M104 Tensioner Center Bolt Stripped!
 

Greetings!

W124 - 300CE

After doing a wonderful job replacing my belt tensioner, and going on a test drive, I noticed a strange squeal coming from under the hood. Having done this repair a couple of times before, it shouldn't have been a problem, but as near as I can tell, the squeal was caused by the tensioner being loose and misaligned. It seems I may have screwed up and forgot to tighten the large center bolt for the tensioner and subsequently it stripped the aluminum threads. I find it odd that it only took a few miles of city driving to completely hose the threads.

Now that I'm done being completely upset with myself, I have resigned to the thought that I either need to replace the (I think it's called the A/C compressor mount) or carefully helicoil new threads into the old one.

I rather like the idea of helicoil and have had success using them before on a myriad of other repairs on other things years ago.

And a few questions...

Has anyone ever seen a stripped tensioner center bolt? And if so, what was the recourse? Helicoil or replace?

Any idea what size the bolt is? Threads and pitch?

I appreciate any response and feedback. Thanks in advance.

Michael B.

I think unless there is something unusual about the location of it, any thread can be helicoiled. If you are unsure of your ability to do it I would recommend having a machinist do it for you. I recently had need of one and have never done one so I got my favorite machinist to do it. It worked fine.

Tom W

So I brought the center (pivot/pinch) bolt to work to check the size and to get a helicoil kit for it and suddenly it came to me that maybe I did tighten the bolt and this has been an issue before. Most posts here talk about the "19mm bolt", but mine does not take a 19mm socket. It does however sport a better hex allen, say around 10mm. This leads me to wonder if it happened before and someone else put in a longer/larger bolt.

This may now explain why the bolt was a little too large for the tensioner hole. With little filing of the hole, the bolt fit fine, but I blamed it on the part manufacturer for not sizing the hole correctly. So, it seems I may have cast blame where I shouldn't have, unless MB used something other than a standard hex cap bolt.

Can anyone confirm the stock measurements for the original bolt?

Help is appreciated. Thanks!

Michael B.

Ok, I now have the size of the bolt. The size is M12 x 1.75 x 120mm.

Assuming this hex allen bolt is not MB OEM, I do wonder though what the original size was.

Off to the helicoil store. More later...

Michael B.

Euro vision is using hex allen bolt for belt tensioner!! I sold one belt tensioner to my customer 2 days ago. His car imported from Japan... He told me that the bolt is hex allen bolt, not 19mm bolt.

Repair Complete...
 

Well, it's done.

Complete disassembly again. I was able to run the tap in without drilling the hole with a 31/64" drill as recommended on the helicoil package. Lining up the tap wasn't especially difficult, but did have to take it slow at first, checking to make sure I was going straight in. Once the threads were cut, I was able to screw in the helicoil and snap off the little tab.

The rest is history (as they say, [whoever "they" is]), because from that point it was all reassembly.

By the way, I'm not sure what some are using for a special tool to hold the fan pulley, but I found that a slightly bent "L" allen wrench worked just fine to hold the pulley. I did have difficulty holding the water pump pulley, however. There's just no easy way to hold it. Maybe if I had an oil filter strap wrench, it might have been easier.

All in all, a belt tensioner gets easier each time I do one. This makes the third time for me (different cars) and it may not be the last.

I hope the information I provided will help someone else in a same or similar situation.

Michael B.