The Death of a Flex Disc ... did I kill my 240D? - PeachParts Mercedes-Benz Forum

BodhiBenz1987 · #1 05-20-2009, 09:08 PM

Update and some pics:
Well, I finished everything this evening. Yesterday I spent most of the day removing/replacing the bushings at the ends of the driveshaft (that included two trips to Lowes and a trip back to my house). I drilled a hole in each one, per Haynes manual procedure (bought a new set of Bosch drill bits that could handle the thickness), stuck a mini allen wrench through it and tried to pry it up using screwdrivers with the shaft in a vice. Upgraded to giant pry bars, no go. So I bought a torch and heated it ... they came right out. You can see the damage on the front one.

Today I put the center support back on, had to tap it on using a wide PVC pipe. The real tough part was torquing on the 12 bolts for the flex discs. My torque wrench is too big, everything kept slipping, and I had no space under there. Plus I have the strength of a preteen girl. But, I toughed it out and got them all done, despite lying in what was clearly not just mud under the back of the car.
Took it for a drive, everything seemed OK.
But here's the kicker (you knew it was coming, right?). When I was cleaning up a I picked up one of the old bolts, which still had the washer on it ... and realized I put the washer on the wrong side of every single bolt (i.e., I put it under the nut. Why? I have no idea). I doubt that will lead to immediate catastrophic failure, but the washers go under the bolt heads for a reason, and I have to go back and re-do all 12 bolts. It makes me so mad, because if I'd stopped and thought about it, logic alone would compel me to put the washer under the bolt head. And I was so careful about everything else. Oh well, it could be worse ... at least it doesn't look like this anymore ...

rs899 · #2 05-21-2009, 07:23 AM

If the washers DO go under the bolt heads, I am at a loss to understand why. I probably have all mine on the way you did it. I would say the "normal" way would be to put the washers under the nuts anyway. I wouldn't change a thing or worry about it, especially if you used nyloc nuts.

Good job!

Rick

Deltacom · #3 05-21-2009, 10:01 AM

Quote:

Originally Posted by BodhiBenz1987

But here's the kicker (you knew it was coming, right?). When I was cleaning up a I picked up one of the old bolts, which still had the washer on it ... and realized I put the washer on the wrong side of every single bolt (i.e., I put it under the nut. Why? I have no idea). I doubt that will lead to immediate catastrophic failure, but the washers go under the bolt heads for a reason, and I have to go back and re-do all 12 bolts. It makes me so mad, because if I'd stopped and thought about it, logic alone would compel me to put the washer under the bolt head. And I was so careful about everything else. Oh well, it could be worse ... at least it doesn't look like this anymore ...

Quote:

Originally Posted by rs899

If the washers DO go under the bolt heads, I am at a loss to understand why. I probably have all mine on the way you did it. I would say the "normal" way would be to put the washers under the nuts anyway. I wouldn't change a thing or worry about it, especially if you used nyloc nuts.

Congratulations on a well presented job, nice write and fantastic pics on step-by-step procedures. Well done!!

Just one point to clarify regarding the washers on the bolts. As you can see from your picture before disassembly the washer either go under the bolt if the bolt goes first through the flex disk to screw on to the lobe of the shaft or axle/tranny.
Conversely, the washer goes under the nut when the bolt goes first through the lobe of the shaft or axle/tranny. The washer always goes resting on the flex disk and not on the lobe of the shaft or axle/tranny.

Here are pics of your flex/shaft before dismantling and one of my own flex that I need to replace soon (thanks to your efforts, write and pics it will be a lot easier for me). BTW, there is 6 grams of pertinent grease to go on the centering sleeves.

BodhiBenz1987 · #4 05-21-2009, 01:55 PM

Quote:

Originally Posted by Deltacom

Congratulations on a well presented job, nice write and fantastic pics on step-by-step procedures. Well done!!

Just one point to clarify regarding the washers on the bolts. As you can see from your picture before disassembly the washer either go under the bolt if the bolt goes first through the flex disk to screw on to the lobe of the shaft or axle/tranny.
Conversely, the washer goes under the nut when the bolt goes first through the lobe of the shaft or axle/tranny. The washer always goes resting on the flex disk and not on the lobe of the shaft or axle/tranny.

Here are pics of your flex/shaft before dismantling and one of my own flex that I need to replace soon (thanks to your efforts, write and pics it will be a lot easier for me). BTW, there is 6 grams of pertinent grease to go on the centering sleeves.

Thanks for pointing that out ... I guess I only got if half wrong.

Now I have to decide whether to go in there and change the six that are on wrong. rs899 ... I know the nuts are self-locking, but I don't think they were nyloc. Can they be removed and refastened without losing their locking function?

rs899 · #5 05-21-2009, 02:16 PM

As long as the "self locking" feature still works, you should be able to re-use them. You can use some liquid threadlocker if need be. Maybe I'm not such a purist as Deltacom, but I would not remove and reconfigure those bolts. I say leave' em alone and find something more important to work on.

As long as those bolts are torqued down well and the nuts don't spin off, nothing bad is ever going to happen. All the side load is in the inserts.

Besides, just how many of these discs were put on by high school dropouts at Meineke mufflers over the years?

Rick

Deltacom · #6 05-22-2009, 07:26 AM

Quote:

Originally Posted by BodhiBenz1987

I doubt that will lead to immediate catastrophic failure, but the washers go under the bolt heads for a reason,

Yes, indeed!, you’re absolutely right!, there is a very good reason for the washer to be either under the bolt head or the nut, whichever is resting directly onto the flex disk.

Very briefly, the six holes on the disk contains 2 sleeves (bosses) each. The outer one is “Vulcanised” to the rubber, the inner one is fit-pressed into the outer. When the bolt is tightened the washer actually pushes slightly the inner sleeve into the shaft/flange/yoke recess in the lobes and bridges the 2 sleeves to the metal disk thereby securing the flex disk onto the shat/flange/yoke.

If the new bolts supplied with the new disk are Allen socket type of bolts, the head of the bolt is not wider than the rim of the inner sleeve (even the hexagonal head bolt barely bridges the 2 sleeves) and without the washer to bridge the 2 sleeves onto the metal disk the bolt is not securing the flex disk but only the inner sleeve. Without that vital support of the washer the bolts could shear off as it happened to your original one when failed the support due to surrounding rubber deterioration. Even the shape of the sides of the washers are important in which way they face, as one side is flat (facing the flex disk) and the other has rounded edges (facing the nut/bolt).

The way I understand you installed the washers just under the nuts only, it means that theoretically you could pull the shaft and would come away from the yokes together with the flex disk attached to it (since those are the only bolts that are secured with the washers to the flex disk) leaving the 3 bolts and inner sleeves attached to the tranny’s yoke/flange and the same at the differential.

Bolt without washer

Bolt with washer

Hex without washer

Hex with washer

rs899 · #7 05-22-2009, 11:12 AM

Looking at the pix from Deltacomm, he is correct in that you need to cover the bonded bit of the disc, whether it is with a washer , head of a bolt or nut doesn't matter. I forgot that the socket headed bolts they give you are so narrow.

BodhiBenz1987 · #8 05-22-2009, 08:06 PM

Thanks for the pics/explanation. I was going to do it anyway, but it's good to understand the significance of the washers ... and I will pay more attention when I do this job on my 300D.