JHZR2
Staff member
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:
I got the old one off, new one on, used the loctite and primer, and it all went fine.
Went for a drive and heard a clang. The tensioner pulley bolt had broken off.
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.
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.
So I went to investigate. Looks like the thread is busted. Apparently threads can pull out at the entry point from over tightening?
I don’t think it’s all bad.
It was very gritty when I first removed it:
And I found this in the threads:
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?
Discussion on removing the 12mm female hex bolt on the tensioner is here:
Removing a hex bolt that is in tight
It’s the belt tensioner on my new 91 350SD. Probably original. I had noticed early on that it was in bad shape. I have a new Mercedes tensioner, it’s the same as the one on there. They are installed with some sort of thread locking sealant. I’ve been using a quality 12mm SK hex socket on a...
bobistheoilguy.com
For reference, the tensioner has a pretty big captive threaded bolt, can be seen here:
I got the old one off, new one on, used the loctite and primer, and it all went fine.
Went for a drive and heard a clang. The tensioner pulley bolt had broken off.
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.
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.
So I went to investigate. Looks like the thread is busted. Apparently threads can pull out at the entry point from over tightening?
I don’t think it’s all bad.
It was very gritty when I first removed it:
And I found this in the threads:
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?
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