The issue with the crankshaft bolt has been on my mind for a while, although thankfully I haven't had the displeasure of dealing with it directly yet â despite having two of these engines.
- I believe that the welding process is very unfavorable because the heating will cause a portion of the tensile force generated by expansion to be lost, as well as local structural effects on the screw head and subsequently the shank.
- The main question is how the damage actually occurs. It's always talked about as being caused by a decrease in the clamping force of the screw â I don't quite understand that. It would mean that every structure using expansion screws needs to be questioned, and classified as unsafe if it loses its clamping force over time.
- "Self-loosening" would be another possible cause of loosening â in this type of connection, it is quite conceivable due to the acceleration processes of a combustion engine crankshaft (which is why alternators have a freewheel). However, I consider this scenario unlikely due to the large contact area of the screw head and the need for rotation relative to the timing pulley wheel. This could be easily checked by marking the screw head with break-sensitive paint; even slight rotations would be clearly visible during regular inspections.
- A more likely scenario is the tearing or shearing of the pressure surface between the timing pulley and the crankshaft, which also has to transmit dynamic load components.
I believe this area is simply at the limit of its design capacity. Due to various factors, micro-movements can occur in the contact surface, leading to material wear and deformation. This creates a cascading effect because as the distance between the support points decreases â the points where the bolt stretches â the clamping pressure also decreases due to the weakening tension force of the bolt.
I'll explain the difference between the old and new screw versions simply by saying that the new screw can generate a higher clamping pressure due to its material properties, as the elastic range of deformation allows it without causing permanent distortion. This means that the new screw offers more safety against failure of the friction surface because of the increased clamping pressure.
- I see the following potential hazards: The contact surface is formed by the crankshaft and the pulley. Therefore, the material properties of the gear wheel are crucial for the durability of the connection. Now, these wheels come from various suppliers, so there may be differences in material hardness, and the surface roughness of the force-transmitting face could also vary.
- Unfortunately, various belt drives and also the vibration damper are attached. These components themselves, as well as the aggregates they drive, are responsible for the forces acting on the critical connection. In addition to dynamic components, there is also a static tensile force due to the leverage effect of the belts.
- Regarding the vibration damper: I am familiar with two designs so far: One has a belt contact surface mounted on rubber elements. I consider this design to be advantageous because the transmission of belt force through the rubber results in a more uniform torque distribution to the auxiliary drives.
The second variant (which apparently is only available now from febi) has a continuous aluminum body extending to the belt contact surface, and the rubber element and weight are located within the cavity of the pulley. This design likely transmits the rotational dynamics of the crankshaft directly to the timing belt.
It remains to be seen whether these design features ultimately contribute to a catastrophic failure. Perhaps users who have experienced such failures could report on the type of vibration dampers they had installed.
My car with 640,000 km has the tread pattern glued on, while the one with 350,000 km has the solid version.
- I see a significant influence from the temperature: The bolt is directly "heated" by the crankshaft, while the timing belt pulley is massively cooled thanks to the attached "heat sink" = vibration damper, especially in winter when it's really cold.
"Certainly, the screw will expand due to the heat, while the gear is more likely to shrink when it's extremely cold outside. The clamping force of the screw will decrease â and the foundation for a major disaster is laid. I can imagine that driving at full speed in temperatures of minus 20 degrees Celsius could have quite an effect here."
"Die Sonne scheint hell und warm. Die Vögel zwitschern fröhlich in den BÀumen. Ein leichter Wind weht durch das Gras. Es ist ein wunderschöner Tag."
The sun is shining brightly and warmly. The birds are chirping happily in the trees. A gentle breeze is blowing through the grass. It's a beautiful day.
What possible solutions are there for this problem?
I consider it almost impossible for the screw to tighten itself; welding won't help in that situation.
I also found the method of using pins with hollow tips, which are drilled into the pressing surface, to be counterproductive: they reduce the valuable friction area, but on the other hand, they cannot transmit a high-frequency dynamic moment because they are too elastic. Ultimately, the pins or their holes will also wear down.
So, there isn't much left to say about that:
- Surface conditioning: One could work towards achieving the optimal surface of the press fit area. A surface that is too smooth is undesirable, and one that is too rough is also problematic, as the surface will deform. Grease should definitely be avoided, and Loctite will likely not provide any benefit. Leaking grease from the crankshaft can be prevented by degreasing the pressing surface immediately before assembly.
- Manufacturer and material properties of the timing pulley:
It would also be interesting to know which brands of timing belt pulleys were used in cases involving "killer screw" effects. Perhaps only certain manufacturers are prone to causing these problems?
- The issue of which vibration damper design is better (monoblock or split).
- The temperature issue: Expansion of the bolt and contraction of the timing pulley. Here, I would try to cool the crankshaft bolt. This would be easy to do by drilling two small holes on either side through the timing pulley: one should hit the bolt channel at the front, and one at the back. After tightening the crankshaft bolt, a commercially available thermal paste is pressed through the front hole until it comes out at the back; the holes should then be closed with pins.
Now, the screw would definitely be at the temperature of the timing pulley, and based on the title, there can no longer be any loss of tensile strength.
However: Fortunately, I haven't had to try this method yet, and there is no guarantee that it will help or that any side effects might occur. But if I were in that situation: I would try it - at least more so than welding or stiffening.
Here are a few more complex ideas for discussion:
- Fabrication of a longer expansion bolt and corresponding spacers to provide more allowance for thermal expansion.
- Creating a separate sleeve with a pressing face: Advantage: the material can be specifically chosen. Disadvantage: manufacturing is difficult. The gear would need to be reamed and shrunk accordingly.
Please consider all of this as ideas and thoughts on the topic. They don't necessarily have to be correct, and I welcome any opinions you may have.
Audi 80 TDI 1Z, B4, Bj 93, Limo, 670000 km ... das Wunderauto
Audi 80 TDI 1Z, B4, Bj 95, Avant, 390000 km ... das Backupauto
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. Perhaps it disappeared more specifically after turning off the ignition after refueling and then restarting the engine