1. Introduction
Engine connecting rod bolts play a crucial role in the operation of an engine. They are responsible for connecting the connecting rod big end cap, the connecting rod big end bearing shell, and the connecting rod body, bearing all the forces within the connecting rod assembly system. However, due to the harsh working conditions they are exposed to, connecting rod bolts are prone to various defects such as cracks, fractures, and deformations. A broken connecting rod bolt can lead to a serious engine breakdown known as “cylinder pounding,” which may cause the entire engine to be severely damaged and even 报废，posing a significant threat to the safety of vehicle occupants. Therefore, it is of utmost importance to pay close attention to the inspection, repair, and installation of connecting rod bolts.
2. Common Defects of Connecting Rod Bolts
2.1 Cracks or Fractures in Connecting Rod Bolts
A broken connecting rod bolt can trigger a major mechanical accident of cylinder pounding. The main causes of this defect are as follows:
| Causes | Descriptions |
|---|---|
| Poor Material Quality | The material of the connecting rod bolt has low tensile strength and rough processing. It may deform or develop cracks when subjected to the standard torque, and then break under impact loads. |
| Excessive Tightening Torque | Maintenance personnel, without using a torque wrench, use a common wrench with an extended arm to increase the torque. This excessive tightening torque causes the connecting rod bolt to stretch beyond the elastic limit of the material. Under impact loads, the bolt material undergoes plastic and yield deformations, eventually leading to fracture. |
| Insufficient Pre – Tightening Force | Using a common wrench to tighten the connecting rod bolt often results in insufficient pre – tightening force. This causes the connecting rod split surface to fit poorly and open, and during operation, the big end of the connecting rod and the cap may become loose. The bolt then withstands strong impacts and is pulled apart. |
| Unidentified Existing Cracks | The connecting rod bolt may have developed surface cracks due to repeated stress. If these cracks are not detected and replaced during maintenance and installation, stress concentration will occur under impact forces, causing the bolt to break. |
| Severe Wear | Long – term use of the connecting rod bolt leads to severe wear. There may be poor contact and looseness between the nut and the bolt, or an excessive clearance between the bolt and its installation screw hole. Under impact loads, the bolt will break. |
| Uneven Tightening | When tightening the connecting rod bolt, instead of following the standard torque and evenly tightening it in 2 – 3 times alternately, some bolts are tightened with excessive torque while others are tightened with too little torque. This uneven torque causes the bolts to break. |
| Lack of Anti – Loosening Measures | Omitting the installation of the connecting rod nut lock washer or using a damaged lock washer without replacement, or failing to install the safety wire correctly (such as not forming an “8” shape as required) can prevent the bolt from being effectively locked. During operation, the connecting rod bolt may loosen and break. |
| Excessive Wear of Bearings | Severe wear of the connecting rod bearing and the connecting rod journal, especially when the roundness exceeds the service limit, increases the alternating stress and impact force during operation. This causes the connecting rod bolt to break due to fatigue. |
| Other Engine Malfunctions | Issues such as piston scuffing, crankshaft bearing seizure, or diesel engine runaway can cause the connecting rod bolt to be pulled apart. |
Inspection Methods:
- For obvious cracks, they can be observed with the help of a magnifying glass.
- For fine cracks, the bolt can be immersed in kerosene for half an hour, then taken out and wiped dry. A layer of chalk powder is sprinkled on the suspected area. After drying, check for black lines.
- Another method is to apply a layer of dye solution on the cleaned bolt. After 10 minutes, wipe off the surface pigment solution, apply a thin layer of white powder paste, and then dry it. If there are cracks, they will become visible.
2.2 Deformation and Elongation of Connecting Rod Bolts and Thread Damage
When the connecting rod bolt is deformed, elongated, or has damaged threads, it can cause the connecting rod cap to be installed loosely. In mild cases, the bearing shell clearance will increase; in severe cases, the cap may fall off, leading to a cylinder pounding accident. The main causes are as follows:
| Causes | Descriptions |
|---|---|
| Excessive Installation Torque | During installation, if the tightening torque is too large, the bolt will be elongated and the thread will be damaged. |
| Abnormal Loads from Engine Malfunctions | When the piston scuffs or seizes in the cylinder during the operation of the diesel engine, the connecting rod bolt will be subjected to a large tensile stress. |
| Frequent Disassembly and Assembly | Multiple disassembly and assembly operations will cause wear to the threads. |
| Poor Manufacturing Quality | Low – quality manufacturing with insufficient strength of the bolt. |
Inspection Methods:
- Measure the length of the bolt with a vernier caliper or micrometer and compare it with the standard bolt. If it is 2% longer than the standard bolt, it should be scrapped.
- If the thread slips during installation and cannot be tightened to the specified torque, it should be scrapped.
- Use a 90° square to check the perpendicularity of the bearing surface of the connecting rod bolt head to the bolt center line.
- Use a thread gauge to measure whether the thread is deformed or has slipped.
3. Precautions for Replacement and Installation of Connecting Rod Bolts
If cracks, deformations, or thread damage are detected during the inspection of connecting rod bolts, they should be replaced. The following precautions should be taken during replacement and installation:
| Precautions | Descriptions |
|---|---|
| Pair – wise Replacement for Flat – Cut Connecting Rod Bolts | For flat – cut connecting rod bolts or nuts that are damaged, they are generally replaced in pairs. The threads of the connecting rod bolts are of high precision. After cleaning, they should be kept clean, and no impurities are allowed to enter the thread gaps. No washers that do not meet the technical requirements are allowed to be added between the bolt head, the nut, and the bearing surface on the connecting rod. |
| Inspection of New Bolts | For newly replaced connecting rod bolts, check whether the top of the bolt is fully inserted into the bolt hole of the connecting rod big end. When turning the bolt or nut, it should be able to be screwed in by hand until the end, and there should be no feeling of jamming or looseness. |
| Proper Tightening | During assembly, if the groove on the nut does not align with the hole on the bolt, the nut should be retracted or replaced and then tightened again. It is absolutely not allowed to tighten it too tightly or too loosely. |
| Avoid Arbitrary Replacement of Bolt – Hole Pairs | The connecting rod bolt and its installed hole are in a transition fit and should be lightly tapped in with a small hammer. Arbitrary replacement may result in some fits being too tight and some too loose. A loose fit will affect the positioning of the rod body and the bearing cap and reduce the connection rigidity. A tight fit will make disassembly and assembly difficult and may damage the bolt and the hole during the process. For connecting rod bolts with copper – plating for anti – loosening, if no damage is found during inspection, they can be reused. However, they should be tested with the threaded holes on the connecting rod. If it is too loose, the connecting rod bolt can be re – plated with copper to achieve the correct fit and anti – loosening requirements. |
| Effective Locking | After installing the connecting rod bolt, use new split pins, wires, or lock washers for locking. When using a split pin, ensure that it is new, and its length and diameter are appropriate. The head of the split pin should be sunk into the groove of the slotted nut, close to the nut and the bolt head, and there should be no cracks or fractures at the bent part. When using a lock washer, choose a lock washer without tears or fractures. Do not place the lock washer on the edges of the nut. Used lock washers should not be bent in the original position to avoid breakage and loss of the locking function. When using a wire for locking, select a wire that is not too thick, too hard, or has no damage. After tightening the wire, gently tap it with a hammer to make the wire ends sink into the groove of the connecting rod big end. Currently, most connecting rod bolts are locked with self – locking nuts. Note that used nuts have lost their self – locking function. If reused, gently tap the nut along the axial direction with a hammer to reduce the groove width to 0.5 – 0.6 mm to restore the self – locking function. |
4. Conclusion
Connecting rod bolts are critical components in an engine. Understanding their common defects, inspection methods, and replacement and installation precautions is essential for ensuring the normal operation and safety of the engine. Regular inspection and proper maintenance of connecting rod bolts can effectively prevent engine failures caused by bolt – related problems, extend the service life of the engine, and safeguard the safety of vehicle operation.