Destroyed Aluminum Threads (and how did I?)

JHZR2

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This all makes me wonder if you got a very real looking counterfeit part. Pulley bolt breaks, threads on block damaged ( slightly undersize bolt/threads) ?

Have to wonder.
It was my first time buying from autohausaz. They also had a mixup with an OE belt.

I don’t think the part was fake, but you never know…
 

CleanSump

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This all makes me wonder if you got a very real looking counterfeit part. Pulley bolt breaks, threads on block damaged ( slightly undersize bolt/threads) ?

Have to wonder.
Not likely counterfeit from Autohaus.
The buggered tensioner bolt thread issue isn't uncommon with the older inline 5 and 6 MB diesesls. Not enough to be called common, but enough that I worried about it when doing my '92 300D Turbo because of the threads I read on a couple of the MB forums. And that was with a ten year old car. Add another 20 years to that and ....... There are threads if you google.
The bolt head could have been a badly heat treated bolt, had an undetetected void/crack, or have another metalurgical fault. I'm not a structures guy.
 
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JHZR2

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Not likely counterfeit from Autohaus.
The buggered tensioner bolt thread issue isn't uncommon with the older inline 5 and 6 MB diesesls. Not enough to be called common, but enough that I worried about it when doing my '92 300D Turbo because of the threads I read on a couple of the MB forums. And that was with a ten year old car. Add another 20 years to that and ....... There are threads if you google.
The bolt head could have been a badly heat treated bolt, had an undetetected void/crack, or have another metalurgical fault. I'm not a structures guy.
It just really stinks that the original one came out ok. I made the other thread out of an abundance of caution.

Then this.

And it’s a comedy of issues. The time sert/helicoil drill bit is big (23/32), requiring a 1/2” right angle drill that I don’t have. Or a different kit that takes longer to get.

It’s always something.
 
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The threads were fine when I pulled the old one. I looked them over, went through each one, etc.

There was oil on the threads when I pulled the old one. No signs of galvanic corrosion.

The little shards came out when I removed the new one.
Just curious, could oil have interfered with the torque process? Doesn’t oil make a false reading in a torque wrench? I would think slippery threads would make 73ft-lbs on a torque wrench actually end up higher on the physical part.

Oh I see that your cleaned the threads with brake clean.
 
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Reading this whole thread, and posted pictures I think the smaller bolt shearing off has something to do with the main thread in the cover being messed up as a result. These forces must’ve been pretty substantial to shear that bolt off so cleanly. The main bolt must’ve experienced these shear forces as well as it is on the same plane.

The case is aluminum so the threads got probably stretched as a result. I would say his has nothing do do with your installation and torque, but whatever caused the pulley bolt to be sheared off.

Edit: these are the type of forces I’m thinking about. It’s exaggerated of course, but I believe it’s a good representation of what might’ve happened.

3A8A081F-3710-49EB-B703-ED29871ABB50.jpeg
 
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JHZR2

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Reading this whole thread, and posted pictures I think the smaller bolt shearing off has something to do with the main thread in the cover being messed up as a result. These forces must’ve been pretty substantial to shear that bolt off so cleanly. The main bolt must’ve experienced these shear forces as well as it is on the same plane.

The case is aluminum so the threads got probably stretched as a result. I would say his has nothing do do with your installation and torque, but whatever caused the pulley bolt to be sheared off.

Edit: these are the type of forces I’m thinking about. It’s exaggerated of course, but I believe it’s a good representation of what might’ve happened.

View attachment 120880
I don’t think the tensioner was that crooked. I also have a hard time rationalizing that a new pulley, that spun freely, with a good bearing, would need to shear its bolt because of a little misalignment, if there was any.

It could be a combination of things. For sure. And the tensioner could have been a bit misaligned if not threaded right (though it went in easily by hand so I don’t see how that could be).
 

JHZR2

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Just curious, could oil have interfered with the torque process? Doesn’t oil make a false reading in a torque wrench? I would think slippery threads would make 73ft-lbs on a torque wrench actually end up higher on the physical part.

Oh I see that your cleaned the threads with brake clean.
That was brought up by @The Critic when it first happened. Thing is, you’re right, I cleaned it, and primed it, and had loctite on the threads.

The FSM indicates the torque with threadlocker, so notionally they realize the lubrication effect.

Unless the OE locker and 243 have vastly different torque characteristics. Or my cleaning it out was insufficient….
 
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How many foot pounds of torque?? On aluminum threads in a casting??

If I were doing that job, I'd have considered these issues. Is the thread 12mm? A full threaded aluminum head bolt of that size would take between 40 and 55 foot pounds. But that's with 30mm++ of thread depth.
 
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JHZR2

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How many foot pounds of torque?? On aluminum threads?
87A689CB-ADC9-4A87-B687-FACFC225F00D.png

FSM says 100nm on threads coated with Omnifit 100M, which crosses to loctite 243.

Unless 243 vs 100M have different torque values????

I’ve read elsewhere that people have done the job with 271 (red), but that didn’t have the performance that aligned to 100M.
 
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lol that is an option, isn’t it? Maybe the best one in the end! If I just torqued it and put JB in there, I’d suspect that we would run a major risk of shreds of metal, and epoxy, down in the engine.
If you are going to goop it up like you're suggesting- best to stay away from JB.
 

JHZR2

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Possible crisis averted?

I only had like 15 minutes to look at this on my lunchtime, but it seems like a positive outcome. I got a quality 18x1.5 thread chaser (Lisle or KD or something US made from my toolbox - forgot I had it). Liked it up and worked it in and out slowly. Worked it up, down, and sideways. It eventually went in.



5DF55C58-A812-4D5A-84C9-206DD184D8F7.jpeg


Remember when I pulled the new tensioner, there was a lot of silverfish powder. Galling? Vibration? I don’t know why…

It got caught in the oil, though I had cleaned the threads already.

4602E134-65E7-4F8F-BFD9-3DF300B7A842.jpeg


There was a small shard on the chaser, so it did have to deal with a bad thread someplace.

EC57F93D-3169-4759-9960-4FB5742D623A.jpeg
A4F50223-C138-4053-BDCE-B1E48639E920.jpeg


I didn’t put the old tensioner (that’s just plugging the hole from dust) all the way in because I didn’t clean the threads properly yet. But it went in easily by hand turning the Allen key, just like before this debacle.

FF54604A-2F59-42CF-BEA7-97AC5464E1EB.jpeg


So… go with it and re-install the new one with primer and 243? 243 only? 271? Dry? Oiled? Still tap and time sert or helicoil?

Thoughts?

Thanks!
 
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Possible crisis averted?

I only had like 15 minutes to look at this on my lunchtime, but it seems like a positive outcome. I got a quality 18x1.5 thread chaser (Lisle or KD or something US made from my toolbox - forgot I had it). Liked it up and worked it in and out slowly. Worked it up, down, and sideways. It eventually went in.



View attachment 120947

Remember when I pulled the new tensioner, there was a lot of silverfish powder. Galling? Vibration? I don’t know why…

It got caught in the oil, though I had cleaned the threads already.

View attachment 120948

There was a small shard on the chaser, so it did have to deal with a bad thread someplace.

View attachment 120949 View attachment 120950

I didn’t put the old tensioner (that’s just plugging the hole from dust) all the way in because I didn’t clean the threads properly yet. But it went in easily by hand turning the Allen key, just like before this debacle.

View attachment 120951

So… go with it and re-install the new one with primer and 243? 243 only? 271? Dry? Oiled? Still tap and time sert or helicoil?

Thoughts?

Thanks!
If you re-install can another failure force you into buying new cover?
 

JHZR2

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If you re-install can another failure force you into buying new cover?
Why would I? The repair would be a helicoil or a time sert… remove if necessary, replace in situ if not.

I’m not sold that I’ll just screw in the tensioner and go…. That’s an option.

Removal of the timing case is a pretty intense activity. It’s not just a couple of screws.

And since the tensioner will probably be good for another 200k miles, and I have 13 cars… it’s dubious this car will get another tensioner…
 

Astro14

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@JHZR2 - I would still feel better with a Timesert.

That said, if you clean the threads well, with brakeclean, and seal them with an appropriate threadlocker, then torque to about half that value, it might be OK. 100NM is an awfully high torque, even for an 18mm. Maybe back off the torque by a bit.
 

JHZR2

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@JHZR2 - I would still feel better with a Timesert.

That said, if you clean the threads well, with brakeclean, and seal them with an appropriate threadlocker, then torque to about half that value, it might be OK. 100NM is an awfully high torque, even for an 18mm. Maybe back off the torque by a bit.
All the tensioner bolt is doing is holding a bearing in place. The tensioner isn’t like the coil,spring loaded ones I’m used to. It’s a spring and a hydraulic shock.

The electrical equivalent would be like an LC circuit somehow I think…

so the bolt need only be there to allow the bearing to move wi5 the oscillations of the system. Tight enough to not back out is tight enough.

I still have to wonder what/why I had all that grey powder on the threads when I removed the tensioner…. I had cleaned, primed, and loctited the threads. Could it be that I didn’t use enough loctite? I’m actually wondering if use of the primer was the issue.

Loctite 7649 is a copper salt in acetone. Copper salt in solvent in an aluminum casting, with heat… what could go wrong? It says for use in low activity metals, and includes anodized aluminum. Maybe this aluminum didn’t have a thick enough oxide layer and the copper and aluminum reacted, damaging the aluminum?

This FAA technical publication describes the powder I saw in the threads. I’m wondering if the primer was really the culprit…

5553E924-F0A8-4029-BF7F-894A543FA261.jpeg


I was essentially doing mechanical polishing…
 
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