Destroyed Aluminum Threads (and how did I?)

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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…
I don't know why it would which is why I asked. If another failure won't make installing a helicoil or time sert more difficult then I would give it a go .
 

JHZR2

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I don't know why it would which is why I asked. If another failure won't make installing a helicoil or time sert more difficult then I would give it a go .
The threads are decently there. If I have a few minutes at lunch I may go look it over or even put it back together again.

I’m thinking no primer - don’t want active Cu salt on the bare aluminum. Just 243. Lower torque. Fingers crossed. It can cure for days.

Hope for the best at least to get to a drivable state. If it comes to it then I’ll drive it into a garage and do a proper time sert repair.
 

D60

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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…

View attachment 121029

I was essentially doing mechanical polishing…
The problem is that it's in single shear.

BUT I'm on your side -- I'd try reassembly with threadlock as-is. I'd use a basic length 3/8" ratchet and snug-by-feel (technical term, of course). I'd argue -- based upon nothing I could back up with any technical docs -- since the threads are potentially suspect you don't want to stretch them too much. Let threadlock do the holding.

Furthermore, while FEAD is ultimately a show-stopper, let's not forget we're talking about a belt tensioner here folks. We're not talking roll cage retention, seat belt anchoring, brake components or even lug nut torque.
 

D60

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Also random musings for Plan B.2:

By my math 18mm is ~.708". Being a crazy machinist I might consider sourcing a 3/4-16 SHCS and turning it to match the overall features of the existing bolt. Run 3/4-16 tap into timing cover.

There are potential snags that I won't go into because it's not ripe for review until said plan is seriously considered and it'll just be a TL;DR
 

JHZR2

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Well after work I put it back on. At lunch I pulled the old one and cleaned the threads really well with brake cleaner.

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This lower of the two pics was older before I got all of the oil out of the threads. I added it to show where the timing chain is relative to the hole.

So then it had time to dry.

I coated the threads with more loctite than I did last time. This was because there may be a bit more gap now between the threads at the opening, and because I’m not entirely sure that I used enough last time which might have actually allowed some sort of galling or removal of aluminum, which resulted in the powder when I pulled the tensioner.

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I limited the torque to 45 ft-lb. That’s like 60nm, so a little bit lower than it would be, even if you assumed the need to reduce torque in a wet thread by 20%. It threaded in by hand very easily, and I took it all the way in until it was snugged up before using a tool.

I bought a new grade 12.9 Allen bolt from the hardware store and installed the pulley. I didn’t like the way that the damper shock sat relative to the head, so I also used a washer, which isn’t really supposed to be there, but may be a discrepancy on the later cars, since I noticed that there is one on my 1993 with the same engine.

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Everything got torqued to spec.

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The belt got routed, but I didn’t install the tensioner spring because I wanted to keep extra forces off of the curing bolt in the timing case.

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So I counted 12 female threads and 12 male threads. Accurate?

Concerned about you getting enough thread depth and engagement. Was going to suggest getting an appropriate stud, getting that secured and loc-tited in 100%, then modifying the tensioner to not use its captive SHCS but to instead slip over the stud and somehow get secured on the outboard side. This is probably infeasible without access to a lathe since your cap screw likely has a custom shoulder to meet the bearing, it sucks not knowing what's inside there.

I think the second attempt is worthwhile. When you get this running put your hand on various parts to see if you're getting a weird vibration frequency that might be murder on these parts.
 

JHZR2

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So I counted 12 female threads and 12 male threads. Accurate?

Concerned about you getting enough thread depth and engagement. Was going to suggest getting an appropriate stud, getting that secured and loc-tited in 100%, then modifying the tensioner to not use its captive SHCS but to instead slip over the stud and somehow get secured on the outboard side. This is probably infeasible without access to a lathe since your cap screw likely has a custom shoulder to meet the bearing, it sucks not knowing what's inside there.

I think the second attempt is worthwhile. When you get this running put your hand on various parts to see if you're getting a weird vibration frequency that might be murder on these parts.
Interesting concept. Would have probably needed m18x1.5 all thread, and some washers and nuts. Not sure if clearance with the water pump pulley would have been an issue.

There are plenty of threads there, I would think. What is the rule of thumb for thread engagement vs bolt diameter?

Thanks!
 
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Reading this thread brings a lot of teenage memories back lol. My parents were old world immigrants so once they got successful all they bought were Mercedes which I remember where crap cars...the 90's ones always needed repairs. Good luck with yours. I'd probably change the timing cover before the engine gets ruined
 

JHZR2

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Reading this thread brings a lot of teenage memories back lol. My parents were old world immigrants so once they got successful all they bought were Mercedes which I remember where crap cars...the 90's ones always needed repairs. Good luck with yours. I'd probably change the timing cover before the engine gets ruined

Given that I’m working on a 200k mile car, and have another in another bay with the same engine with 294k miles, and two more with the 5 cyl version of the same engine with 228k and 337k…. And all but the new one Id drive across country tomorrow… I’d hardly call them crap cars. But I’d also not buy most newer ones other than maybe an S600 or SL600.

My 80s/90s MB diesels don’t need repairs because of randomly failing parts. I’ve owned quite a few of them. Some rubber parts that age… sure. Of my 7 MB diesels I currently own, 6 have fully working AC systems. Of the number of units I have owned in the past, AC worked on all those too. I have 40 year old MB cars with working and sealed R-12 systems. Crap?

These 3.5L 6 cyl ones are rare. Only 2066 were made. They do have a design defect from boring out the engine, but the 3.0L and 5 cyl versions didn’t. Design issue sure. Crap cars not really.

Regarding the cover - sky’s the limit in terms of what repairs could be done. If I need to time sert or replace the part I can in time. Getting it back to somewhat functional and figuring things out from there is fine…
 
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If it holds proper torque I would run it as is. What do the threads look like after chasing?
I’ve had a situation where the threads were damaged, but held torque after being cleaned. Unfortunately after some heat cycles, the bolt worked itself loose.
 

JHZR2

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I’ve had a situation where the threads were damaged, but held torque after being cleaned. Unfortunately after some heat cycles, the bolt worked itself loose.
What was the application? This bolt is really the pivot of a bearing that the tensioner oscillates on. I would assume that if the bearing stays in good condition that the lack of friction should keep such a case from happening. Did your application have as many large threads and use loctite? Not sure those things are a true resolution, but I can hope…

I didn’t torque it full tight in the interest of preserving the threads. I’m not sure how much it really matters.

Knocking on wood.
 
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What was the application? This bolt is really the pivot of a bearing that the tensioner oscillates on. I would assume that if the bearing stays in good condition that the lack of friction should keep such a case from happening. Did your application have as many large threads and use loctite? Not sure those things are a true resolution, but I can hope…

I didn’t torque it full tight in the interest of preserving the threads. I’m not sure how much it really matters.

Knocking on wood.
Mine was a 12x1.5, brake caliper
 
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What was the application? This bolt is really the pivot of a bearing that the tensioner oscillates on. I would assume that if the bearing stays in good condition that the lack of friction should keep such a case from happening. Did your application have as many large threads and use loctite? Not sure those things are a true resolution, but I can hope…

I didn’t torque it full tight in the interest of preserving the threads. I’m not sure how much it really matters.

Knocking on wood.
It matters a bunch. You're counting on bolt stretch to put enough spring tension on the fastener to have friction on the threads so it won't vibrate out. If the bolt widens to a shoulder inside the bearing it has even less "spring capacity" or ability to absorb normal vibrations. This is an engineering/ design failure. The fix looks like really good loctite. Maybe even the red stuff, LOL. I suspect with how you cleaned the threads to hospital-grade the 2nd time around you should have better luck with it sticking.

As for thread depth, the general rule of thumb is as deep as things are wide, which your setup meets. I don't know what "class fit" you have in the threads, but hope things don't rattle around in there. Proper torque will be your friend even though it's hairy at this point to pursue.
 

JHZR2

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It matters a bunch. You're counting on bolt stretch to put enough spring tension on the fastener to have friction on the threads so it won't vibrate out. If the bolt widens to a shoulder inside the bearing it has even less "spring capacity" or ability to absorb normal vibrations. This is an engineering/ design failure. The fix looks like really good loctite. Maybe even the red stuff, LOL. I suspect with how you cleaned the threads to hospital-grade the 2nd time around you should have better luck with it sticking.

As for thread depth, the general rule of thumb is as deep as things are wide, which your setup meets. I don't know what "class fit" you have in the threads, but hope things don't rattle around in there. Proper torque will be your friend even though it's hairy at this point to pursue.
Yeah I’m a little uneasy with it, but I’ll keep an eye on it. Drove the car a few miles today and all seemed well. Of course it may be hard to know there’s an issue until there’s an issue….

I suspect that if it doesn’t back out in the first many hundreds of miles and heat cycles, it will be good enough. I only torqued it to 45 ft-lb, vs 73…
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I put the dirt cap over the large bolt that holds the tensioner to the timing case. I’m thinking maybe I should leave it off and mark the bolt with a paint pen so I can watch if it starts backing out.
 
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It matters a bunch. You're counting on bolt stretch to put enough spring tension on the fastener to have friction on the threads so it won't vibrate out.
I wonder now, if the reason he lost a few threads, was due to bolt stretch. Probably loaded the first row of threads heavily in one direction when using the breaker bar to loosen, loading the front threads to get the back threads to release.

Then when torquing in the new tensioner, you load the front weakened threads in the opposite direction and they let go. You could not see it until you took the tensioner out the second time, and some threads came with it.
 

JHZR2

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I wonder now, if the reason he lost a few threads, was due to bolt stretch. Probably loaded the first row of threads heavily in one direction when using the breaker bar to loosen, loading the front threads to get the back threads to release.

Then when torquing in the new tensioner, you load the front weakened threads in the opposite direction and they let go. You could not see it until you took the tensioner out the second time, and some threads came with it.
You may be onto something. I know when I pulled and cleaned that second time after chasing the threads, it was hard to get all the oil out of the deep part of the threads. I used gun cleaning patches and q-tips and towels on sticks.

I have to wonder if some of the threads still had lol residue the first time. Enough to allow it to be both overtorqued, and for some of the initial threads to gall.

I did some more reading, and 243 cures to a grey-white powder…. So I’m thinking that some of what I cleaned out was not metal but rather cured 243.

My suspicion is that I “lost” a fraction of one thread, the outermost one. That’s where most of the extra 243 will reside as it squeezed out of the threads. So hopefully it stays for the long run!
 
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