Z3 M Coupe Rear Subframe Reinforcement: Part 3

posted in: 1999 M Coupe | 5

Z3 M Coupe Rear Subframe Reinforcement: Part 3

In continuation of part 2, part 3 covers the completion of the reinforcement with seam sealer, paint and sound deadening.

Step 1: Paint all exposed metal.

All exposed metal was painted with 2K epoxy primer.

Step 2: Spray cavity wax into frame rails.

Welding on the frame rails burned off the paint and e-coat on the metal as seen below. 3M Cavity Wax Plus was sprayed liberally inside of each frame rail. The part number for the 3M cavity wax is 08852. The applicator wand kit was used to reach inside of the frame rail. Part number for the applicator wand kit is 08851.

Applicator wand can be seen in the photo below.

Frame rail coated in cavity wax.


The holes inside of the body were covered in painter’s tape to keep it from getting inside of the car. Any excess dripped out of holes in the bottom of the frame rail.

To be able to install the false floor bracket on the driver’s side a small section has to be cut out and bent flat to sit flush against the floor. The small section to be cut out is seen below.

Starting to cut.

Cut out.

Bent flat and welded up.

Ground smooth and ready for welding.

Step 3: Undercoat and seam seal.

The underside and inside of the car was coated with Fusor 806D beige undercoating.

All seams inside were taped off, sprayed and then brushed to give an OE finish.

The undercoating was painted over with SEM olive green ez coat paint to give an appearance that matched the factory e coat color.

After the seam sealer was applied, OE sound deadening material was added to give an factory look.

OE BMW sound deadening with recognizable honeycomb pattern.

The sound deadening was heated and pressed down to mold to the shape of the floor.

Sound deadening installed.

Prepped for final paint.

Masked and ready for final coat of BMW 668 Schwarz II basecoat.

Painting the final coat with light passes to give a factory appearance.

Reinforcement completed.

The battery box was reinstalled. 3M window weld was used to replace the butyl sealant the factory used to seal the plastic box against the trunk floor.

Butyl sealant installed and ready.

Battery box being installed.

Wondering where this started? Check out Part 1 and Part 2 of the Z3 M Coupe Subframe Reinforcement.

5 Responses

  1. Christopher Hagopian

    Just purchased a 2000 z3 m coupe that has yet to be reinforced, is there any chance that you would replicate this job if I provided a Randy Forbes kit and payed for additional parts/materials/labor? If so, please let me know!

    Thanks in advance!

    • TC2

      Hello Christopher, yes we are open to helping you out. We are constantly busy with various matters going on in our lives with business and so forth, but can usually work something out. You can reach out to us directly through our contact form. Thanks!

  2. Phil Wood

    Bimmer Brothers,

    You guys have done an excellent job of describing/sharing the process you use to install Randy’s kit.
    I currently own a 1998 M Roadster with a Dinan S3 package installed. I do not track my car or drive it overly aggressively.

    Currently, I have no broken spot welds or torn metal at the differential mount. However, I am thinking I need to be pro-active about getting this situation eliminated before things go wrong.

    I have communicated with Randy about his ceasing the installation of his kit and I am in the process of trying to find someone like you guys to install the kit. I live in North Carolina.

    1) How long does it take for you to install a single ear kit?
    2) What would your charge be?
    3) When might you be able to do the job?

    Thanks very much for your time and consideration,
    Phil Wood

  3. Paul

    Brilliant write up and documentation of the procedure, challenges, and wins! Cannot thank you enough for the info capture. Were you able to find a used 312 spray gun or did you buy new? I’m trying to track one down but am not sure if the ones on ebay are legitimate

    • TC2

      Good morning Paul, I purchased the Fusor 312 new off of eBay. My only gripe about it is that I had a hard time matching the factory texture of the undercoating. Could have been my technique, but it did work well overall.

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