The time has come for a drivetrain refreshment. New flex disks, driveshaft center support and bearing, along with mounts (engine, trans, diff) which I'm planning to fill with 30a shore urethane rubber before installing.

Thoughts on keeping the mount durometer consistent throughout the drivetrain by urethane filling the driveshaft center support? I feel like the weakest mount will get destroyed very quickly.

Honestly, this is my first rwd and I'm not familiar with the function of the center support. Does it need to have play? Does the driveshaft move over time?

Should the support need play, maybe a pattern of urethane would be better than solid?. Trying to think outside the box for longevity's sake. Is it worth a shot? Or do I man up and buy MB oem? or both
Lou

PS feel free to add to my refreshment list while I'm in there

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83 300D Silberblau

The driveshaft center support is generally pretty trouble-free. Even if you tried to "fill" it, I suspect that the urethane would just peel off in short order. Mine was torn when I got the car and, not knowing any better at the time, replaced it with what I believe to be a Uro one. I turned 100k miles on the car since I got it and so far so good. I know someone who actually repaired a completely torn one with some two-part jet plane tank sealer. That stuff makes urethane look like cellophane tape by comparison.

I wouldn't sweat the center support. There's almost no stress on the part...unless the driveshaft is out of balance. Make sure you mark the two driveshaft halves before you pull it out. There's a rubber boot that tends to wear out covering the splines - I'd replace that and maybe the centering bushings too.

The only other thing in the drivetrain worth considering is the rear axles if the boots are cracked or leaking.

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Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

I may fill my motor mounts the next time. They compress sometimes too much putting the upper oil cooler line in jeopardy. Do use urethane to refurbish the engine shock mounts. The rubber will be crumbling by now.

I screwed the base to a sheet of plastic from a bag mounted on a board. Then filled the mount keeping the hole open with a properly sized drill bit.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

good looks on the centering bushings, is that something I can inspect while on the car?

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83 300D Silberblau

First, the tranny mount is not rocket science, so don't over-think it. It is only resilient for passenger comfort. Racers sometimes substitute rigid metal engine mounts, but that would shake your teeth in daily driving.

If you have a can of urethane, go for it. It is not easy to buy. I looked over caulks at Home Depot and chose PL1 polyurethane black "gutter seal". I used poly caulk previously on my trailer and found it very rugged and sticks well, plus 50 yr warranty. Google Images will show many used this or similar to fill their motor mounts long before I tried.

The tranny mount should be OK, but if you fill the engine mounts, you will likely experience what I did. I removed one for other work and found the base sheet-metal totally cracked. I posted w/ photos. It is too thin to support the weight of the engine as the poly filling acts on it. You need a ~3/16" steel plate under it to span that area. If you can wait, I am working on a better solution. If you do replace the rubber hose in your oil cooler lines, cut the top one 1" longer, as I did. Had M-B done so, the horrors of "p.s. belt cuts oil hose" would have been avoided (happened to me also). The filled mounts do make the engine feel slightly buzzier in the cabin. My plan should fix that too.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Don't bother filling your center support with urethane - it's job is only to keep the shaft straight, it's a relatively low stress area in regards to rubber. Solidifying it won't do you any good, chances are it'll just fall out considering there really isn't anything there to hold it in place.

I will also advise against filling your motor mounts with any sort of urethane - you don't have a race car and while you will have the vibration problem in the cabin, the vibrations aren't necessarily good for the motor. You WILL have nuts and bolts working themselves loose over time - OM60X's have the problem with collapsed mounts. Besides, how many miles do you think the current mounts have lasted without much problem? Run the stock ones, you'll be happy.

As for the trans mount, do it. I fill mine with urethane on every car. My track car benefited from the firmer mount more than anything. Helps keep the drive line aligned when under load, which will keep those flex disks from imploding on you in any sort of high stress situation (I realize you have a 617, hardly high stress, but still).

FWIW, using stock mounts and just filling the trans mount - my driveline was invincible. Welded 3.27, coilovers, fully adjustable rear, etc etc etc. Was a fun car.

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Allen Kroliczek
Oak Grove Autosport | Oak Grove Autosport
01 G500, 82 300TD, quite a few more.....

Based on BillGrissom's experience, it's probably prudent to put a 1/8-1/4 inch piece of metal under the mount to prevent his problem. I happen to have a 1/2 inch spacer between my subframe and motor mount, not for this problem but for clearance issues, so that's why I haven't had any problems or issues.

Speculation about hypothetical bolts falling out and other catastrophes, while interesting as forum fodder, is completely unfounded in practice. As I just mentioned in an update to my build thread, I just passed 100k miles on the swap this week. All bolts are intact, no non-normal vibration or other issues have been experienced - though it's critical to use a soft Shore 30 urethane to fill the mounts, otherwise you could experience these unwanted side effects. Oh, and no visible collapsing of the mounts - try that with a "normal" mount.

Filling the transmission mount is something I would consider optional. There's almost no weight on the mount and collapse is not critical. In the same 100k miles referenced above my unfilled transmission mount is no worse for the wear and tear. Like the engine mounts, if a hard urethane is used it WILL produce noticeable and annoying vibration.

Hope this helps.

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Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

Forgive me for taking the thread on tangent OP, I'm bored on lunch and love getting the folks on this list explaining their process, will hopefully help you and others out as some folks are very vague (not to say those posting here are).

I'll start documenting the 2-4 cars a year I get through the shop that have had bolts loosening for no reason other than vibrations caused by collapsed mounts - that's not to mention any of the track cars that come in with similar problems from urethane mounts. Happens to the G wagens and to the Pagoda's I play with in addition to many W126's (for some reason) - hardly hypothetical forum fodder. Happens to heavy equipment, etc all the time. While you may not have experienced it in 100k miles of leisurely driving, that's hardly a testament to what can happen.

EDIT: Just read sig and saw the R107 w/ diesel. Good for you! Disregard any clearance questioning. 100k miles also doesn't mean much - over the course of how many YEARS has that urethane been in there? How are you gauging a collapsed mount, when you have an additional 1/2" plate for "clearance"? Would you not have any clearance problems with your urethane filled mount? I don't follow build threads here, I'd love to know. Over the course of my W124's track life, I've put it through more hell in a weekend then daily driving will do in 200k miles. It's only pitfall was structural cracking in the unibody from fatigue - resulting in me scrapping it to build a different chassis. That was on STOCK motor mounts/urethane trans mount. Seems like you guys are counteracting the front sub frame fatigue by adding plates.

Have you ever experienced a collapsed trans mount? I'm not so sure you have as they absolutely wreak havoc on drive line alignment. They shift like crazy with any torque.

To clarify, I'm not against urethane filling motor mounts - just unnecessary.

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Allen Kroliczek
Oak Grove Autosport | Oak Grove Autosport
01 G500, 82 300TD, quite a few more.....

As a small data point on structure, my 1984 300D had been stolen and crashed into a high curb before I bought it. I planned to just use its engine (for my 85), but ended up fixing the car. Anyway, the crash showed some structural weaknesses. The K-frame had separated at the upper attachment welds (near shock tower) on both sides, which a shop had repaired. I expect they also replaced the LCA guide rods. When I bought it, the L front wheel was leaning in, which the PO thought due to failed weld repairs, but was actually due to a crack thru the LCA pivot bracket hole on the K-frame (shop hadn't touched). I fixed that by bending back and welding a thicker support strap across that tear.

Several years later the shop's welds did separate from the front frame at the top. The right side failed while my son was turning in a parking lot (highest stresses in normal street driving). Just the weight of the tire and suspension (jacked up frame) was enough to rotate the top K-frame back into position, so that top attachment is critical and it appears the factory used just a few spot welds there (based on my unmolested 1985). I re-welded the top K-frame to the frame, all across the top, then welded an L-bracket across that as extra support. I removed the turbo and mount bracket for full access. I later did the same on the left side pre-emptively, removing the mount and oil cooler tubes for access. Of course, I changed the oil cooler hoses while I had them out (used AC hose & crimper).

In sum, the K-frame and its attachments appear fairly weak. I wouldn't be surprised if owners in the rusty north have the few spot-welded attachments rust away or the thin metal where the LCA's attach rust thru. I am comparing this to 60's Mopars I have where even the light A-bodies have much thicker sheet metal in the K-frame. Somebody apparently crashed both, in one case ripping the strut rod (similar to guide rod, but goes fwd) right thru the K-frame, but not distorting it. The filled motor mount breaking thru the thin M-B K-frame sheet metal probably wouldn't happen in my old Mopar K-frames since they appear to be ~1/8" thick sheet-metal. Not that M-B in general is inferior. 1980's Mopars also went to much thinner structures to save weight. Today's cars use even thinner "high strength steel", which can crumple easier in accidents. That is intended and better for safety, but makes the cars less able to be repaired.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans