This afternoon I decided to finish the center spar supports, after clamping and back drilling the extrusions, I removed the forward extrusion and used a drill slightly smaller than the hole in the spar to mark the hole positions on the rear extrusions, I did the same for the top and bottom holes, removed the angle drilled it and re-installed it in the fuse. I then re-isntalled the front angle, clamped and back drilled from the rear through the front angle. The instructions carry on about drill guides etc. but I found that with a little care you can get away with just marking the extrusions with a sharp drill bit.
