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Old 08-08-2012, 12:20 AM   #297
Registered User
Join Date: Oct 2006
Location: Springfield OH
Posts: 740
My Ride: '00 323i + '75 2002
Thanks guys!

The next order of business was to fabricate the new mounts for the rear subframe. After taking some measurements I made a quick sketch of what I needed to turn and got to work.

The Sketch:

Shown in the sketch is the 'bottom plug'. This is what will hold the mounting studs for the two forward mounts on the rear subframe. The bottom plugs will be welded to a piece of 1.5"-0.120 wall DOM tubing, which will then allow me to tie the mounts into the roll cage and unibody very easily. Also, by leaving the top of the tubing open, the studs will be able to be removed/replaced if necessary. This could come in handy if the threads get stripped or rusty over time.

The material I used for the bottom plugs was 416 Stainless, which has good machinability while being very corrosion and wear-resistant. Starting with 1.5" bar stock, a couple hours later I had these:

And here are all the parts of the subframe mount in an exploded view. From left to right: DOM Tubing, Mounting Stud, Bottom Plug, Subframe Bushing, Lower bracing Plate.

This is how the plugs slide into the DOM tubing. The chamfered edge is to help the weld penetrate better, as well as give the filler material somewhere to go.

And here is how the plug fits onto the subframe bushing. The aluminum insert in the bushing fits up into the plug, and when bolted in the aluminum is tight up against the counter bore in the plug.

Everything put together to double check before welding:

Next it was time to press in the mounting studs. For this I used the stock e30 studs pulled from the parts car. The studs themselves have a knurled shoulder right under the head. This is press fit into the plug to hold the stud in place.

The stud loose in the plug, before being pressed in:

After being pressed into the plugs:

After welding the plugs into the tubes:

The two assemblies installed onto the subframe. Here you can also see the jig I made up to help position the subframe under the car:

Finally, with the subframe roughly in position, you can see the DOM tubing sticking up into the cabin area. I'll need to trim the top of the tube down to be even with the floor, I left it extra long since I didn't know how much I would need.

Hoping to get the subframe squared up and welded in place along with the diff mounts tomorrow. Then it's on to the trunk floor!
Tinker Engineering - 2014

Mica - 2000 BMW 323i - The one that started it all
Fiona - 1975 BMW 2002 - The Definition of Project Creep
Heidi - 1988 BMW M5 - The piece of BMW history
Silvia - 2013 Subaru WRX - Stock, for now
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