Poly-filled motor mounts - problems - PeachParts Mercedes-Benz Forum

BillGrissom · #1 07-26-2015, 06:03 PM

This may be of interest to those who have filled their motor mounts w/ poly-urethane to prevent sagging. Just one small data point. Your experiences may differ.

1st photo shows the cracked base-plate I found after removing the left poly-filled motor mount in my 1984 300D after ~9 months. The sheet-metal is only 0.060" thick, so perhaps cannot take the load of the motor as the polyurethane fill acts on it. The right mount appears OK. This isn't the best test case since this car had the K-frame damaged before I bought it (stolen, crashed into curb). Perhaps the cracks were already there and the poly mount just made them deflect. I haven't noticed issues in my 1985 which also has filled mounts.

To repair, I welded a 0.25" th plate on top (2nd photo, Home Depot framing support). It spans between the supports underneath so should relieve bending forces on the factory sheet-metal. I also welded thick washers around the mounting holes, which add more structure (photos 3 & 4). It raises the engine up ~1/8", which helps. I used a wire-flux welder ($90 HF). Even with many parts removed it was difficult getting the welder tip oriented and see what I was doing. I re-installed the poly-filled mounts and hope no more cracks.

BillGrissom · #2 07-26-2015, 06:05 PM

1st photo shows the K-frame repair which had separated at the top. This was done by a shop. They might have tried working around parts, since the welds are sloppy. The right side was similar, and more of the K-frame had separated and rotated, moving the LCA pivot and causing severe toe-out. I had to retrieve the car from San Diego when my son called about it (post ~Aug 2014). He swears he didn't hit the speed bumps around the university hard, but I have other reasons to wonder. After pushing the top flush and welding, I secured it with a welded L-bracket, as I did for the R side. I first painted all hidden surfaces w/ high-zinc primer since it works as "weld-thru". 2nd photo and photo in 1st post shows the new repair.

Diesel911 · #3 07-26-2015, 08:57 PM

Wow and I got raked over the Coals for welding up the Rear Crossmember under the Trunk and adding some Metal plates to it.

I blew up part of the pic. It is not badly welded it was barely welded.

Maybe the polyurethane has to be softer aprximating the origial Shore rating.

mach4 · #4 07-26-2015, 09:38 PM

That's very interesting...

Unrelated to filling the mounts with urethane on my project, I have a 1/2 inch steel plate between the subframe and the mount. This is soley for the purpose of additional clearance so obviously I've got no issues. I just looked at the mount point on the 123 donor car and was surprised to see how flimsy that is. I can't imagine how urethane filling could do that, but I've seen some pretty strange stuff over the years. It would probably make sense to add 1/8 to 3/16 reinforcement under the motor mount as a matter of course to spread the load.

w123fanman · #5 07-27-2015, 01:23 AM

Is there a particular reason for refilling the mounts with polyurethane?

The German-made Phoenix mounts for the turbo 617 are only $18.75 here on Pelican.

Rogviler · #6 07-27-2015, 02:14 AM

Quote:

Originally Posted by w123fanman

Is there a particular reason for refilling the mounts with polyurethane?

The German-made Phoenix mounts for the turbo 617 are only $18.75 here on Pelican.

It's not about cheaping out and trying to reuse old mounts, it's simply an extra measure to prevent collapse of new mounts in the future and all the headaches that can bring.

I just put in new mounts filled with windshield urethane, so we'll see how they hold up. At first the vibration was crazy, but once I took out all the stock air filter stuff (which was heavy and no longer attached anyway), most of it went away so I'm a lot happier with it.

-Rog

Maxbumpo · #7 07-27-2015, 10:26 AM

Quote:

Originally Posted by Rogviler

It's not about cheaping out and trying to reuse old mounts, it's simply an extra measure to prevent collapse of new mounts in the future and all the headaches that can bring.

Count me among the skeptical. If MB star-marked-from-the-dealer last for 10 or 15 years, why not simply use them? Last I checked, after-market rubber parts for our cars have very poor reputations (race to the bottom price, quality = made in China to minimum spec).

mach4 · #8 07-27-2015, 10:41 AM

The polyurethane filled motor mount concept is one that I pioneered a couple years ago when I was engineering the 617 engine into a 107 chassis project. Because I didn't want to cut into the subframe, I determined that I could raise the engine 1/2 inch and drop the subframe 1/2 inch. Even though the engine-subframe clearance was adequate at that point it wouldn't be with even a partially collapsed engine mount. After reading all the horror stories about mounts collapsing in a few months, cut oil cooler lines, vibration, noise etc, I came up with the filled mount as a solution to my problem. I put it out on the forum and others have adapted it as a way to extend the life of their mounts indefinitely even when using mounts of unknown quality.

Maxbumpo · #9 07-27-2015, 10:55 AM

Thanks mach4, now I understand the genesis of this modification.

BillGrissom · #10 07-27-2015, 03:18 PM

Quote:

Originally Posted by Diesel911

...
I blew up part of the pic. It is not badly welded it was barely welded.

Maybe the polyurethane has to be softer aprximating the origial Shore rating.

Your zoom is of the L-bracket I added over the top of the factory K-frame, to help insure it will never pop loose again. It isn't the prettiest weld line, but best I could do without much visibility, and is definitely bonded. Before that, I pushed the top back and re-welded it, which most would figure "good enough", but I went for a belt & suspenders fix. My 1985 has never been touched, and it appears the factory just used a few spot welds to attach the K-frame, so I would be concerned if I lived in the rust zone.

The polyurethane is fairly flexible. I can push it in with my thumb. I used black PL1 gutter caulk from Home Depot. But, it is surrounded by metal so still applies force like a hydraulic cylinder. The cracks might have been existing and just exposed by this, but 60 mil sheet metal can't take much load.

Zacharias · #11 07-27-2015, 08:47 PM

Quote:

Originally Posted by Diesel911

Maybe the polyurethane has to be softer aprximating the origial Shore rating.

I am wondering about this too.

What Shore rating was the poly that you used?

Diesel911 · #12 07-27-2015, 09:54 PM

Quote:

Originally Posted by Zacharias

I am wondering about this too.

What Shore rating was the poly that you used?

You would likely have to go to the site of the company that made it and they often have a spec sheets on the items they make.

If you bought a product like Flexanne (spelling) you can purchase it in different shore ratings (hardness cured).

BillGrissom · #13 07-28-2015, 12:47 AM

I would say the poly is softer than the factory mount rubber, though there is more of it. I can push it in w/ my thumb, which isn't true of the rubber. As I said, it is polyurethane caulk, not the cans of urethane others have used. I inserted a section of 3/4" ID rubber "heater hose" before filling, to leave a hole for the big allen bolt. I am confident it won't crack the 1/4" th plate I welded in, though other parts of the K-frame may eventually crack. They used much thinner gage metal than in my 60's cars, as was true of most 1980's cars as they strived to lighten them for better mileage.

Zacharias · #14 07-28-2015, 10:28 AM

Okay well as I recall, Mach originally spec'd Shore 30 but it sounds like you may be under that.

I was just wondering if perhaps you had used something super hard.

BillGrissom · #15 04-02-2018, 04:59 PM

New data point to add to this post. Several related posts out there are often discussed, one being: http://www.peachparts.com/shopforum/diesel-discussion/374388-w123-engine-mounts-drivers-side-squished-way-down-despite-being-brand-new.html
Before swapping my 1984 300D w/ the 1985 for my son, I decided to replace the polyurethane filled motor mounts with regular ones, to see if I could smooth the engine shakes at idle. It made an amazing difference. Before, it would shake aggressively when cold and still a bit at idle once warm, though smooth at higher rpm. After the change, I could barely feel the engine running in the cabin after a cold start, and no resonance points when driving in a parking lot as before.

The motor mounts looked a bit "pregnant". Removed, they sat ~0.5" lower than the new ones. The poly-fill was definitely taking much of the load since it had conformed to the K-frame plate it sat on. The new motor mounts are Febi "of Germany", but a white sticker added "Made in China". I was angry that they differed from the ebay photo, not having the large embossing in the rubber shown (like Phoenix - Turkey OE type - many posts here), after paying a premium ($48/pair). They did have small embossed characters, unlike the cheapest smooth rubber ones from URO or Anchor (India), which are notorious for sagging in a few years.

Don't know how long these will last, but I can confirm that motor mounts make a tremendous difference in how it feels in the cabin. BTW, my transmission mount is poly-filled, but unlikely it affects the main side-side rocking vibrations.

Re recent questions about "how easy to change motor mounts?", it is never easy for me. I do one side at a time so I don't get the engine mis-aligned. It is always hard getting the last upward bolt started (8mm allen). I leave the 2 downward bolts (6 mm allen) loose. I also had to shift the engine slightly w/ a crowbar and loosen the tranny mount to get the bolt started. Be real careful to not get a bolt started cross-threaded.