Chassis Preparation

The idea of this swap is to put a big engine into a small car. The concept is simple, but proper execution takes a bit of work. Clearance has to be made in the engine bay to make room for the engine itself and the transmission tunnel needs to be expanded ~2″ on each side to accept the larger LS/t56 bell housing. Many people leave it at that. I wanted to stiffen up the chassis some via stitch welding because the new engine will produce three to four times more torque than a stock BP. That, and stiffness is good for handling. Since viagra doesn’t work on cars, I grabbed my trusty Harbor Freight angle grinder, borrowed a MIG welder, and got to work.

The chassis comes packed with seam sealer. There’s a lot of the stuff and it’s everywhere–particularly near the seams. The seam sealer is important as it prevents moisture from reaching the crevices and rotting the metal, but you can’t weld over the stuff. In fact, you can’t weld over anything. All of the seam sealer, paint, and primer needs to be removed from the seams so that clean, contaminant-free, welds can be made. Although removing the seam sealer is messy business, an angle grinder with a good wire wheel gets through the stuff relatively quickly.

Not fun

99% of the seam sealer is gone. The remaining 1% is stuck in the seams and takes about as long to remove as the first 99%. More paint and primer need to be stripped before welding can take place.

The engine bay needs to be stripped as well. The process is the same. You can see that the gusset in the corner of the bay has been removed. That makes space for the engine.

I took care of the stitch welding once all of the contaminants were stripped from the seams. Stitch welding involves putting short lines of weld along all of the seams at short intervals. I had ~1″ of space between most of my welds. Don’t stich forward of the front strut towers or behing the rear strut towers. This will provide no benefit and will compromise crumple zones–making the vehicle less safe. I had never welded prior to this project. Fortunately, MIG is easy to pick up.

I then stitched the pinch protrusions that are revealed when the doors are open. Numerous layers of steel merge at this point, and stitch welding them together is supposed to make a tangible difference in itself. Plus, they’re easy to get to.

I cleaned up the seams once again after the stitch welding was complete and packed them with new seam sealer.

Making clearance in the engine bay is really quite simple. There are gussets in the back corners that need to be removed. I removed mine with an angle grinder and cutoff wheel. That said, my tool of choice would have been a plasma cutter. You can see that the firewall seam is gone and that a number of holes are filled. This isn’t at all necessary–it’s strictly for aesthetics. I’ll write a post about the engine bay shave once I’m further along.

The transmission tunnel needs to be expanded as well. If you do this swap, I recommend that you not start this process until your engine and transmission are present. The tunnel needs to be expanded for, and only for, the bell housing. The bell housing is wide where it meets the engine, and quickly tapers down. Accordingly, the transmission tunnel only needs to be flared out where it meets the firewall. I didn’t do enough research and started before my drivetrain was present. I expanded the tunnel to the widths that were specified in the Flyin’ Miata instruction manual, but I went too deep with the expansion. Now, the gas pedal hits the tunnel. A v8 Miata with a gas pedal that can only be depressed 20% isn’t particularly useful. Bashing the metal back in will cause more fatigue and the finished product would be an eyesore. Now I need to cut out those sections of beat up metal and fabricate new ones. That should be reasonably straight forward, but it is more work, and it’s the kind of work that could have easily been avoided.

Check back–there’s more to come.

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