For this one I consulted with my IA. The plans are clear in using AN4 Bolts to attach the extrusions to the longerons/spar. I did this for all but 4 places. The square 4130 steel extrusions that I installed are fantastic except for 1 major issue. The problem is that I have to fit a small socket inside the extrusion in order to access the engine mount bolts. Now this is fine until I install the inside longeron bolts. These bolt heads restrict access to the engine mount bolts so that if I ever had to replace them it would mean drilling out the extrusion bolts/nuts which in turn means damaging the airframe. This was NOT an option in my mind so I asked my IA if I could replace the AN4 Bolts with structural flush head screws, and his answer was simply yes. I therefore took my countersink tool and slowly countersunk the extrusions in these hard to reach bolt holes. I then installed the flush head screws and stop nuts. This solves the problem. I can now access the engine mount bolts at all times. Finally, I filled all of the Stop nut access holes with flox and applied 1 ply BID and peel ply. Glad this step is finished. I can now hang my engine!