According to the "Plot Crotch Strap Assembly" detail drawing the crotch strap is secured to the left pre-punched hole in the face of the aft crotch strap bracket. To do this the pre-punched hole must be match drilled into the front bracket, then drilled out for an AN5-5A bolt. The right hole is drilled out for the passenger crotch strap. To do this I need to keep the drill bit perpendicular to the aft bracket while drilling. To do this I made a 3/4" composite wood block about 1.5x1.5 size and drilled a number of guide holes for the bit. By placing this wood block against the bracket face I am assured that the holes are perpendicular in both bracket faces. First I match drilled the brackets using a #30 bit. Working out quite well. Tomorrow I will finish the job.
