My father has a 99 E300 and today I noticed the belt tensioner is about to fall out. I knew the threads were partially stripped out and would have to deal with this eventually. Now is the time. Judging from the wear marks on the fan blades its been like this a while. Lucky I caught it before it came completely out. What is the best approach to this? Drill it out and use either a time-sert or a helicoil? If so, what size is the bolt? Or, would it be better or easier to just pull the cover and replace it?

Drill it out and use either a time-sert or a helicoil would be the normal way to deal withit.

I don't know the bolt size.

I don't know what the stripped hole is in but your comment about the cover makes me think the stripped hole is in the cover. Part if the issue with thread inserts is drilling the hole straight in if you are doing that on the engine.

The drilling issue also has to do with if there is enough room for the drill motor with drill bit to fit in.

Personally, I have trouble drilling straight with drill motor even when everything is out in the open and on a bench. However, I have seen others do it fine.

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84 300D, 82 Volvo 244Gl Diesel

Some prefer TimeSert or KeenSert over HeliCoil. Whichever you choose put Threadlocker on the insert.

Used thread inserts on a 104 belt tensioner, it worked well. Car had over 360k miles.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Nobody knows what size the bolt is?

If you fix the stripped hole or you buy a new cover you are going to need a new bolt. Get the new bolt and when you have that you can find out what threads it has.

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84 300D, 82 Volvo 244Gl Diesel

Thanks for the reply. The point of the question and post was to find out what size the bolt was BEFORE I got the car into the shop and started tearing it down so I could go ahead and get the proper time sert kit ordered. I figured surely somebody has done this repair before and would know what size it is but so far not finding an answer.

I do understand but I have seen similar issues on the forum before. People simply do not know all the sizes and threads of the bolts on their vehicles.

If I take a bolt off of my Car I would not know what size it was. I would have to measure the outside diameter and get out my thread gauge to find out the pitch of the threads. That is likely what someone would have to do to answer your question.

Best wishes.

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84 300D, 82 Volvo 244Gl Diesel

Be very careful drilling or tapping that hole, its open to the inside of the engine (you literally see the timing chain once you remove the arm bolt).
This is why the fsm also instructs to use thread sealer to prevent oil wicking through the threads.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

Not sure I follow. Do you mean the bolt that holds the tensioner is about to fall out? If "yes", then you still have the bolt, correct? Take the bolt to where "heli-coils" are sold and they should be able to help you out. You could replace the bolt, as well, if it's worn or of questionable condition. You should be able to find a "heli-coil" for the same size bolt.
I'd try to find a knowledgeable independent parts store and avoid national chains that begin with the letter "A".
I apologize if I'm failing to understand your problem. Just trying to help.....

So I got the car to the house and into the shop. Basically the situation is about the first 3rd or half of the threads are stripped. The thing was probably not tightened down enough and backed out. If you tighten the bolt all the way down the tensioner is straight and the bolt seems to grab. I am not sure off hand what the torque spec is on it at the moment but the bolt did tighten good but I did NOT put a ton of pressure on it or try to torque it to whatever the spec is. I am half tempted to apply either some blue or red lock tight to the bolt and hope it lasts a while. Since it is my dads daily driver I would really prefer to fix it right, even though I would bet the lock tight would hold for a while. With the bolt removed the timing chain is right there. I have a bore camera I might be able to get down there thru the oil filler hole, or worse case pull the valve cover. I am thinking maybe if I do the time sert repair I can drill it out manually rather than using a power drill which would give me more control of it since I would be able to feel what is going on. I would bet I would end up having to pull the radiator and possibly the condenser to have room for the drill.

Anyway back to the lock tight, if I did try the route we could keep a very close eye on it and first sign of the thing becoming cockeyed, would then do the right fix. Heck I don't know.

Loctite would probably work from your description. Perhaps it could be Loctite'd now and you could search for a good used cover and bolt in the interim.

Is installing a stud with Loctite Stud and Bearing Mount so it is sort of permanent and then securing the other end with a nut a possibility?

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

I am worried about making it worse doing the time sert repair at the moment. I am thinking about applying locktite to the front half of the bolt that will engage the good threads and JB Weld on the back half where they are stripped out to provide a little extra hold. I will then see if it will torque down to 73ftlbs per spec. If it does or will at least get close I think it might last a while. I think I have a good used timing cover off a 603 around here somewhere but really not wanting to do that replacement unless it is last resort.

Sounds like your plan is more than worth a try. Since you'll be using a Threadlocker you could probably stop at 60 ft/lb if the torquing process doesn't feel correct. What is the bolt diameter and depth of the good threads?

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

The thread locker/JB weld attempt did not work. It is basically fully stripped out now. I am getting ready to get this stripped belt tensioner bolt fixed on the 99 606 of my fathers. My current plan is to repair it with the cover in place with a time sert. These things are expense and the kit for this is $258. bolt is M18x1.5 btw. Of course that includes 5 inserts. I am planning on having to possibly remove the radiator and condenser. Doing that the timing chain is right behind the hole so would have to mark the depth on the drill bit so I know how far I can go.


Pricing a new cover, its around $193 for a new cover from various MB parts dealers online. Should I attempt to repair the threads or just bite the bullet and start pulling the front of the engine apart? I assume water pump, vacuum pump, pulley, alternator bracket, ps pump and bracket, crank pulley? What else would have to come off. Is it removable without pulling the head and oil pan?

I can't answer your question specifically about the engine you are working on. Calculate the time difference between doing the timesert and the cover replacement to help make your decision. The new cover will be a better repair and perhaps you can freshen up other items in that process. A helicoil kit would cost a lot less but isn't quite as good IMO.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer