A centreline was marked on the plank at 4.625″ (4 5/8) from each side.
Two sets of two 2x4s were nailed to the 10′ plank perpendicular to the centerline, level, and exactly 101.75″ apart. The 101.75″ is measured from the outboard side of both sets and 101.75″ was confirmed across multiple locations along the 2x4s.
The temporary firewall will be added to complete the jig, then fuselage assembly can start.
