Z3 M Coupe Rear Subframe Reinforcement: Part 2

posted in: 1999 M Coupe | 6

Z3 M Coupe Rear Subframe Reinforcement: Part 2

In continuation of part 1, part 2 covers the reinforcement of the rear trunk floor.

Step 1: Weld chassis floor flat.

First, we stop drilled the existing crack near the battery tray box in the center of the floor. We attempted small spot welds around the bar stock used to level the floor and the crack. As this was our first time welding, there were some issues on the smaller gauge metal. We ended up figuring out the appropriate settings and cleaned this up substantially before completion.

Continuation of welding up the sheet metal we cut and hammered flat in part 1.

The sheet metal welded and sanded smooth. In the cavity, you can see where good penetration was completed by the welds on the diff ear reinforcement.

Step 2: Prepare and weld cavity reinforcement sheet.

The reinforcement bar for the cavity in the trunk floor is substantial and takes many plug welds. Here, it is clamped down and the holes are marked with a sharpie to prepare the metal for welding. We tried to remove as little factory e-coat as possible.

The dual diff ear reinforcements from the Randy Forbes kit were clamped to the bottom cavity in order to prevent bending the factory sheet metal.

Good weld penetration.


Step 3: Prepare and weld top chassis floor reinforcements.

Before the top plates were welded the inside of the cavity was masked off and painted with 2K epoxy primer to provide as much corrosion protection in the future as possible.

SprayMax 2K epoxy primer. 3680032.

After the cavity was sprayed with epoxy primer.

The back side of the reinforcement plates were painted to provide additional corrosion protection.

The top edge of the center plate was sanded back down to metal so a clean weld could be achieved when the top plates were welded. The cracked spot welds were then repaired by drilling them out. They were then welded and ground flat.

The top plates were clamped so they would sit flush against the center plate.

The top plates were designed so the holes would connect to the top of the chassis cavity when welded.


Spot welds completed and stitch welds going in tying in the top of the chassis floor to the bottom of the cavity through the center vertical reinforcement plate.

Step 4: Weld the frame rail reinforcements to the chassis and floor plates.

Peeking inside the frame rail as the frame rail reinforcements were spot welded.

Drilling the remaining popped spot welds on the driver’s side of the floor.

Reinforcement kit completely welded up.

Sanding the spot welds smooth in preparation of paint, seam sealer, and sound deadening.

Part 3 to include the completion of the reinforcement with seam sealer, paint, and sound deadening.

Wondering where this started? Check out Part 1 of the Z3 M Coupe rear subframe reinforcement. Check out the finishing step here.

6 Responses

  1. Bradley Zafris

    This is an amazing documentation on the process. I’m in need of getting this done for my 2000 m roadster. Really with you guys we’re based in Pittsburgh. Did you have trouble getting in touch with Randy Forbes? I’ve tried contacting with no luck. I am interested in sourcing one of his kits before I find a shop willing in the area.

    • TC2

      Good afternoon Bradley, I appreciate the positive comment!

      Are you trying to email him? I remember when I contacted him it would take a few days to respond. It stay fairly busy.

      Best regards!

  2. Mark

    Amazing job guys good for you for new welders , I’m contemplating doing this myself but am hesitant . I have a 2.8 roadster and the floor is coming away and have a nice crack in the diff ear 🙁 . Have contacted Randy just contemplating where I have the courage to do it . Nice to see two brothers with a passion . Great job !Montreal .

    • TC2

      Thanks for the kind words Mark! I know it is frustrating that your trunk floor is tearing away. Good luck with the repair! It will be solid once completed and Randy’s kit is the way to go. Best regards

  3. Randy Forbes

    Good job guys! Very interesting to see the pictures from the opposite side(s) while the fusion is taking place__working by myself and taking the pictures, I can’t do that.

    I can only hope that all the kits I sell are being installed as conscientiously as this one.

    Thanks for posting the story and pictures of you doing this yourselves.

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