Correction of hole sizes for mass dampers

The mass dampers had 4mm holes, with a 6mm counterbore. Rib 4 had 4.8mm holes, and this larger number was mentioned in the instructions, and intending the use of 4.8mm x 10mm rivets. The dampers were drilled out to 4.8mm. Experiments showed the 6mm counterbore was insufficient to allow proper rivet operation, 7mm was considered a minimum. The counterbore was increased to 7mm with a nearly flat tip (cobalt) drill to retain a flattish step at the base of the counterbore.