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E46 M3 (2001-2006) Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006.


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Old Fri, Jun-02-2017, 02:01:50 PM   #1
GotThrottle
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Smile Crack repair methods

Hi All,

After 6 years of ownership doing HPDE and AutoX, I finally found my first crack on the rear subframe of my E46 M3. It's about 1" long and found at the typical left-rear mount location. This appears to be the beginning of bad things to come unless I take action now.

Note: I've been searching and reading pages until my eyes bleed, and have realized there are a ton of options members have chosen to strengthen their e46 M3 rear subframe.

My question is: which method(s) are the best to pursue on strengthening the rear RACP to prevent cracks from propagating? It's really hard to come to a clear conclusion on this issue.

I've gathered the following methods as found on this forum:
  1. Structural foam on the rear mounts, per TIS
  2. Structural foam on the front mounts
  3. Weld in plates (like Turner)
  4. Epoxy in plates (no welding)
  5. Mason 3pc subframe support bar
  6. VinceBar, cut apart and weld-in

I'm leaning towards choosing foam on the front and rear mounts, and purchasing a Mason bar. I've already purchased $300 worth of foam, but have not installed it yet. I'm heading to a track day this weekend, which is a bit scarier now.

Any feedback is appreciated.
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Last edited by GotThrottle; Fri, Jun-02-2017 at 07:10:29 PM.
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Old Fri, Jun-02-2017, 03:04:04 PM   #2
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Default Re: Crack repair methods

I think the foam is ok in principal but once it's in you can't weld ever without a big smokey mess and a fire. Also if the foam was bonded to the metal it would be great but on a 10 year old car and with the seam sealer and dirt etc inside the cavity you can't guarantee a bond of the epoxy foam to the metal surface.

I have Turner plates but if I ever get a crack outside of the plates I will go with something like a welded in Vince bar that ties the frame in better like it should have from the factory
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Old Fri, Jun-02-2017, 04:19:04 PM   #3
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Default Re: Crack repair methods

Your going to need to weld the cracks anyway, so while you have the welder out, do a proper job. Your going to need to weld more than just that crack because there will 1000% more areas that need attention. So don't be surprised. And if you can't find them then your not looking properly. Sell the foam and buy some proper plates ( weld or epoxy using dp420 epoxy) and again while the welder is out , attend to the front mounts upper anchors and the rear mount upper anchors and install some sort of load relieving apparatus of which the choice is Vincebrace, cmp brace or mason or custom such a a tube welded to each side of the chassis legs and pick up the RACP in process.

Weld the rear left arch and the spot welds that run along the chassis leg - inner racp rectangle ( the rectangle with the sound deadening). These spot welds penetrate and can be seen under the car and run along the inner portion of the Spring perch. These fail! And often under seam sealer.

Ps. It's the rear left that is typical not rear right
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Old Fri, Jun-02-2017, 07:25:13 PM   #4
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Default Re: Crack repair methods

You're correct, left-rear is the typical starting crack location and where I've found mine. I've made the correction above.

So you're conclusion is: plates (either welded, or epoxy) + weld the rear left arch + attend to the front mount upper anchors + stress relieving bar for the rear mounts (ie: Mason bar). That's a lot!

Do indy shops even know about all of these areas, or do they only weld in the plates and call it good?

I too am skeptical about wasting time with foam. I will consider selling it then.

I have a track day this weekend that I cannot get a refund for, so I'm going. I was hoping to at least use the foam to prevent the crack from propagating, but the feedback is that it has little value to a car this old... sigh. I will just have to fix it after my track day is over.

I don't have a welder, so will pay a shop to do it. Any indy shop recommendations for the Madison/Milwaukee/Chicago area?


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Originally Posted by schoonerm3 View Post
Your going to need to weld the cracks anyway, so while you have the welder out, do a proper job. Your going to need to weld more than just that crack because there will 1000% more areas that need attention. So don't be surprised. And if you can't find them then your not looking properly. Sell the foam and buy some proper plates ( weld or epoxy using dp420 epoxy) and again while the welder is out , attend to the front mounts upper anchors and the rear mount upper anchors and install some sort of load relieving apparatus of which the choice is Vincebrace, cmp brace or mason or custom such a a tube welded to each side of the chassis legs and pick up the RACP in process.

Weld the rear left arch and the spot welds that run along the chassis leg - inner racp rectangle ( the rectangle with the sound deadening). These spot welds penetrate and can be seen under the car and run along the inner portion of the Spring perch. These fail! And often under seam sealer.

Ps. It's the rear left that is typical not rear right
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Old Fri, Jun-02-2017, 09:43:46 PM   #5
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Default Re: Crack repair methods

Just go on your track day and enjoy, your not going to Rio the floor out. Your going to fix it it anyway.

Alot of garages weld plates and call it a day. It's not enough
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Old Fri, Jun-02-2017, 09:58:52 PM   #6
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Default Re: Crack repair methods

You're absolutely right. Just drive it this weekend and fix it after.

So I just received a quote from a motosport fabrication shop, their estimate is 10 hours of labor, plus the price of their reinforcement kit ($139), and brake fluid to re-bleed the brakes. They claim to perform these repairs often, so I will discuss the next-steps with them, and if they ever delve into the front mounts, or left rear arches.

Thanks for the replies.

Anybody want to buy 4 cartridges of structural foam? lol


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Originally Posted by schoonerm3 View Post
Just go on your track day and enjoy, your not going to Rio the floor out. Your going to fix it it anyway.

Alot of garages weld plates and call it a day. It's not enough
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Old Sat, Jun-03-2017, 01:15:21 PM   #7
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Default Re: Crack repair methods

I just epoxied in the VinceBar, Front Gussets and Vince Skins. I really like the design and thought that went in to them. In the next month or so I will be putting together my thoughts for the complete install, but I would recommend all of Vince's stuff. He hasn't made it idiot proof, but he has done a great job at making everything fit. The biggest part of the job is deciding how much stuff you want to replace if you drop the whole RACP. I did all ball joints, bushings, ebrake cables, short shift kit, rebuilding brakes, stainless steel brake lines and decided that was enough.

Jonathan
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Old Sat, Jun-03-2017, 04:19:20 PM   #8
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Default Re: Crack repair methods

Damn. Seems like every time I step away from the forum for a bit and come back to read up on what's current, somebody else is posting about their M3 developing cracks in the RACP. GL OP.
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Old Sun, Jun-04-2017, 01:53:20 AM   #9
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Default Re: Crack repair methods

If you can fit a drill in that tight space, I'd put a hole at the end of the crack to stop the spreading. Afterwards, attend your next event and get it fixed. I'd install redish motorsport floor plates because they cover a larger area than the turner plates and install the vince bar kit on the inside. Also, spot weld all the weak points on the subframe floor that many have pointed out. The last step would be use the epoxy foam to fill the empty cavity between the RACP.
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Old Sun, Jun-04-2017, 02:28:42 AM   #10
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Default Re: Crack repair methods

It seems that the consensus now is that foam is ineffective. I didn't inject foam a few years back because I wanted the option to weld in the future if necessary, so I guess I got lucky.

As mentioned above, you should try to drill a stop hole.
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Discussing Crack repair methods in the E46 M3 (2001-2006) Forum - Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006. at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)