Hello All,

I am new to this forum, and am glad I found it. I have been a member of the Benzworld forum for awhile but the tech expertise of the users on that one does not compare with the members posts that I have been reading here.

Anyways, on to my challenge...

...I have an '82 300SD bought it with 270K US miles. Drove it like I stole it for over thirty-thousand miles. One day I lost all accessory drive ( as in all the belts on the engine stopped turning, but engine is still running), turns out that the two dowell pins that locate the harmonic balancer onto the end of the crank-shaft cut in half. Replaced them and drove again for a few thousand miles. It happened again, I repaired again. It happened about five times and then I determined that the 'keyways' that the pins sit in were wore from cutting all those pins and that I was going to need to get my crank-shaft replaced or machined. Figuring that was going to be the same or more expensive than finding and replacing the engine with a used one, I did.

When I installed the engine I put brand new shear pins ( as I have learned what the official name for them is here in this forum) in when re-installing the engine.

I thought all my problems were solved, enjoyed the newer less miled engine for about five thousand miles and it just happened again!!!!!!

I have found topics that dealt with this happening to one of the members cars, and read all the detail in the repair. Nothing too much new in these as I have done this repair so many times I could do it blind-folded!

Does anyone know what causes this to occur?

What can I do when repairing this (yet again!!! ) to avoid it re-ocurring?

Unfortunately for a more permanent solution you may need to replace the Crankshaft. More than likely the centering grooves are too elongated to be effective at maintaining the harmonic balancers fixed position under load, long term. You could try Green loc-tite on the shaft and balancer to see if it provides better results. Just keep in mind that the pins have sheared more than once, the balancer as well has slipped more than once. Each time it slips it causes material (metal) on both surfaces, which is crucial to the interference fit, to be lost.

Since you have a "new" engine with a "new" crankshaft, I would look in an expanding search for the problem. I'm assuming you are using the harmonic balancer that came with the engine swap.

Could one of the driven auxilliary pulleys be so eccentric that it is setting up a vibration that killed the new pins?

__________________
The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

Good point. It is possible that an auxiliary drive component is contributing to the failure of the pins. It is also possible that something is unintentionally being overlooked while performing the repair.

I have witnessed this failure at different times through out my tenor and more often than not, it is a repair procedure that is inadvertently overlooked. What is the torque specification of the front bolt?

MrCJames,

Could you give some examples of the missed procedures that you have observed with this failure.

I can then check these off as I do this repair again.

The tourge specification on the front bolt is probably the most crucial element to this repair. I believe it should be tourqed close to 300Lb/ft, I will need to check to make sure I am giving you the correct information.

I will get back to you!

Thank you,

I will await your reply.

The crank bolt should have three concave washers on it (crown to the bolt head) oil the threads lightly and torque bolt to 250 lb/ft. There is other information regarding the replacement of the balancing hub/disc, if it is replaced it must be dynamically balanced before fitting to an engine. The dampener itself does not call for any balancing.

If these procedures were not mistakenly overlooked than you may need to question the calibration of the torque wrench you are using. If your newer/replacement engine suffered from the same failure than you may be looking for another engine, if not now than in the near future.

In my experience once the balancer spins on the crankshaft it is pretty much a done deal for both pieces.

Keep me posted

I see sheared pins on alot of different types of equipment and you can bet that something "makes" it shear whether it is a problem with the torque or with the "keyway". You need to find out what is making it shear and resolve that problem. I would be real surprised if it were the auxiliary drive compontents. If they were causing this issue you can bet that you would have some issues with the belts too.

On the 300SE I just replaced the timing chain/guides on the torque on the crank bolt was 300Nm and it does have the three washers. Not sure of exact value on your model. It is not unheard of to hear of a torque wrench being out of calibration. That is why it pays to have them checked once/year if they see heavy usage.

__________________
Jim

This appears to be a fairly common problem on 126s through 1987. The proper term in the U S of A is "woodruff Key."

__________________
N. Hodges
1994 S350
1987 300 SDL (for sale)

Thank you all for your input on this challenge that I am having. I will use this over the coming weekend in my repair.

I do have some questions though:

How/where do I dynamically balance my HB? At a machine shop? Does this require the engine to be removed/torn down? Is it right to assume that this is moderately expensive?

I have been wrenching for myself and for various companies in the heavy equipment world for several years, and I understand the ramifications of not doing all procedures properly, in regards to resulting failure. So in this case, I ummm...well I...ahhh...hmm hmmm....
Ididn'thaveatourquewrenchandsoIdidnottightenthe27mmbolttotheproperspecinanyofthesepastreairs

And so...well..uhhh, with that said we can all just move along here nothing to see, and lets all go out there and remember to ALWAYS do the proper procedures when repairing our beloved Mercedes Benz's, and avoid any embarassment.

Over a 15 year tenor with MB I would strongly disagree with you about it being a "fairly common problem!" I worked on a lot of diesel engines during that time, some with low mileage and some with over 300k. I saw this failure five times. After reviewing the vehicle records the most common element in those cases was a new front crank seal within 5k to 10k of the failure. This particular failure is a result of something being overlooked during the repair or there is a significant imbalance that is causing it to shear.

After a second failure on a second engine the question that needs to be asked is "what do these two engines have in common that could be causing the problem?" It can only be: Auxiliary drives, Flywheel, Torque Converter, Tools being used to perform the repair, and the person doing the repair. If it were an auxiliary drive I personally do not believe it could have an effect on the balancer to this magnitude, I could be wrong. The flywheel and torque converter would be my first suspicion since more than likely they were transferred to the second engine. If it were one of these two items I imagine the engine would have a very annoying vibration that would make it unpleasant to drive as well as tell the driver that something is not right!. Lastly is the tools and or the person.

If a person is performing something for the first time they inherently question themselves through out the experience, proceeding cautiously so as to avoid mishaps or damage. Until you have some experience and fully understand all the steps involved with what you are doing things can go wrong. The more you do something the more familiar you become with it. Simply put it is education.

The half moon style key-way is commonly referred to as the "woodruff," in this particular application the balancing hub is fitted to the crankshaft with two semi-soft steel "dowel pins.” The grooves on the crankshaft where the pins reside are offset by nearly 180 degrees. They are not exactly 180 degrees apart because the balancer is marked roughly 40 BTDC thru 10 ATDC in order to time the fuel injection system. The more common problem would have been the balancer being reinstalled 180 degrees out of position making it difficult to set the injection timing to any degree of accuracy.
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Keep us posted and wouldyouliketoborrowmytorquewrench? Good Luck to you.