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It started out about 7 pounds heavier than the cast aluminum Cobra diff cover, which you can see above. But we needed thicker material to resist the torque of this LS motor, so the 1/4" thick steel unit was the right way to go. Buying a pre-made CNC laser cut and welded cover saved us a dozen hours of hand making this piece.
This bare steel cover was easy to modify to have "ears" attached that bolted to the E46 M3 rear subframe. Unlike the front, the rear mounting modifications were all done to the cover - nothing on the back of subframe was modified. Care was taken to maintain access for the rear cover mounting bolts as well as the existing fill plug.
The structure was set and plated, tack welded, then fully TIG welded to make a strong set of mounting ears on this 1/4" thick plate steel rear cover. Strong as an ox, yet compact. This cover came with a fill plug (threaded bung welded in, with a screw in plug) and we later added a drain plug as well.
The modified M3 rear subframe was also stitch welded to cover any gaps in the factory welds, just like the front. Then the rear subframe, the lower front mounting cross bar, and the reinforced front subframe were all powder coated gloss black.
So while the 210mm M3 diff housing might have worked, it was always going to be heavier, weaker, and have fewer & costlier options for gearing and LSDs. We would be having custom halfshafts built for either the 210mm BMW unit or the Ford 8.8", so that's a wash.
Aluminum subframe mounting bushings were installed into the shiny black M3 rear subframe. Instead of taking our chances with polyurethane or even Delrin, we're going "full stuff" on the rear diff and subframe mounts on this car.
We added a second threaded bung and plug to the diff cover before it was powder coated gloss black. This will allow us to run a dedicated diff fluid cooler, which will help keep the 8.8" unit happy under high torque abuse. The completed rear subframe - minus the diff housing - was installed in the car at this point to allow the rear suspension parts to be added.
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