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Any help is appreciated. This is the 6th time I'm doing this job. The bolts for the fan belt tensioner mount keep breaking. I'm out of ideas. I have replaced everything. I've tried different grade bolts and even studs. I think the problem is with the metal surface you can see breaking away at the 2 bolt holes. If that's the case it will require a new engine as it is part of the block and not a cover. Any ideas or advice? The fan hub turns freely and is working properly. That's all this belt runs.

Is it a clutch fan? Fan clutch not disengaging at high RPM?

This one will not disengage at high rpm,

I tried to find some info on this but there is no service bulletins or any real info on this prob.
I worked for Detroit Diesel for 8 year (they build the MBE 900 and 4000) and another 3 now for Freightliner, ive worked on a lot of MBE's and i cant ever recall having this prob.
As for a fix i don't think going to a bigger bolt is a good idea but maybe a threaded insert with some bearing retainer might work, also is there a locating dowel on your tensioner?

The bolts appear to be cracking from fatigue. To my eye, they look to be starting to crack on the "bottom" and the last bit of failure is the top section of each bolt (at about 10:30). Replace them with something from ARP? Are the bolts shanked bolts? Or fully threaded? If they are shanked, are they breaking at the last thread or are there threads left on the section that's not stuck in the block? Are they staying tight? If they were to start to come loose, then the belt flopping/vibrating the mount could start to fatigue crack the bolts.

Is the block aluminum? If so, you should be able to add material and then drill and retap the holes. I know we used to repair rocker stands on L88 heads all the time that way.

Are electric fans an option?

Yeah no one else is having this problem. I have talked to Freightliner and Detroit Diesel as well. There is a dowel on the bracket put it just sits on the block. Unfortunately I had to go from a 10mm bolt to a helicoil and now I am using a 7/16 stud. I talked to a local bolt company (kind of like a Fastenal) and he was thinking the grade 8 hardware I was using may be too brittle. I never thought that would be an issue. I forget the grade of stud he gave me but it is supposed to be able to handle any vibration better than a grade 8. I do not now I am at the end of my knowledge on this one

This is the 3rd time in 2 months I have done this job on this truck. They are staying tight. the factory bolts are not shanked.

The block is not aluminum. Funny I said something about electric fans too but I do not think they make them big enough that will fit. I would have to fill the front and rear of the radiator with them probably lol. Every time I look at the engine running on this truck, which is a lot now everything looks fine and then one evening I get the call or the warehouse will bring it to me with a belt squeal and bam! the tensioner and bracket are in the fan shroud. This truck is acting like the 2 damn Internationals I have lol

It's not the bolt's fault. If the mounting surface is no longer flat and square, I don't care what kind of bolt you use it's going to break. That means a different block or you will have to have this one welded to build back up the surface and then machined. Cast iron can be welded by someone who knows what they are doing. Probably cheaper just to find a good used engine or block, I'm guessing. Also is there anything non-stock about the engine or accessories?

That is what I am thinking has to be done. But my managers and the customer do not want to hear that. This is the last time I do a temp repair. That's all I fell like I have been doing since the 3rd time it happened and I started seeing the block chipping away. It gets worse and worse every time so I am pretty confident saying its the surface like you are saying as well. Looking at around $30,000 for a new/reman engine. Good thing Coke has plenty of money lol. I would have to talk to the local machine shops about a possible repair option. Lord know they will probably make me pull the engine to repair.

It's a little "hillbilly", but I wonder about installing it with a skim coat of epoxy (or similar) to fill the voids in the metal. The goal is to create a "good" surface for it to interface to the block (and not to just "glue it on"). I wonder if it might stabilize the surfaces enough to show some improvement. I might also find bolts from ARP and send them pics of what you have going on. See if they have a fatigue resistant bolt that might do the trick? Possibly worth a phone call.

Hmm I did not think of that either. It is back together and on the road now. If it happens I will call ARP up and have pictures ready for them. Maybe if they hear it from a company like them they will listen to me lol. But then I will probably have to explain to the Coke guys who ARP is. I do not think any of the managers are car guys

Do you know if the idle speed has been lowered? we have seen problems belts on Cummins wearing out prematurely due to too low of idle speed and the belt will start flopping, could be causing your prob in the first place

It is idling at the same speed as the other ones I have. This one is a 2007 CL120. I have 3 other 2005 CL120's that have the exact same set up and have never done this.

And you've gone ahead and replaced the idler assembly and fan, just in case there's some balance issue?

everytime

Wow. Well, I'm out of talent. (IMG:http://www.frrax.com/rrforum/style_emoticons/default/blink.gif)

One last thing. Does this truck idle with no surging at all? I had a 7.3 PS that had a ever so lightly surge at idle. You could watch the bet tensioner move back and forth with the surge. Eventually it would break tensioner/bolt off.