To maintain alignment while installing flush head rivets, what should be installed in the middle row of the assembly?

Using clecos in the middle row of the assembly is crucial for maintaining alignment while installing flush head rivets. Clecos are temporary fasteners that help hold parts together securely and ensure accurate fit and alignment during the assembly process. By inserting clecos, you can keep the components in the correct position until the rivets are installed, which allows for precision and prevents misalignment that could compromise the integrity of the assembly.

Installing other items like clamps, blocks, or brackets might not provide the same level of alignment assistance as clecos. Clamps might exert pressure in a way that could shift the parts, while blocks may not provide sufficient hold or flexibility to support proper rivet placement. Brackets, similarly, are more rigid and less adaptable for maintaining the precise alignment required during the flush rivet installation process. Therefore, clecos stand out as the optimal choice for achieving the necessary alignment during assembly.