Destroyed Aluminum Threads (and how did I?)

JHZR2

Staff member
Joined
Dec 14, 2002
Messages
49,488
Location
New Jersey
The saga begins with the need to replace my belt tensioner on my new auction buy 1991 350SD.

Discussion on removing the 12mm female hex bolt on the tensioner is here:


For reference, the tensioner has a pretty big captive threaded bolt, can be seen here:

5DEA98F7-4867-4007-AA64-DB087568201F.jpeg


I got the old one off, new one on, used the loctite and primer, and it all went fine.

7E0408AB-27F8-443A-9D1E-C753487D2A73.jpeg


Went for a drive and heard a clang. The tensioner pulley bolt had broken off.

BAEFF738-E07B-46A2-97D0-9CAFA228C735.jpeg


Figuring that out is a problem for another day.

I pulled the tensioner, and it came out fine. Just like it went in (I threaded it in by hand).

But since I want going to mess with it more today, and it’s a hole to the engine interior, I wanted to thread the tensioner back in. I could only get it to go a couple threads.

A8083D0E-9BDE-48BA-8EF0-894E56AC9295.jpeg


Crap. I didn’t force it. I never did. In fact, the only time a tool touched the thing was when I did the 73 ft-lb (100nm) to final tighten it. The FSM says 100nm with threadlocker.

3AE17F0C-5CEA-483D-AD2C-38A638952D94.png


So I went to investigate. Looks like the thread is busted. Apparently threads can pull out at the entry point from over tightening?

FBB95D72-6C8A-4AC0-AD5B-78EA02F15BC6.jpeg

8CD5FA8D-C6FF-4071-B321-0BD1CE5A23F5.jpeg
3C4630DF-F8E9-4D0A-AE46-1477E1CD324A.jpeg


I don’t think it’s all bad.

8CD5FA8D-C6FF-4071-B321-0BD1CE5A23F5.jpeg


It was very gritty when I first removed it:

922E309F-43EC-444F-B633-568DDB6F7FC8.jpeg


And I found this in the threads:

8D91C325-9621-455C-8D66-857789A0B4CD.jpeg

So, what did I do wrong?

I made sure everything was completely clean.
I went over the threads, dry fit the part by hand.
I degreased everything.
I primed both sides.
I used loctite 243 but sparingly.
I installed by hand until the very end.
I torqued to spec with a quality wrench.
I removed it by hand.

And to add insult to injury, I could remove the sheared off bolt with a fingernail. Had I let the tensioner be, I’d have no knowledge of any issues right now…


So… what did I do and how do I fix it?
 
Last edited:
Joined
Jul 11, 2014
Messages
3,399
Location
Paradise of Florida
The quality wrench might need a cleaning and calibration

Is that a new OE part? Doesn't have the word "Germany" on it. If you grabbed a sharpie and wrote the word "Germany" on it, the engine wouldn't have rejected the part.

The new tensioner was offended by the belt condition.

I'd run a die over the new bolt and see if it hangs up. Could've just been a bad part.
 

JHZR2

Staff member
Thread starter
Joined
Dec 14, 2002
Messages
49,488
Location
New Jersey
The quality wrench might need a cleaning and calibration

Is that a new OE part? Doesn't have the word "Germany" on it. If you grabbed a sharpie and wrote the word "Germany" on it, the engine wouldn't have rejected the part.

The new tensioner was offended by the belt condition.

I'd run a die over the new bolt and see if it hangs up. Could've just been a bad part.
It indeed was an OE part.

09350F98-A3FD-40A0-837B-6153DD8F8C84.jpeg

Has the part number and star. Came from an MB package.
 
Joined
Jul 13, 2003
Messages
2,581
Location
Tracy, CA
The hole goes into the engine internals - it's not a blind hole? What's on the other side of the hole? If it's engine oil or coolant, I'm thinking the thread locker may be more for sealing than locking (MB is killing two birds with one stone?).

IMO, there should still be more than enough thread engagement to allow for proper torque.

However, chasing threads with taps and dies can open another can of worms; I was told more than a few times that taps and dies are cutting tools, not repair tools. I have gotten away with doing it many times. I would first use a tap in the hole only enough to "clean" up the offending threads, only chase the bolt threads if it needs it.
 

JHZR2

Staff member
Thread starter
Joined
Dec 14, 2002
Messages
49,488
Location
New Jersey
You sure ? How is it sealed ? I don't see an o-ring on the tensioner so I suspect that's a blind hole.
It’s not. That’s why oil can seep out.

I don’t know what’s in there, timing stuff…

50488092-0071-4ECF-9EE2-A023BAA1EE2F.jpeg


You can kind of see there that it’s half open, half occluded. The threadlocker is supposed to be used as a sealant.

8BE3E277-4A94-4D49-BB48-E96FC4309204.jpg


Regardless, now I need to fix it. I’m not sure I can drill it properly because it’s not a through hole.
 

JHZR2

Staff member
Thread starter
Joined
Dec 14, 2002
Messages
49,488
Location
New Jersey
For that - TimeSert.

Won't be cheap. Will be as good as new.

If TimeSert is good enough for BMW head bolts in aluminum blocks, they're good enough for this.
It’s a half occluded hole. Open one side so oil can seep out, covered the other half. Seems kind of dumb. I think you need to be able to drill through to install a time sert. Pretty sure I can’t.
 
Joined
Feb 20, 2007
Messages
6,591
Location
Southeast
I don’t think you did this. I’m wondering if the old bolt stretched while it was in there- putting more stress on the exterior-facing threads than the interior facing. The damage would have been done before you took it off.

im really wondering if you can just clean them up and get by? I only see 4mm of one thread torn off? If the others are still good there’d be plenty left.
 

Astro14

$100 Site Donor
Staff member
Joined
Oct 10, 2010
Messages
16,594
Location
Virginia Beach
It’s a half occluded hole. Open one side so oil can seep out, covered the other half. Seems kind of dumb. I think you need to be able to drill through to install a time sert. Pretty sure I can’t.
It looked open. TimeSerts work in a blind hole. You can drill deep enough. If you can do a helicoil, you can do a TimeSert.

You have to get the appropriate depth of thread insert. What's the bolt thread size? How deep?
 

JHZR2

Staff member
Thread starter
Joined
Dec 14, 2002
Messages
49,488
Location
New Jersey
It looked open. TimeSerts work in a blind hole. You can drill deep enough. If you can do a helicoil, you can do a TimeSert.

You have to get the appropriate depth of thread insert. What's the bolt thread size? How deep?
I don’t know the bolt/thread off the top of my head.

What I can’t visualize with a blind hole application is this - there are threads in there. You either drill all of them out, or some. Say you drill some out. Then you put the insert in. At some point the insert threads end and the original ones start. Then what?

I guess the answer is that you must drill it out enough to have the full time sert or helicoil engage the male threads. Not sure if that’s 100% the case though.

I’ve installed a time sert in an oil pan before. This one is pretty lousy because it needs to be done with right angle tools in minimum space. If any one part of the kit is too long, it will require removing the entire front end systems so it can be done straight in.

01E21B1E-E4F8-481A-939C-1064E7894C51.jpeg
 

JHZR2

Staff member
Thread starter
Joined
Dec 14, 2002
Messages
49,488
Location
New Jersey
I don’t think you did this. I’m wondering if the old bolt stretched while it was in there- putting more stress on the exterior-facing threads than the interior facing. The damage would have been done before you took it off.

im really wondering if you can just clean them up and get by? I only see 4mm of one thread torn off? If the others are still good there’d be plenty left.
It was a new bolt because they’re captive to the tensioner.

I did torque it properly, to the FSM value, and the FSM prescribes threadlocker. So notionally the torque is right.

But it could still be enough to damage the threads. The question really is if I can chase the threads in such a way that I engage the good ones further back and then smooth out the bad ones at the opening. Essentially some kind of tapered chaser thatI can somehow get aligned into the hood threads a few rows back?
 
Joined
Feb 20, 2007
Messages
6,591
Location
Southeast
I see what you are saying. yikes, you really want to try to thread it from the back side… but it doesn’t look that that’s a thing.

dumb thought. Thread it to where it stops, try tapping inward with a hammer, see if that could gently coerce the bolt to properly engage the next ramp?
 

JHZR2

Staff member
Thread starter
Joined
Dec 14, 2002
Messages
49,488
Location
New Jersey
It looked open. TimeSerts work in a blind hole. You can drill deep enough. If you can do a helicoil, you can do a TimeSert.

You have to get the appropriate depth of thread insert. What's the bolt thread size? How deep?
Apparently it is open, it’s just that some part of the timing chain system is directly behind it.

19C2D974-795C-4CB8-BCE3-FB6A6956AB97.jpg
 
Joined
Sep 23, 2008
Messages
10,927
Location
Ontario, Canada
I'd think you are still fine without going to an insert. Check how many threads that are engaged that are still good and then look up how many you actually need for the torque required in an engineering table. I'd think MB probably left some margin on that.
Also I'd get a normal short bolt of the right size so you can play around with starting it threading that in by hand or tools, and you can see what you are doing, and practice getting it started with the first thread missing. If it came out by hand, it should go in by hand, you just have to figure out how without messing up more threads.
I imagine in aluminum you might be able to clean up the first couple threads with pick too, may help start the bolt again.
 
Joined
Nov 20, 2006
Messages
29,194
Location
MA, Mittelfranken.de
It was a new bolt because they’re captive to the tensioner.

I did torque it properly, to the FSM value, and the FSM prescribes threadlocker. So notionally the torque is right.

But it could still be enough to damage the threads. The question really is if I can chase the threads in such a way that I engage the good ones further back and then smooth out the bad ones at the opening. Essentially some kind of tapered chaser thatI can somehow get aligned into the hood threads a few rows back?
I think the threads got weakened when you removed it. I see it a lot in aluminum especially spark plugs on some cars, no matter but I don't think you can fix this to handle that kind of torque by chasing the threads with a tap.
Time Serts are fine but not as strong as a coil but your problem is the swarf going down in the timing chain area, if I were faced with this I would first find out the size and thread pitch and see what coils/inserts and STI spiral fluted taps are available, they pull the swarf out not push it through and in STI sizes are more available for coils.

The other alternative is to remove the cover and do it on a drill press or give it to a machine shop or just replace the cover but I am guessing that is about 800-1000 bucks new, it may be less I haven't checked but that seems to be the going rate. Post the size and thread pitch.
 

JHZR2

Staff member
Thread starter
Joined
Dec 14, 2002
Messages
49,488
Location
New Jersey
I think the threads got weakened when you removed it. I see it a lot in aluminum especially spark plugs on some cars, no matter but I don't think you can fix this to handle that kind of torque by chasing the threads with a tap.
Time Serts are fine but not as strong as a coil but your problem is the swarf going down in the timing chain area, if I were faced with this I would first find out the size and thread pitch and see what coils/inserts and STI spiral fluted taps are available, they pull the swarf out not push it through and in STI sizes are more available for coils.

The other alternative is to remove the cover and do it on a drill press or give it to a machine shop or just replace the cover but I am guessing that is about 800-1000 bucks new, it may be less I haven't checked but that seems to be the going rate. Post the size and thread pitch.
My searching last night seemed to indicate that it is M18x1.5 but I don’t really have a way to really validate that. I’ll put some calipers on it tonight.

Supposedly the timing cover is like $330 or so, new, also not validated. The real question is the amount of work to replace that.

There is a thread on benzworld where someone did the job with a time sert recently on a 606 engine (same timing cover), and they did remove the whole front end to drill it straight.
 

JHZR2

Staff member
Thread starter
Joined
Dec 14, 2002
Messages
49,488
Location
New Jersey
Quite a job to remove that cover.

I’m in disbelief that I screwed this up so bad. Completely disgusted.

Had the pulley bolt not sheared off (not sure why that happened in like 1-4 mile of use), it would be in there, installed, no issues that I’d know. And good to go another 200k.

To add insult to injury. The broken screw could be removed with my fingernail backing it out.

Ugh.

1C04562B-FCB5-4D92-A6A9-4F7EFFA6ED39.jpeg
 
Top