between the piston and cylinder and push the oil film away. Boundary lubrication occurs because the lubricating oil film is broken by the high temperature. Dry friction occurs. The shirt of the piston (the side) becomes damaged (feeding marks) and possibly pieces of piston break off at the level of the piston rings or the piston material melts. Possible causes are:
- Glow ignition, detonation or a dripping atomizer;
- Prolonged high load during the engine run-in phase;
- Malfunctions in the engine cooling system, such as lack of coolant, defective coolant pump, insufficient cooling of the coolant, etc.
- Faults in the oil supply (oil nozzles under the piston).
Breakage of the piston
When driving for a long time at a (too) high speed or under too heavy a load, such as after software tuning without mechanical adjustments, especially when the engine is not yet at operating temperature, there is a mechanical overload. This can be done by:
- Detonation: Gasoline engines can detonate if they have an incorrect octane number, a compression ratio that is too high, a mixture that is too lean, an incorrect ignition timing or if the intake temperatures are too high. Detonation creates very high pressures, pushing the lubricating oil film away and causing high temperatures. The result is that piston material breaks off between the piston rings, or that a hole is created in the piston;
- After chip tuning: with a software modified engine, the pressure for the non-adapted engine parts can be too high. The piston may break due to the combustion pressure;
- Dripping injector of a diesel engine: as a result, too much fuel enters the combustion chamber and part of the fuel ignites on / in the piston bottom. Metal particles in the piston bottom can come off due to the mass forces and erosion caused by the combustion gases.
Piston coating and cylinder liner wear
In engines with high oil consumption or tilting pistons, we often see wear of the piston coating and bright spots in the cylinder liner. The honing grooves have become worn and slippery in certain parts. Causes can be:
- high mileage;
- idling regularly and driving short distances;
- had too little maintenance, resulting in increased wear due to outdated oil.
This wear can be recognized by one or more consequences:
- increased oil consumption because oil can easily get past the piston rings into the combustion chamber;
- blue smoke or black soot particles in the exhaust gases;
- rattling noises at idle and increased revs due to increased clearance between piston and cylinder. We also call this a “tilting” piston.
Not only due to the aforementioned circumstances and consequential damages, but also after a repair or. overhaul, new piston damage can occur:
- Cylinder wall contains irregularities: perhaps an old wear process or defect has not been properly noticed and the piston has been mounted in a damaged cylinder;
- Improper mounting: by not mounting the piston rings and the pistons carefully, a (minor) damage can occur, which in the longer term causes consequential damage. Too small a clearance between the piston and cylinder also has a high risk of consequential damage: the expansion of the piston can cause eating marks. Usually, food traces can be recognized by a small clearance by shiny spots with a dark-colored edge.
Tightening the cylinder head bolts too tight or unevenly can also lead to piston damage because the cylinder liner can be deformed;
- Fitting incorrect piston rings: if the lock clearance is too small, the piston ring can get stuck during expansion after heating and start to scrape in the cylinder;
- Pistons hitting the valves: Installing the wrong type of piston with incorrect valve recesses, a head gasket that is too thin, insufficient valve clearance or an incorrectly installed timing belt or chain can cause the piston to hit the valves.
Curve connecting rod:
One or more curves can be connecting rods as a result of a defect or event. A bent connecting rod leads to a lower compression final pressure, because the piston can no longer reach the TDC of the cylinder. Some causes of a bent connecting rod are:
As one of the most common causes, a liquid in the combustion chamber leads to a bent connecting rod during the compression stroke. Unlike air, a liquid is not compressible. We call this a “water hammer”. This can be encountered in the following situations:
- The head gasket is cracked between a coolant duct and the cylinder space. The coolant can leak freely into the combustion chamber. While the engine is running, there is little chance that a water hammer will occur. The coolant is forced through the crack, especially during pressure testing (pressurizing the cooling system). If we suspect that a ruptured head gasket is the cause of coolant leakage, the cylinder may be tested with a during the pressure test endoscope are inspected. In the event of a leak, there may be a puddle of coolant on the piston;
- (rain)water has been drawn in from outside. With heavy rainfall, deep puddles can form on the street. Also consider a high water level in a tunnel. When driving through a deep puddle, a considerable amount of water can enter the engine via the air filter;
- A physical part has entered the cylinder, such as a screw or other material being drawn in through the inlet.
The moment the damage occurs, there is an enormous pressure above the piston. The connecting rod is pushed onto the crank pin of the crankshaft with an unusually high force. This force allows the lubricating oil film between the plain bearings and bearing journals to be squeezed away from both the connecting rod and the main crankshaft bearings. After the lubricating oil film has been pushed away, mechanical friction immediately occurs, with bearing damage and possibly also crankshaft damage.
In addition to the friction between the bearings and the bearing journals, with such damage to the connecting rod, there is a good chance that direct damage will also occur to the piston pin.
The above image shows the cross section of the piston, clearly showing the effects of a bent connecting rod. The piston pin and the connecting rod bearing journal are no longer loaded centrally, but at an angle. The piston pin can break and the bearings will immediately wear in and start to “cheat”.
Broken Piston Pin:
A broken piston pin can be caused by overload due to abnormal combustion when, for example, detonation occurs, or from foreign objects or liquid in the combustion chamber during the compression stroke. The overload can also be caused by too high combustion pressures as a result of performance improvement (chip tuning, turbo, etc.).
Careless assembly can also cause the piston pin to break. The moment one makes an impact damage with a hammer, this can lead to a small crack.
This incipient crack can lead to the piston pin breaking even under normal load.