Project BMW M3: E46 Bushings and Subframe Mounts

What’s one of the biggest weak points on the E46-chassis BMW? The rear subframe mounts.

Our 2004 BMW M3 came from a trusted source: an old friend. It’s been a Southern car, at least in recent years.

Upon picking up the M3 from its Tennessee home, we immediately headed to Dublin, Virginia, to visit BimmerWorld’s HQ.

James Clay, BimmerWorld’s owner and our personal guide on this journey, put the car up on the lift. Pry bar in one hand and a flashlight in the other, he started to inspect those often-criticized mounts.

“Rear subframe and floor failures are quite common on the E36 and E46 models. Unlike the E36 M3, the E46 models did not receive sheetmetal reinforcements from the factory, and subframe and floor failures continued to be a problem well into the E46 production. BMW even has an official fix for non-damaged cars. However, every E46 on the road potentially needs rear floor repair and reinforcement, and it's a absolute must-do for any track or race car project.”

What causes the problem? Again, the TL;DR from the BimmerWorld page: “The rear floor of the E46 is actually two sheets of steel with a gap between them. The rear subframe mount gussets (where the subframe is bolted to) are tack welded to the bottom layer. If the two sheetmetal layers were joined or braced together, this problem may never have developed in the first place. BMW tried to eliminate flexing of the floor by bracing the front mounts, but this only transferred the stress to another point. Since there is this gap between the layers, with tremendous torsional load at these points, the bottom layer twists and deforms and the spot welds eventually pop and fail.”

And if someone keeps driving after popping those welds, the situation only gets worse.

The seller included a trunk full of parts, including OE bushings, but before shoving off we grabbed some pieces. Since BimmerWorld also serves as Powerflex’s U.S. importer, we grabbed some replacement urethane bushings as well.

Once home in Florida, the car went up on the lift. (Yes, this installment is a bit of a flashback. Insert special effects here.)

Now, finally, we had a clear look at the subframe mounts. And they looked good. Really good.

So, what did we mean when we said we bought the wrong car? Part of this project’s purpose is to walk people through the M3 ownership experience. Good or bad, depending on your view, we got to skip that common rite of passage.

Fortunately for you, though, we know people—people who know about E46s. Before we discuss the actual bushing replacement process (it’s basically out with the old, in with the new), let’s do a Q&A with James Clay on this ever-popular topic.

GRM: What’s the real problem here?
James: The root of the issue is the bushings. The four subframe bushings in that car almost fully contain the rear suspension loads, and it's a real balance to have enough bushing material there to support that load and yet minimize the noise transfer from the tires and differential—and of course, from a manufacturer standpoint, to use a part with a moderate cost for the hundreds of thousands of these chassis all over the world.

The stock bushing life of course depends on use, but also age of the rubber—which is at least 14 years now—and some level of happenstance. But these cars are largely moving to enthusiast hands now, especially if you're reading this, and saving a few dollars across a million cars isn't your concern. So, putting in a nicer bushing set now is a wise move, whether you have a problem or not.

GRM: How does it happen?
James: Unless it's a full-solid mount (these aren't), a bushing allows some small amount of movement to help with NVH—noise, vibration and harshness—but it's really small.

But when the bushings begin to deteriorate, the movement becomes larger and larger, and this movement has momentum: Would you rather be hit by a hammer someone is holding half an inch away from your finger or 10 inches?

The chassis feels the same way. When you hit the brakes or whack the throttle or load the car into a corner, there's a big difference in the movement a factory bushing allows to minimize NVH with an OE-type material and the movement of a worn bushing, and the results over time are very similar to the hammer hitting your finger, so it's no surprise the chassis starts to show the effects of this hammering.

GRM: How do you fully inspect the subframe mounts?
James: This is slightly tough because of their position in the car, but the best way is visual inspection with a very bright light while the car is on a lift, and then using a pry bar on the subframe to look at the level of movement.

We see these cars regularly, so it's pretty easy for us to visually put them on a scale. But if they are starting to get really bad, anyone can see that level of excessive movement.

But the best answer if you plan to drive the car is to just replace them, along with the trailing arm bushings. I feel strongly that replacing the bushings and eliminating that excessive movement of the subframe from failing parts can eliminate the need for the reinforcement so many people have had to do—and is much more expensive and invasive.

GRM: How come ours looked good?
James: A clean car that wasn't pounded on goes a long way. Also, having a car stored properly goes a long way.

GRM: What’s the reality about weld-in reinforcements?
James: We designed a weld-in reinforcement kit to handle the most common failure areas we have seen. It's a pretty extensive kit, but you have to do both top and bottom of the area and dig down to the metal that fails.

The chassis that I've seen that were the most wrecked had failed bushings that were ignored. Sometimes you really have to piece it all back together, and our reinforcement kit is more like the glue that stitches it all back in place.

Note: This a welding operation. Glue or even high-tech adhesives do not work here.

The typical install cost for a street car is a little over $1000 in addition to the parts, but do yourself a favor and replace the bushings and you might avoid this process!

GRM: What should you do if the mounts are torn?
James: Replace them! And I would prefer something nicer than OE, like a Powerflex, which is a premium material that minimizes NVH through material properties, not voids in the bushing like the factory parts.

And do a very heavy inspection of the subframe area of the chassis and make sure you don't have to fix that also. Replacing mounts preventatively is pretty easy with the proper tool, requiring that you only drop the subframe a few inches. But when the problem spreads, the job becomes a lot bigger.

GRM: If the mounts look good, do you still install the reinforcement plates?
James: Personally, I don't, given my strong feeling that the chassis is made bad by bad bushings primarily. Other people would say do it while you are in there, and that depends on approach to bushing check.

For yours, we did it on the lift with a big pry bar, which I am comfortable with. On a race car, where I'm upgrading the bushings already and have the subframe off, I do the reinforcements at the same time.

One last tip from James: “It's worth noting that any ‘problems’—like this or rod bearings—don't affect 100% or even 25% of the cars out there—probably not even 10% or anywhere close given that there are close to 1,000,000 of them worldwide.

“It's more the ones that have been pounded on or just have had really bad luck. So it's not a part of the E46 experience, just a possible pitfall.”

Enough talk. Time to get the car up on the lift. Thanks, Jesse Spiker, for the help here so we could take photos.

While in Virginia, though, we picked up some Powerflex hybrid aluminum/urethane front lower control arm bushings. Why the hybrid designation? These bushings feature a urethane center inside an aluminum housing. The urethane can rotate inside the aluminum, and reducing the amount of compliant material further reduces deflection. So in theory we’re talking about sharper handling without a big NVH penalty. Call the stock pieces just an interim step.

And look, the Powerflex front control arm bushings bushings are ready—even zip-tied together so we don’t lose one. (True story: We lost both for a while.)

Our car also came with a new differential cover, as this is the only way to replace the differential bushings with the stock rubber pieces. Since our old bushings (above) looked crusty and we had the new diff cover, we installed it.

He even included the BimmerWorld trailing arm bushing installation tool. How could we not try it out?

What about those all-important rear subframe bushings and attachment points? All looked really good.

To replace the rear subframe bushings, we first needed to remove the originals—meaning the subframe had to come all the way out.

James adds a caveat: “I do personally think the stock mounts allow too much movement, and if your stock mounts are good, these are a good idea. If your stock mounts are questionable and you're lazy, this is a Band-Aid over a problem—the easy way out.”

Once the subframe is lowered a tad, the reinforcement inserts simply slip into place.

Considering we mostly replaced stock with stock, the ride feels no different—less slop up front, though, since those lower control arm bushings were trashed. The biggie here: We nipped any potential problems in the bud and now have a solid baseline.

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