Welding Timing Belt Pulley: How To & Tips

Hello everyone,

I'm currently facing a decision.

My old Audi 80 (which already has 270,000 km on the odometer) has now experienced the problem of the timing belt pulley becoming loose for the fourth time. This, of course, comes with all the usual issues (difficult cold starts, white smoke, etc.).
The entire thing has been repaired twice privately and twice at the Audi workshop. The intervals during which it worked properly were getting shorter and shorter.
Okay, since the crankshaft stub is already so worn out, I'm thinking about welding the timing belt pulley directly to the stub. After all, this part has already seen a lot of miles, and I just want to get it to last for the next couple of years. Besides, I'm not going to scrap the whole car just because a stupid timing belt pulley broke. Everything else is actually in top condition.

My question to you: Has anyone ever done something like this before? Or is it fundamentally something that should be avoided?

Here's how I figured it out: First, adjust all the settings (timing, etc.) – the timing belt was replaced 8,000 km ago. Then, securely attach the timing belt pulley with locking compound. Check to make sure everything is working perfectly. Loosen the timing belt pulley so that it's only held in place by the locking compound, and then weld it in place. Of course, constantly cool it with water to prevent damage to the crankshaft seal.

What do you think about it?

Fuschia.

Hi,

"So, BMW once welded the crankshaft to the flywheel in their 3.5-liter racing engines in the 635 CSI because the crankshaft would regularly detach from the shaft. This was caused by the 1-meter-long 6-cylinder crankshaft, which would vibrate excessively and then cause the attached components to come loose."

While the result was a complete elimination of rotor wobble, the ease of service was also lost (which, of course, doesn't matter with racing engines).
I don't think you'll be able to install that KW wheel without any issues onto the KW suspension. You're more likely to cause more damage than you'll fix.

I don't think that will work.
1. The material of the KW (likely referring to a component) may not be suitable for welding. This means that rapid cooling or localized heating could cause fine cracks to form, which would eventually lead to the component, along with the weld and a remaining piece, detaching after some time.
2. You probably want to weld through the screw hole. In that case, the weld will be on the inside of the shaft instead of the outside. This will create a very high stress concentration, which will likely lead to a fracture.

As mentioned before, there are engine rebuilders who can repair the crankshaft stub while the engine is still installed, so that the screw can once again fulfill its purpose.

Hi Fuschi,

Basically, that sounds good. However, you need to properly dimension the weld, otherwise it won't hold. Also, you have to consider that when loosening the screw, forces will also be transmitted to the KW (dampers), and no amount of paste will withstand those forces (assuming you're tightening the KW wheel with the VW tool -> in that case, the forces act between the KW wheel and the KW, especially if the groove in the bucket is damaged!!!) -> resulting in a wobbly KW wheel (see Bertil). Instead, try to properly grind the KW stub flat -> that should be possible .

Were the timing chain wheels also replaced during the previous replacements? They should be!

"Yes, I've heard about engine rebuilding before. Does anyone know someone reliable (with good quality work) in the Munich or Würzburg area? So far, in my search, I've only found companies that specialize in cylinder head rebuilding."

Greetings,
Fuschia.

Quote:

Were the timing belt wheels also generally replaced during the previous replacements?

That is something that should be done!!!

Everything was done. New KW wheel, sealant, new KW screw. } Nevertheless, it kept coming loose.

Best regards,
Fuschia.

Fuschi68 wrote:

Everything was done. A new KW wheel, sealant paste, and a new KW screw were installed.

Nevertheless, it still didn't hold up properly.

aaah ok, if I understand correctly, the KW has never been re-sharpened (I thought it might have been worked on before).

I'm curious about how the groove on the crankshaft stub is machined by these companies . Is it then ground larger and connected to a corresponding crankshaft gear?? Probably, the machining of the contact surface is sufficient .

@Fuschi:

How did it all start for you, and most importantly, at what mileage?? Was the original 8.8 screw still in place at that time ?

haehnlein wrote:

How did it all start for you, and most importantly, at what mileage?? Was the original 8.8 screw still in place at that time

?

I can't say for sure whether the original 8.8 timing belt was still in place, as the first timing belt replacement was done by the workshop. The car was manufactured in December 1994, so it should have already had the more durable version.

It started approximately 20,000 km after the first timing belt replacement, so at 90,000 + 20,000 km. The next timing belt was replaced at 187,000 km. Then it happened again at 200,000 km. The last time it happened was now at 260,000 km. This time, I had the timing belt replaced at the same time. It lasted only 7,000 km, and I was already warned that the crankshaft stub was quite worn. This Saturday, I'm going to do it myself.
My trusted diesel mechanic (who unfortunately doesn't have time) told me I can safely tighten the KW screw to 400 Nm to ensure it holds. Is that correct? That seems like a lot of torque. I really don't want to strip the threads.

Best regards,
Fuschia.

Fuschi68 wrote:

I can't say for sure whether the original 8.8 was still installed, as the first timing belt replacement was done by the workshop. The car was manufactured in 12/94. So, it should actually already have the more durable one installed.

Hi Fuschi,

At that time, the wrong screw was still installed from the factory!!! -> So, the first workshop was negligent and did not replace this screw, with the known consequences .

Fuschi68 wrote:

My new trusted diesel mechanic (who unfortunately doesn't have time) told me that I can safely tighten the KW screw to 400 Nm, so that it will hold. Is that correct? That's quite a lot. I actually don't want to strip the KW screw.

I would be extremely cautious about that . There have already been cases in this forum where the vibration damper, along with the KW wheel, came loose after changing the screw, leading to a crankshaft fracture . According to the manufacturer, the specified torque is 90 Nm plus 90 degrees . However, that 90-degree rotation is quite something . Even with strength training, I couldn't manage the rotation . Only leg power helped. If the torque for that rotation had been measured, it might have resulted in 400 Nm .

One more important thing :

If you use thread locker, please use it sparingly . If too much is applied during tightening, a part may not be able to move, and a crankshaft fracture is almost inevitable due to the formation of hairline cracks .

It should be possible to create a grinding disc with a centering pin to then grind the surface flat.

Deleted upon request.

diesel.gustav wrote:

The connection is achieved through frictional force by surface pressure, generated by the preload force of the screw.

Surface pressure is clear; if the contact area isn't flat, it won't last as long. But what about the contact area itself? Let's say the contact area is perfectly flat. Wouldn't even a slight roughening of the surfaces increase the frictional force between the parts?

Best regards,
Fuschia.

Fuschi68 wrote:

Wouldn't a slight surface roughness increase the friction between the parts?

More likely in the short term. If the unevenness is smoothed out under constant or alternating loads, the preload of the screw connection will be reduced, meaning it will loosen even further!

christians wrote:

More likely in the short term. If the unevenness is smoothed out under constant or alternating loads, the preload of the screw connection is reduced, meaning it will loosen even more!

Okay, then I'll do it differently:
Make sure the KW spindle and timing belt pulley are perfectly smooth and flat, and attach them completely free of grease and dirt. Then, there should be no Loctite, no air pockets, and no unevenness between the surfaces, and the clamping force should be perfect. I'll then see if it holds.
Regarding the screw, I was thinking of applying a bit of threadlocker to it, so that if it becomes loose, it won't come completely out and ruin everything. Also, I thought of lightly oiling the contact surface of the screw to make it slide more easily when tightening. As I learned before, with the same tightening torque, the tensile forces of the screw should be significantly higher. I'll tighten it as specified, with 90 Nm plus an additional 1/4 turn.

This is now my last, and hopefully best, attempt to figure out how to properly secure this stubborn wheel. It seems like, aside from an expensive KW repair or replacement, there's no other proven solution to this problem.

Will it actually work? If not, I will post it here to show that it didn't help either.

Best regards,
Fuschia.

Fuschi68 wrote:

Also, I was thinking of lightly lubricating the contact surface of the screw with oil, so that this surface would slide easily when screwing it in.

As I learned before, with the same tightening torque, the tensile forces on the screw should be significantly higher.

Hi Fuschi,

I don't believe that oil will help (it shouldn't get on the threads anyway } -> so, first screw the screw in with turpentine to make sure everything is grease-free -> better for the screw's tightness). The tensile forces remain approximately constant, whether with or without oil on the contact surface.

haehnlein wrote:

The tensile forces remain approximately constant, whether with or without oil on the contact surface.

Well, what does "approximately" mean? The friction coefficients with and without oil differ by about 10-20%. Now, the tightening torque doesn't solely come from friction, but estimated 5-10% is still significant given these high forces and torques.

christians wrote:

Now, the tightening torque does not solely result from friction, but an estimated 5-10% is still due to friction, even with these high forces and torques.

Hi Christian,

Okay, the rotational angle (2nd stage) is also not 100% predictable, and it's this angle that generates the enormous forces. The resulting tensile forces are independent of friction on the contact surface at the rotational angle of the 2nd stage. Therefore, the oil only plays a role at the 90 Nm level. Whether it's 94 Nm or 90 Nm (same friction) might result in a 3% difference in the rotational angle deviations (1st stage). However, the deviations in the rotational angle of the second stage are more significant for the tensile forces. Also, only the contact surface is lubricated, not the threads, so I don't think it makes a big difference.