Crankshaft Screw Problem: Causes & Solutions

Hi,

"Due to the increasing number of reports in the forum regarding stripped wheel bolts of the new generation 12.9 grade, I have come up with the following idea:"

In the Erwin (siehe Fehlerdatenbank) manual, it states that the threads and collar of the torque wrench screw must be free of oil and grease . In the old screwdrivers' manuals, it mentions lubricating the threads . Perhaps this explains the causes of the problems with the new 12.9 screws . It's possible that some workshops don't know that these screws are now apparently used without oil, which could lead to them loosening .

What do you mean?

Best regards, Christian.

Hi,
I don't believe that oil will help to loosen it. That would be achieved with anti-seize compound, graphite, or molybdenum disulfide. Oil will be displaced when the screw is tightened. However, especially when tightening "quickly," it's possible that the screw is tightened too much, which could damage the screw itself or, worse, the threads of the crankshaft.

Hi,

So, this problem is really bothering me .

@ Christian:

I don't think that oil can be to blame .

I found something about that in this forum.

http://www.motor-talk.de/showthread.php?s=4cf98fad5f68cb43277240808549c10c&forumid=146&threadid=53600&highlight=kurbelwellenrad

Some people there say that a worn-out vibration damper is the cause of the problems with the crankshaft pulley... others only see the problem with the central screw. There must be a reason for these problems . I can hardly imagine that this screw would simply come loose or break off without external influences . But perhaps the whole thing is still too tightly dimensioned, even with a new screw .

There, another employee from an instant typesetting company writes:
"Approximately 80% of all D/TD/TDI engines, including five-cylinder engines, that we overhaul have this type of damage."

That's quite something, isn't it?!?

By the way, here are two pictures of a broken and a new crankshaft! How can I upload the pictures?


Best regards, Christian.

Hi,
Oil is certainly not to blame for the general problem.
However: If there is a regulation for tightening the screws, it must be followed COMPLETELY. The torque required when tightening with oil is different from the torque required when tightening "without oil."
Otherwise, either the material will be damaged, or the connection will not hold.
Ultimately, torque is not an end in itself, but rather the simplest method for achieving a desired preload in the connection. Unfortunately, it is quite inaccurate. Where the highest reliability is required, such as in nuclear power plants, other methods are used, such as hydraulic or thermal preloading. In these cases, the screw or nut is only tightened by hand.
The newer method, which involves tightening with 90 Nm of torque and rotating 90 degrees, also aims for precise adherence to the target tension.

I looked at the photo from the link, and there's definitely nothing to do with "Anknallen" in that picture.
One could determine the cause of the screws breaking by examining the fracture surface, if one had a broken screw available.
I suspect two possible causes: 1. The pulley, and consequently the screw beneath it, might be pulled slightly towards the engine due to the tension of the belts, which, when the engine is running, would create a fluctuating load on the screw. This is definitely something the screw cannot withstand due to the stress concentration effect in the thread root.
2. If the friction between the wheel and the shaft becomes too low (especially if the vibration damper is also worn), the disc could move relative to the shaft. This initial movement is likely small, but it will increase as the groove wears out.
This could then lead to a fluctuating torsional stress on the screw.
Both can lead to permanent breakage.
If the screw doesn't break, it will eventually stretch, become loose, and may eventually unscrew completely.

Hi Christian,

That sounds good . However, I still don't understand the issue with the vibration damper . Does it break by loosening the central screw, or can it simply fail on its own??? We've discussed this topic before and haven't found a solution .

Now, the question arises whether this screw should be proactively replaced during the timing belt change. I know someone who replaces this screw plus the sprocket every 90,000 km . There's certainly a risk that the thread is damaged, and then you have a problem .

Could one not detect damage early by measuring the oscillation amplitude at the outer edge of the vibration damper (axially to the KW) ? The question is only how one can measure the play with DIY tools .

Best regards, Christian.

Hi,
Unfortunately, I haven't had a broken vibration damper in my hands yet. However, I can imagine that if the rubber is deteriorating, the rotating mass would continue to oscillate, while simultaneously being braked more strongly at the endpoints. This would loosen the normally fixed screw connection in the same way as if you were using an impact wrench for an extended period.

Hello.

Mich hat's nun auch erwischt! Nach 275 Tkm hat sich die KW-Schraube gelockert. Ich weiß nicht, ob die meine Werkstatt beim ersten ZR-Wechsel die neue 12.9er Schraube rein hat oder ob sie schon von vorherein drin war. Jedenfalls hab ich sie nir angerührt, da ich der Meinung war, neue Schraube und alles fest = alles OK. Seit einiger Zeit hatte ich Probleme beim 'Kaltstart'. Dies schob ich jedoch auf eine nicht ganz korrekte Einstellung der ESP nach dem Reinigen des MSW. An dieser Stelle gleich noch ein großes DANKE an die Anleitung dazu. Nach einem Jahr eindlich wieder Ruckelfrei!! Jedenfalls hab ich die Pumpe nochmal eingestellt und es hatte sich nicht gebessert. Nach dem der Motor auf der Autobahn gestern des öfteren in Notlauf gegangen ist und mir VAG - COM danach einen Einspritzbeginn von 0 im Diagramm angezeigt hat, hab ich den Riementrieb zerlegt. Es sah alles noch gut aus. Jedoch kam mir eine Menge Schwingungsrost entgegen. Die Schraube mußte noch mit etwas Kraft gelöst werden. Also richtig locker war sie noch nicht. Als Ursache des Ganzen vermute ich den Schwinungsdämpfer. Mir ist schon vor einiger Zeit aufgefallen, das dieser recht unrund läuft. Viel ist da aber gar nicht dran, war verschleißen kann. Leider kostet das Teil ca. 250 Euro. D'I could just buy another used engine right away and drive that one until it breaks down!'

Does the crankshaft stub *necessarily* need to be reworked, and is it advisable to use Loctite when installing the crankshaft bolt?

My condolences!

I would just plan the strut mount and that would be good enough. The Loctite experiment seems too risky to me! There have already been several cases where it went wrong...

How does the timing belt replacement actually work? Can the engine stay in place during the process, and can any VW workshop perform this repair?

Hi,
VW has this small wooden box (they even lent it to me once so I could make my own).
It consists of a mandrel that is screwed into the thread of the KW (likely a component), and a milling cutter that is guided by the mandrel and constantly rotated in a circular motion.
Gibt herrlich lange Arme und macht besonders im Winter bei Schnee unter nem Bulli fürchterlich viel Spaß. FIt works, but...
You'll need approximately 10cm of space in front of the rear axle; after that, it should fit when installed.
Hi, Marcus.

Hello!
The process of smoothing the crankshaft journal only works up to a certain point. The problem is that the profile on the shaft wears down, against the direction of rotation. Even if the contact surface is perfectly smooth }, it will only last for a certain amount of time. In my Passat }, it was only 30,000 km.
However, there are companies that specialize in repairing such crankshafts. But, in my opinion, it's something to reconsider if the vehicle has more than 200,000 km on the odometer, as it might be more cost-effective to overhaul the entire engine. This is a tricky cost issue, and it might not be worth it for the specific car in question.

Best regards,

Dominique.

D. Kurz wrote:

The contact surface can be as flat as it wants... but it will still only last for a certain amount of time. In my Passat, it was only 30,000 km.

Hello Dominique,

I always thought that the force between the KW wheel and the KW shaft is only transmitted through the contact surface (not through the screw). Therefore, the profile of the KW stub should be quite irrelevant, as long as the contact surface is flat .

...
The process of smoothing the crankshaft journal only works up to a certain point. The problem is that the profile on the shaft wears down against the direction of rotation. No matter how smooth the contact surface is, it will only last for a certain amount of time. In my Passat, it was only good for about 30,000 kilometers.
...

What kind of profile?
If you're referring to the notch for the KW wheel, it's not necessary. The notch has no mechanical function whatsoever; it only indicates the orientation for mounting the KW wheel. The forces are transmitted solely through the surface pressure between the KW component and the KW wheel.
If it only lasted 30,000 km for you, I would definitely confront the person who did the work. I've planned several maintenance schedules, and for some of them, the timing belt needs to be replaced again after the incident.

@Goettmann
I'm not comfortable doing things that might cause problems for myself... In principle, you're right, but there's still a certain degree of guidance provided by the profile. The missing segment with its corresponding counterpart in the wheel hub probably has a reason. I've had to do this kind of work on several 1Z cars, due to my job, and unfortunately, I've had the painful experience that if there's not enough surface area on the segment to provide sufficient support for fixing the wheel, then a press fit and the contact area won't help, and the wheel will inevitably come loose. This has happened a few times in workshops I know. Of course, it's possible that further loosening of the wheel after repairing the wheel hub could be caused by a worn-out shock absorber, for example, but I can't judge that.
That was just an experience made by myself and by others. Of course, it's possible that my Passat, in the hands of its new owner, is still running 'green' thanks to Bern assembly adhesive, but I don't care about that, thanks to my Golf . As long as there are no signs of a loose rear wheel bearing on the AFN (which supposedly shouldn't happen...), I'll just leave it alone and only change the timing belt regularly.

Best regards,

Dominique.

Since my engine already had the M12.9 screw installed, and the timing belt pulley still came loose, I would be cautious with the AFN engine as well. As far as I know, the belt drive system is the same for both engines.

I see the problem in the circumferential bending stress on the screw connection. The pre-tension of the screw can decrease, and the sprocket wheel can easily become loose. The vibration damper then further exacerbates this issue when the shaft rotates.

Well, in my opinion, as confirmed by two very knowledgeable people, Loctite is the only solution to prevent the KW screw from coming loose again... I had this problem 15,000 km ago, and now it's the exact same problem again... Poor starting when the engine is cold. This time, I'm using the red Loctite.