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Rear Flex Coupling Disc Replacement
 
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Pelican Technical Article:

Rear Flex Coupling Disc Replacement

Tom Morr

Time:

2 hours2 hrs

Tab:

$75 to $120

Talent:

**

Tools:

Floor jack, torque wrench, 19mm wrenches, pry bar(s)

Applicable Models:

Mercedes-Benz SLK230 (1998-04)

Parts Required:

Replacement flex disc coupling

Hot Tip:

An air ratchet and 19mm ratchet wrench might save time

Performance Gain:

Corrects driveline clunks and vibration.

Complementary Modification:

Replace front flex coupling disc, inspect the exhaust and driveshaft, repair or replace if necessary

The flex disc coupling at each end of the propeller/driveshaft absorbs shock loads when torque is transferred from the engine to the driveline. These rubber discs also mitigate driveline vibration. The flex disc should be inspected every 80,000 km / 49,000 miles.

Centrifugal force and contaminants eventually take their tolls on flex discs, with rubber cracks and degrading of the rubber. In SLKs with automatic transmissions, an early tip-off to flex-disc failure is noticeably harder shifting between gears. Knocking or vibrations that change sound and intensity with speed are other indications of flex-disc problems. A vibration in the center console is another common symptom.

The rear flex disc is easy to inspect. With the car raised and secured with jackstands, the rear flex disc is visible between the driveshaft and differential. With the transmission in Neutral and the parking brake released, the driveshaft can be rotated to inspect the rubber.

Degradation typically begins with cracks forming around the bolt holes. The holes can also become egg-shaped as their steel sleeves begin to float in the rubber.

The flex disc's intermediate webs will also crack over time, usually spelling the end of the disc's service life. In extreme situations, the rubber's fabric reinforcing cords become visible, making the disc look like it has whiskers. The worst-case scenario: the flex disc completely shears and the driveshaft comes loose, potentially causing a lot of collateral damage.

See our articles on driveshaft replacement, if you decide to perform the comp modification listed for this article.

Inspect the flex disc every 80,000 km / 49,000 miles. The driveshaft is balanced to minimize vibration. A good practice whenever working with driveshafts: make references marks so that the components can be reinstalled in the same position. Some pro mechanics put masking tape on the length of the driveshaft. This allows the driveshaft to be properly re-aligned should the two halves separate at the slip joint.

The disc's steel sleeves can corrode inside the yoke/flange holes. Spraying penetrating lubricant such as PB Blaster on the bolt holes on both sides of the disc can help free the disc.

We had success by removing the three bolts that secure the driveshaft to the flex disc, prying the driveshaft off the flex disc, retracting the driveshaft off the centering stud, then unbolt and pry the disc off the differential's pinion yoke.

If you have an air ratchet and 19mm ratchet wrench you will be able to save some time. The standard shop labor time for removing and replacing the rear flex disc is listed as 1.3 hours. New flex discs typically include new bolts and nuts.

You should replace the flex disc hardware. The replacement flex disc might come with bolts that have internal 10mm or 12mm hex bolts.

The flat washers go on the flex disc side, regardless of whether they're against a nut or a bolt head.

The bolt torque specs are as follows:

10mm - 40 Nm

12mm - 60 Nm

With the vehicle raised and secured on jackstands, the rear flex disc can be easily inspected.
Figure 1

With the vehicle raised and secured on jackstands, the rear flex disc can be easily inspected. Putting the car in Park with the parking brake applied allows the bolts to be loosened and removed with 19mm wrenches. Putting the car in Neutral and releasing the parking brake allows the driveshaft to be rotated for better bolt access. Then, put the transmission in Park and set the parking brake again to remove the fasteners.

Marking the driveshaft's phasing is a good habit, even when it isn't being removed.
Figure 2

Marking the driveshaft's phasing is a good habit, even when it isn't being removed. Wives, daughters and/or other family members often have excess nail polish that works well for marking purposes. Consider marking the shaft on both sides of the intermediate bearing or even running masking tape lengthwise on the driveshaft in case the rear half's splines slide out.

The flex disc has protruding steel sleeves in its bolt holes (green arrow).
Figure 3

The flex disc has protruding steel sleeves in its bolt holes (green arrow). These press-fit into the driveshaft and axle yoke when the bolts are tightened. Spraying penetrating lubricant on both sides of the flex disc around the bolt holes can help the sleeves come out easier.

Flex discs often don't come out without a fight.
Figure 4

Flex discs often don't come out without a fight. After some trial and error, we bolted the disc to the differential. Then we used a pry bar wedged between the bolts and the driveshaft and a large screwdriver at the driveshaft holes to separate the flex disc's bolt sleeves from the driveshaft's flange.

After the driveshaft is de-coupled from the flex disc, it needs to be collapsed away from the pinion yoke's centering pin (green arrow).
Figure 5

After the driveshaft is de-coupled from the flex disc, it needs to be collapsed away from the pinion yoke's centering pin (green arrow).

An optional step is loosening the two 13mm-headed bolts that secure the driveshaft's center bearing to allow more clearance for slipping the shaft off the differential's center pin.
Figure 6

An optional step is loosening the two 13mm-headed bolts that secure the driveshaft's center bearing to allow more clearance for slipping the shaft off the differential's center pin.

After removing the three remaining 19mm bolts, the flex disc can be separated from the differential.
Figure 7

After removing the three remaining 19mm bolts, the flex disc can be separated from the differential.

Finally, the flex disc is free.
Figure 8

Finally, the flex disc is free.

This flex disc is beginning to crack around the bolt sleeves, but it's still functional.
Figure 9

This flex disc is beginning to crack around the bolt sleeves, but it's still functional. The rear disc is easy to assess visually. It doesn't necessarily wear at the same rate as the front, but problems here warrant inspection there. Ideally, the flex discs are replaced in pairs.

Replacing the rear flex disc requires more patience than skill.
Figure 10

Replacing the rear flex disc requires more patience than skill. The bulk of the work is performed with 19mm wrenches; pry bars and penetrating lubricant can help. We used a 13mm socket to loosen the driveshaft's center bearing to give a little more play. Installation of the replacement flex disc is the reverse of removal. Grease should be applied to the pinion yoke's centering pin. The replacement flex disc might come with bolts that have internal 10mm or 12mm hex bolts. The flat washers go on the flex disc side, regardless of whether they're against a nut or a bolt head. The bolt torque spec is: 10mm - 40 Nm; 12mm - 60 Nm.

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Comments and Suggestions:
Gavin Comments: Nice and clear write-up. Thank you!

Question: What is the torque spec for the two 13mm-headed bolts that secure the driveshaft's center bearing?
February 18, 2016
  Followup from the Pelican Staff: I don’t have that info.

I would grab a repair manual. It will have the procedure, special tools and torque specs.

Give our parts specialists a call at 1-888-280-7799 - Nick at Pelican Parts
 
Joe Comments: I have just bought a Febi replacement kit which contains both T60 torx bolts and 10mm internal hex headed bolts. May be an idea for all readers to have or purchase both before starting.
July 23, 2015
  Followup from the Pelican Staff: Thanks for the additional info and feedback. We appreciate it.
- Nick at Pelican Parts
 
BivDog Comments: SLK230 - Rear Flex Disc - Bolts are T60 instead of 10/12mm hex bolts.
July 17, 2015
  Followup from the Pelican Staff: You didn't mention a year, so I went with 2002. Torque is by fastener size:


M10 40 NM
M12 60 NM - Nick at Pelican Parts
 
BivDog Comments: Great write up, was able to complete the project without any problems but I want to note a two items and I have one question. 1. The new Rear Flex Disc from MB comes with T60 bolts instead of the mentioned mentioned hex bolts so make sure you have a T60 socket before getting started. 2. If I had wrote this article I would put the center pin greasing as step 10 and create a recap as 11...yeah I put the disc back on before seeing that I needed to grease.

Question: What torque spec do I need for the T60 bolts? 60 Nm, 40 Nm or something else?
July 12, 2015
  Followup from the Pelican Staff: I can see if I have the specs for your vehicle. What are you working on?

Thanks for the feedback. - Nick at Pelican Parts
 

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