An M.G. Locost Build
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May 18, 2012
Measure Twice

I guess things were going too well. We were making great progress and were really close to attaching the suspension. We'd overcome a few minor setbacks, but the frame was almost done. A few joints to reweld and we'd be good to go. The new upright for the transmission tunnel was tacked in place and all I had left to do was confirm the differential would fit, and we could make the final welds on everything. So we dragged the axle out and wrangled it into the frame and that's when it all fell apart.

  Almost fits
click to enlarge

At first it looked like everything would fit. The new upright was an inch and a half outboard from its original position, providing plenty of clearance on that side, which allowed the axle to be moved all the way forward into its normal position. The upright looked a little funny sitting out there, but after the tunnel sheeting was in place you'd never notice, so we didn't worry about it. However, with the nose of the differential sticking farther into the transmission tunnel than it ever had before, a couple of other problems surfaced.

The first disappointment was the right side upright. As we suspected, a gusset rib on that side of the differential case was less than 1/16" from the upright, and that was at static height. Any upward movement and it was going to hit. So something would have to go. I originally thought we'd grind away the gusset, but looking at it there, nice and solid and keeping the differential from flexing, it didn't seem right to tamper with it. So we resigned to move the right side upright out another inch, as we did with the left upright. Just about then, however, we saw the other problem.

The O3 tube was only a little more than an inch above the top of the differential. The O3 tube bisects the rear bulkhead and forms the top of the tunnel opening. In a turn, the diff doesn't move up or down much, so not a big deal. In a single bump on one side, the diff only moves half the height of the bump, so one inch of clearance is good for a two inch bump. On one side. But what about a 2" bump on one side and a 1" bump on the other? Or a 3" bump? Or some other combination? Those would be rare of course. An inch of clearance would be okay 99% of the time. But not all the time.

Plenty of room for the differential now  
click to enlarge

I agonized over this one. This was bad. I went through most of the seven stages of grief, getting alternately angry and sad, and trying to bargain a compromise, before finally resolving to do the right thing, which was to tear out the old bulkhead and the rear transmission tunnel rails, and rebuild it. It was not an easy decision, but I figured if we're going to spend two years and a couple of thousand hours building a car, we want to have a car that works all the time, not 99% of the time.

  New O3 tube in position
click to enlarge

Before I could change my mind, we got out the old hacksaw and extracted the offending O3 tube, along with both rear transmission tunnel uprights and the upper tunnel rails. A total of five tubes. Not so bad. We left the bulkhead diagonals in place, with about an inch lopped off the ends where they join the O3 tube. We cut a new O3 tube and ground the diagonals until the new tube fit 1-1/4" higher than the old tube, and tacked it in place. That immediately felt better. Then we fit two new tunnel uprights, and tacked them on. Then we ran out of 1"x1/2" RHS.

Good as new. Or better.  
click to enlarge

So we're done for now, but the frame is almost back together, and the new welds look a lot better than the old ones. As soon as we get the tunnel rails welded in, it'll be like it never happened. It only cost us about ten hours, so it's not even that much of a setback. Plus, we won't have to grind down the differential gusset, and as a bonus the new tunnel rails will meet up with the new outboard uprights, making it a lot easier to sheet the tunnel. So it's all good. Now I'm almost glad it happened.

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Comments:  
posted December 29, 2016 at 03:07:34  
BS low - rainitaloty high! Really good answer!