I did have a bit of trouble with the diagonals on the seat back. For some reason I'd cut them identically, but where they met the suspension top mount on one side, it was a good inch or so off on the other side. I actually left this one for a couple of days, so frustrated with myself that the chassis was that twisted. I'd measured everything as close as possible with my eye, each of the horizontal bars on the rear were identical and the verticals were a perfect match as well. But of course, same length on parallel sides either makes a rectangle or rhombus; the former being good and the latter being bad.
I finally came back to it and had a good look at what I was doing. Then I found the problem. The horizontal bars are at different angles, the middle one being 'flat' to the seat back and the top one being horizontal. This means that the diagonals are cut at quite a sharp angle on either end, with a 'diamond' profile being presented to the top bar. I'd basically been measuring the wrong part, the wrong end of the diamond. Once I started comparing the centre of the diagonals rather than the corners, it all lined up. I now have a matching set of welded in diagonals, and there is roughly 1mm difference between the two. That does suggest there is still an element of error, but 1mm is a huge improvement over 1 inch!!
I'm quite happy with how the diagonals have gone in all over the chassis to be honest, I cut them all as matching pairs and they all fitted correctly with no trimming. That means that with the laser alignment checking the horizontals, the verticals are similarly accurate.
Anyway, enough chassis waffle, the MX5 is nearly stripped and might be losing it's bodyshell this weekend, so it's all steam ahead for getting the last few donor bits off the car!
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