Flex Disk Replacement

[George Des]

Got that dreaded vibration in the driveline again and put the 230 Sl up on the ramps to check things out. Sure enough, the flex disk is starting to disintegrate.... again. I replaced the first one I had installed about 10 years ago after about 2000 miles. Did a little better with this one. Maybe 6000 miles or so, but still not good. I think I had everything installed correctly but I was a little worried about driveshaft alignment and how to make sure the disk is totally stress free once things are tightened down. Looks like I was right. I'll be extra careful this time around. I just happened to have a brand new kit hanging around and will put that in. Just out of curiousity though, I started looking around and noticed there is now a more rounded disk being sold as a replacement. Has anyone here used that kit and is it any better that the originally designed disk?

[ja17]

Hello George,

The new version is improved. It is more robust, and symmetrical, so it cannot be installed incorrectly. It will last longer. Just make sure that the driveshaft centering flange  bushing is ok. Also the long and short bolts must be in the correct locations with washers in the proper position.

[mdsalemi]

George--

Don't feel bad that you have not received a better life out of the flex disk.  It's organic, like a hose or belt--and as such can wear out when sitting around doing nothing. Ozone and age, not to mention the horrible life living underneath a car doesn't give these parts the same life as a solid metal part.

Be thankful--very, very thankful--that you caught this before it let loose on you.

[George Des]

Thanks, Joe for the info on the new flex disk. Any idea if there are any special instructions as to how it gets oriented? Michael, I hear you! Just not sure why the origial lasted so long compared to the two replacements. Something I did was obviously not right.

[awolff280sl]

George,
I've been running for about 10,000 miles so far on what I believe is the W108 chassis flex disk.
It required no special orientation, both sides being identical.
I do remember an issue with the hardware and using an extra washer or two to avoid the bolts hitting the tranny flange.
Hope this helps.

[George Des]

Thanks, Andy. I have the old style on hand but just ordered one of the new style ones and will install that. I really don't feel much like having to go back in there again any time soon!

George

[mdsalemi]

I really don't feel much like having to go back in there again any time soon!

Shouldn't we all be thankful that there are no flex disks on the [defrost/heater] blower motor? 

[George Des]

Michael--that's another job I have done several times and happy I have a relatively new motor in there. Anything having to do with the dash is a real pain. Regarding the flex disk. After much swearing and knuckle busting I finally got the old one out. The big issue was having to lower the headpipe to get at the bolts on the transmission plate. Very difficult to get your hands in to remove the headpipe bolts. Once that was done the plate bolts came out easy enough. Took off the rear transmission mount with no problem. Even the flex disk bolts were no big problem. The real fun started with the large nut on the driveshaft. This has to be loosened in order to push the driveshaft back far enough to clear the connection with the transmission. The nut would not budge. I finally resorted to splitting the nut with a dremel tool and abrasive disk. Once his was done, the driveshaft moved back easily and the front driveshaft taken out along with a completely shredded flex disk. Now I need to find a replacement driveshaft nut. Anyone have one of these hanging around? If not, I'll call Tom Hanson tomorrow--looks like they run about $20.00 or so. Now I will use this opportunity to replace the rear cover plate on my ZF 5 Speed transmission. The pinch joint where the speedometer cable plugs in had broken years ago and has been JB Welded together ever since. Has held out well but I was able to finally get a brand new rear plate after years of trying to get one through ZF. It was not cheap!! The new production of the ZF 5 Speed has made available many spare parts that were pretty much impossible to obtain until now--well almost--ZF is not very responsive. Maybe the Classic Center will take over the spare parts supply for these--let's hope!

George Desiderio

[ja17]

Hello George,

I have lots of those nuts lying around here.  Sometimes the drive shaft nuts can be stubborn. The special drive shaft wrenches help a lot. Also consider hanging a big pipe wrench on the nut and turn one rear wheel. (The other wheel must be on the ground). Most of the time the nut will loosen. Be sure to turn it in the right direction.  Take a blunt drift and bang on the nut a few times first to loosen up the rust.

I never disconnect the front pipes. That is a real job. I unhook all the muffler hangars, let the exhaust move downward, then I remove the heat shield when needed. I have several custom  bent wrenches I use to unfasten the 17mm head bolts over the exhaust. Don't forget the spacer shim between the rear mount and the cover plate before closing things up.

[Tomnistuff]

...  Just make sure that the driveshaft centering flange  bushing is ok. ...

Joe, how close is the fit supposed to be between the driveshaft centering flange bushing (in my 230SL case, the centering ball) and the transmission output shaft?

Mine is 0.624 inch shaft OD and 0.634 inch centering ball ID for a 0.010 inch diametral clearance.  It seems sloppy to me, but if it were tighter, would it not cause a problem with the flange bolt hole alignment with those tight fit bolts?  Anyway, I cannot see how the centering ball can wear since I cannot see a source of relative movement.

Am I missing something?

Tom Kizer

[mdsalemi]

George, you might consider replacing all the nuts and or bolts in the project. You can generally take them to a fastener supply house, and figure out what they are (if you don't have the ability to determine this) and get all new ones. I want to assume they are high strength steel, maybe Grade 8, and thus probably have a bit of a zinc coating/plating on them. This of course doesn't last. I do this most of the time I take things apart so it makes things easier next time. If you don't have a local place, there are plenty of online fastener houses.

[ja17]

Hello Tom,

Wear is not usually an issue with the centering bushing. Damaged bolt holes from loose bolts or improper assembly can bend or damage the bushing. Your clearance should be fine Tom. After all the drives haft bolts have been removed,  separate the drive shaft from the three flange centering bushing first. In this way, you will only have to move the drive shaft back  about a quarter inch. The drive shaft can then be moved out of the way so that the flex disc can  be removed along with the three flange centering bushing.  

When you order the flex-disc kit it comes with all the new bolts, nuts  and rubber seals. It is especially nice to get the new locking nuts.

[Tomnistuff]

Thanks Joe.   I have already ordered a new (MB hex shaped) flex-disc kit and was not looking forward to having to order a new centering ball bushing, thus extending the wait for parts. (somehow I have to figure out how to order $10 parts tiny enough to fit in a matchbox without paying $30 for shipping).

George, I have been trying to figure out exactly how to align the driveline to minimize the “flexing” of the flex-disc.  The SLS site provides an excellent sketch but does not say HOW to do it.  My plan is to install the flex disc to the transmission side then move the propeller shaft side up close to it and try to measure the thickness of the gap between the flanges all around.  If the gap is equal, then the alignment should be correct – I think!

Does anyone know of an established "technique" of aligning the engine and transmission to the propeller shaft?  I can't find it in the BBB.

General Comment regarding the driveline:  Has anyone noticed in the EPC, in the SLS sketches or in the BBB, Chiltons, and Haynes manuals just how many iterations of propeller shaft and joint designs there have been over those years of the Pagoda?  I suspect that MB had trouble getting it right during those years.  That could explain the often short life of the flex discs.  My experience working in the engineering of automobile drivetrains was that once you got a design right, you were really reluctant to change it.  Likewise, if there arose a problem, you changed it quickly, often without verification because there is not much risk of changing something that doesn't work anyway.  If, once verified, the solution didn't work out, you changed it again.  Sometimes a bad design changed many times before it got fixed for good.

Perhaps in the interest of making our cars reliable, we should be a little less purist in replacing failed parts with identical parts, especially those parts that are difficult to see or on cars that are not subject to being judged.


Tom Kizer


 

[George Des]

Tom,

I'm with you on that SLS diagram. I understand what it is saying completely but it falls short in explaining how you get there and verifying it. Not like you can get some kind of a straight-edge in there. As far as the drive shaft goes, I have one of the older slip joint versions that I plan to use should I need to install a spare 4 speed I have here. The design of this without he locking joint seems to make so much more sense since the drveshaft can self adjust its length as required while the car is being driven.

[Tomnistuff]

George,

Something like you describe (a slip joint like the right axle shaft) would be better, I think, but leaving off the slip-joint nut won`t work because the female part of the joint is split and would open up under torque.  I just reread that and it sounds obscene so keep your mind on the car when reading it.  Maybe one could figure a way to lightly tighten the slip-joint nut so the joint can still slide, then locking the nut in place somehow so it will keep the spline from flaring open but not grip the spline too tight.   I don`t expect that there is a solution except to keep changing flex-discs as they fail.

In the same era, Ferrari used a rigid torque tube arrangement on their V12 drivelines (no open drive shaft), but they had a fully independent rear suspension, not a swing axle.  That also solved the drive line alignment problem.

Tom K.

[awolff280sl]

Tom, I'm not sure this helps, but I remember going through the same issue of alignment when I replaced my disk.
All I could figure out was, like you said, try to minimize the stress on the disk. So what I did was painstakingly roll the car back and forth each time that I slowly tightened up everything a bit at a time.
Maybe this helps.

[George Des]

The big question I have is why MB changed over from the slip joint driveshaft to the one with the locknut. This happened somewhere in te 230SL production.

[Tomnistuff]

awolff280sl, thanks for that tip.  I`ll plan to do it the same way.

George, I didn`t know that there was once a slip joint without a lock nut.  It`s time for me to do some library-type research.

Tom Kizer

[George Des]

Tom,

The early versions of the 230SL had this. Not sure when the cutover was made. Like I said, I have one in the garage that I will use if I need to go to the 4 speed transmission I have while I have the ZF out to do some repairs on it. The driveshaft used with the ZF 5 speed is a shortened version of the two piece locknut d/s.  Actually, it is just the front section of the D/S that is shorter, so I would not be able to just put the 4 speed in without switching out the d/s. The slip joint d/s can adjust itself lengthwise. A much better design in my opinion.

[ja17]

The 190SL and all the sedans of the era had slip joints without a lock nut.  As time went on, the cars developed more horsepower. In later years the design of the flex disc and the diameter changed to larger to handle more horsepower. On the other hand the 300hp, 6.3 liter MB sedans of the era had the same configuration as the W113. However the flex disc itself was much more robust. In fact it looks like the latest and greatest one now supplied for the W113 cars. I suspect it is.

You got to understand how much complexity and engineering the factory uses to limit VIBRATION.  They used two rubber flex discs and a slip joint with nut on the later cars!  Look at all the bushings and rubber/plastic parts that are just in the shifter of a W113 alone! All this complexity and expense just to limit vibration!  Many manufacturers would have metal bolted to metal all the way back to your shaking grip! Even the air filter canister on these cars have rubber mountings. And then there is the expensive rubber mounting gaskets under each dash instrument. Felt strips under the dash wood. I can go on and on.

[George Des]

I'll post a picture later today of my latest shredded flex joint. I heard the vibration start about 15 miles ago and wasn't completely sure that was what was going on. The last 2-3 miles it started to get worse and I figured at that point that I was playing with fire so I jacked up the car, took one look in that locations and immediately saw what it was. I'm surprised the car was still driveable since the disk was literally being held together by shreds! All in all, this would not be a terribly difficult job to do except having to deal with the headpipe. Joe, I know you said you have been able to get at the mount plate by lowering the exhaust further back, but I was afraid to put a lot of undue stress on those cast iron exhaust manifolds. I had even enlarged the access holes in the heat shield the last time I did this job to get at the bolt heads a little easier, but no such luck, so the headpipe had to be disconnected. That and the big d/s nut were the worst part of the job so far. Joe, I agree, MB goes to great lengths to limit vibration. I just wish they had gone to the same great lengths to simplify some of these jobs and limit the use of special tools! I worked for 35 years in the Defense weapons systems design and acquisition business and one of the big things we pushed with our suppliers was simplifying design for easy maintenance and limiting the number of special tools and test equipment. The T-700 turbine engine for example for one of the new generation helicopters was designed to be fully maintainable with 7 common hand tools and no special gauges or jigs!

George

[ja17]

Hello George,

If you let the exhaust hang down and just support it at the muffler at the last inch or two of its travel, you will have no problem with damaging the exhaust manifolds. Both of them together can handle a lot of stress. I used to have access to a CNC laser machine. I made some nice drive shaft wrenches from 1/2" steel plate that work nicely. I know what you mean. These cars were never known for their design simplicity. I have a huge assortment of special factory tools and tools I designed and made over the years. Many are specific to just one procedure.

[garymand]

I'm coming in late and maybe missed it: Can the flex disc can be put in wrong?  I believe there is internal bracing between every other hole.  I thik the leading hole should be bolted to the tranny and the trailling (braced) hole bolted to the shaft.  The result is the disc is pulling the shaft and not pushing it?

[ja17]

The answer is yes and no Gary. The early now obsolete original style can be put in incorrectly. There is a tab on the rubber which must be aligned to the transmission flange. As you perceive some section are designed for tension and others are for compression. See the tech manual for photos and details. The newly designed flex disc is more robust and is completely symmetrical and does not have to be oriented. MB is supplying only the later discs these days. I would find a new design if you are replacing yours, and order the whole kit with all the hardware and seals. 

[garymand]

Learning something new every day.  Love these cars.