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DIY: Do It Yourself
Post here to share or improve your wrench turning skills! All BMW E46 DIY tips, tales, and projects discussed inside. Learn to work on your car and know the right BMW parts you will need!

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Old 03-23-2015, 08:25 PM   #1
BMWPower8
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Join Date: Dec 2014
Location: Connecticut
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My Ride: E46 BMW 328i
Damaged Subframe Mount Repair DIY - No Rear Dissembly Required

I know the issue of cracked and torn subframes has been talked over and detailed many times in these forums. However, I still see the occasional recent post where someone is still inquiring about how to go about fixing this issue. I figured I would share the way that I did it which is somewhat different than how it is traditionally done.

After purchasing my E46 328i, I set about looking into how to repair the torn subframe mounts. My car was badly damaged. The driver side rear mount, the one most common to fail first, had basically no metal holding it in place anymore. The mount was pretty much just hanging in the air.
As a 18 year old student, I did not have the time to set aside to drop the whole rear of the car as it is typically suggested to do. I kept searching and read a post that suggested that someone had successfully reinforced their subframe mounts from the top; by cutting through the floor of the trunk. I never did find a specific post that detailed how this person had done this but I figured I would try this method and hope that it worked out. In the end, it was largely successful.
Below I have detailed the steps I took to get this done; all without having to disassemble the rear of the car. I am missing some pictures but I hope everyone can get the general idea of what I did.
This method is great to use if you have a badly damaged subframe mount like mine. I don't think the traditional method would have been sufficient at repairing my car's damage.

Note: this method will only work in reinforcing the rear mounts; there is no way that I know of to access the front mounts from the top of the car.



Step 1:
The rear mounts are directly below the top left and right of the trunk floor. As shown in the picture, I started in the top left (driver side rear mount).
Cut away the top layer of metal from the trunk floor. This will expose a metal housing that holds the subframe mount.






Step 2:
Cut away a section of the second layer of metal that is seen. This will expose the subframe mount.






Step 3:
Jack the car up and remove the bolt that goes through the bushing of the subframe up into the subframe mount that was just exposed from above. If your car was damaged as badly as mine, this will allow the mount the simply be lifted out from above.

Mount removed looking from above:



Step 4:
Prepare the area for welding by grinding the metal to remove all paint. Here is a picture from the early stages of grinding:




Step 5:
Take your reinforcement plates and fit them below the car. There should be enough room between the subframe bushing and bottom of the car to maneuver the plate. If your car is damaged like mine, you will most likely have to create your own custom plates. This is what I did.

Test fitting plate under the car:



Step 6:
Put a jack underneath the subframe bushing and elevate it so it traps the plate between the bushing and the plate on the underside of the car to hold the plate in place.
Weld the plate in from above (inside the trunk). Typically, the plates are welded onto the bottom of the car. Going in through the trunk makes it so you don't have to drop the entire rear of the car.
After welding, make sure to prime and paint the area to prevent rust.

My plate welded in and covered with primer before painting:



Note: I know that my welds to not look very pretty. This project was the first time I ever welded and I wasn't able to use the best welder for the job. What matters is that each of the plates are attached strongly, no matter how ugly the welds.


Step 7:
I now made a second plate that I welded in on top of the first. I wanted the double reinforcement because it had been so badly damaged already. Also, this mount is pulled down (towards the ground) when under load from the drivetrain/differential. I figured that having the second plate on the top would really help strengthen the mount when it is pulled down.
If you are using this method as a preventative measure (you don't already have a hole in your floor) you can look at this step and adapt it to your needs. You will likely be successful with one plate on the top.

Plate #2:


Plate welded in:



Step 8:
Now place the mount back in the car. Screw bolt into the mount from under the car just enough to hold the mount in place. Then weld the floor of the mount to the plate that you just put in.




Step 9:
Now all that is left to do is to weld the two layers of metal that you originally took out back into the truck floor. Be neat about it and you can make it look as though you never worked in the area. Then go and reinforce the mount on the other side of the car (rear passenger mount).


I hope that people find this method helpful if they are looking to reinforce their subframe without having the drop the entire rear of the car. Unfortunately, this method does not allow for the front mounts to be reinforced. However, I inspected the ones on my car and found no evidence of cracking or breaking. Since my car is only a 328i, meaning less power and less strain on the mounts, I am hoping that with the rear reinforced, the front will remain intact. At the end of the day, this project only cost me $55 to rent the welder for the day.
Best wishes to everyone in working on your E46.
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Old 03-23-2015, 08:50 PM   #2
cosmos328Ci
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Location: Australia
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Quote:
Originally Posted by BMWPower8 View Post
I know the issue of cracked and torn subframes has been talked over and detailed many times in these forums. However, I still see the occasional recent post where someone is still inquiring about how to go about fixing this issue. I figured I would share the way that I did it which is somewhat different than how it is traditionally done.

After purchasing my E46 328i, I set about looking into how to repair the torn subframe mounts. My car was badly damaged. The driver side rear mount, the one most common to fail first, had basically no metal holding it in place anymore. The mount was pretty much just hanging in the air.
As a 18 year old student, I did not have the time to set aside to drop the whole rear of the car as it is typically suggested to do. I kept searching and read a post that suggested that someone had successfully reinforced their subframe mounts from the top; by cutting through the floor of the trunk. I never did find a specific post that detailed how this person had done this but I figured I would try this method and hope that it worked out. In the end, it was largely successful.
Below I have detailed the steps I took to get this done; all without having to disassemble the rear of the car. I am missing some pictures but I hope everyone can get the general idea of what I did.
This method is great to use if you have a badly damaged subframe mount like mine. I don't think the traditional method would have been sufficient at repairing my car's damage.

Note: this method will only work in reinforcing the rear mounts; there is no way that I know of to access the front mounts from the top of the car.



Step 1:
The rear mounts are directly below the top left and right of the trunk floor. As shown in the picture, I started in the top left (driver side rear mount).
Cut away the top layer of metal from the trunk floor. This will expose a metal housing that holds the subframe mount.






Step 2:
Cut away a section of the second layer of metal that is seen. This will expose the subframe mount.






Step 3:
Jack the car up and remove the bolt that goes through the bushing of the subframe up into the subframe mount that was just exposed from above. If your car was damaged as badly as mine, this will allow the mount the simply be lifted out from above.

Mount removed looking from above:



Step 4:
Prepare the area for welding by grinding the metal to remove all paint. Here is a picture from the early stages of grinding:




Step 5:
Take your reinforcement plates and fit them below the car. There should be enough room between the subframe bushing and bottom of the car to maneuver the plate. If your car is damaged like mine, you will most likely have to create your own custom plates. This is what I did.

Test fitting plate under the car:



Step 6:
Put a jack underneath the subframe bushing and elevate it so it traps the plate between the bushing and the plate on the underside of the car to hold the plate in place.
Weld the plate in from above (inside the trunk). Typically, the plates are welded onto the bottom of the car. Going in through the trunk makes it so you don't have to drop the entire rear of the car.
After welding, make sure to prime and paint the area to prevent rust.

My plate welded in and covered with primer before painting:



Note: I know that my welds to not look very pretty. This project was the first time I ever welded and I wasn't able to use the best welder for the job. What matters is that each of the plates are attached strongly, no matter how ugly the welds.


Step 7:
I now made a second plate that I welded in on top of the first. I wanted the double reinforcement because it had been so badly damaged already. Also, this mount is pulled down (towards the ground) when under load from the drivetrain/differential. I figured that having the second plate on the top would really help strengthen the mount when it is pulled down.
If you are using this method as a preventative measure (you don't already have a hole in your floor) you can look at this step and adapt it to your needs. You will likely be successful with one plate on the top.

Plate #2:


Plate welded in:



Step 8:
Now place the mount back in the car. Screw bolt into the mount from under the car just enough to hold the mount in place. Then weld the floor of the mount to the plate that you just put in.




Step 9:
Now all that is left to do is to weld the two layers of metal that you originally took out back into the truck floor. Be neat about it and you can make it look as though you never worked in the area. Then go and reinforce the mount on the other side of the car (rear passenger mount).


I hope that people find this method helpful if they are looking to reinforce their subframe without having the drop the entire rear of the car. Unfortunately, this method does not allow for the front mounts to be reinforced. However, I inspected the ones on my car and found no evidence of cracking or breaking. Since my car is only a 328i, meaning less power and less strain on the mounts, I am hoping that with the rear reinforced, the front will remain intact. At the end of the day, this project only cost me $55 to rent the welder for the day.
Best wishes to everyone in working on your E46.
Great write up.
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Old 03-23-2015, 09:06 PM   #3
Rob43
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OP, great write up.

Question, have you buttoned everything back together 100% ?



Rob43
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Old 03-23-2015, 09:36 PM   #4
BMWPower8
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Quote:
Originally Posted by Rob43 View Post
OP, great write up.



Question, have you buttoned everything back together 100% ?







Rob43


Yes, it has all been put back together. Unfortunately, when I was taking out the outermost layer of the trunk floor (the blue one) I didn't have the best tools for it at that time. The edges of the cut were very rough and so it was difficult to weld that piece back in place at the end. It just looks a little messy for now, haven't taken the time to clean it up just yet. I'll probably end up cutting a fresh piece of sheet metal to put into that area and then paint it blue.
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Old 03-23-2015, 09:46 PM   #5
Rob43
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Quote:
Originally Posted by BMWPower8 View Post
Yes, it has all been put back together. Unfortunately, when I was taking out the outermost layer of the trunk floor (the blue one) I didn't have the best tools for it at that time. The edges of the cut were very rough and so it was difficult to weld that piece back in place at the end. It just looks a little messy for now, haven't taken the time to clean it up just yet. I'll probably end up cutting a fresh piece of sheet metal to put into that area and then paint it blue.
OK, the reason I was asking was this, there's a fix call the "Bolt Through" subframe fix. I think what you've done is great, but if you wanted to take it a step further for greater strength, you'd finish off what you have done with a Bolt Through design. If you're interested in this just search, of course you'll need 2 long grade 8 bolts.


Rob43
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SUMMIT POINT WV 1:24:229 S.C.C.A.
(DynoDynamics https:vimeo.com/8486878 Dyno Video)
"Chance Favors The Prepared Mind"

Need Help With Your Nitrous Ambitions ?.....PM ME
Quote:
Originally Posted by RacerX View Post
Nitrous is a little trickier than boost, but it's not the spray that kills motors, it's STUPIDITY!!
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Old 03-24-2015, 10:59 AM   #6
BMWPower8
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Quote:
Originally Posted by Rob43 View Post
OK, the reason I was asking was this, there's a fix call the "Bolt Through" subframe fix. I think what you've done is great, but if you wanted to take it a step further for greater strength, you'd finish off what you have done with a Bolt Through design. If you're interested in this just search, of course you'll need 2 long grade 8 bolts.





Rob43


Thanks for the tip, I've seen a couple of those threads and it's definitely something I've been considering. Seems like a good route to take for max strength.
Thanks again
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