Countersunk Rivets

Solid shank rivets are available in two standard head styles, universal and countersunk/flush. AN 426 countersunk rivets were developed to streamline airfoils and permit a smooth flow over an aircraft's wings or control surfaces. However, before a countersunk rivet can be installed, the metal must be countersunk or dimpled.

Countersinking : a process in which the metal in the top sheet is cut away in the shape of the rivet head.
Dimpling : a process that mechanically "dents" the sheets being joined to accommodate the rivet head.

Sheet thickness and rivet size determine which method is best suited for a particular application.

Joints utilizing countersunk rivets generally lack the strength of protruding head rivet joints. One reason is that a portion of the material being riveted is cut away to allow for the countersunk head. Another reason is that when riveted, the gunset may not make direct contact with the rivet head if the rivet hole was not countersunk or dimpled correctly, resulting in the rivet not expanding to fill the entire hole. To ensure head-to-gunset contact, it is recommended that countersunk heads be installed with the manufacturer head protruding above the skin's surface about .005 to .007 of an inch. This ensures that the gunset makes direct contact with the rivet head. To provide a smooth finish after the rivet is driven, the protruding rivet head is removed using a microshaver. This rotary cutter shaves the rivet head flush with the skin, leaving an aerodynamically clean surface.

Here is the example of what an AN426 countersunk rivet would look like,refer to figure 1 :

Figure 1

 This figure (figure 2) explains the meaning of the rivet numbers :

Figure 2

And this photo shows the finish product of a riveted countersunk on a sheet metal (figure 3) :

Figure 3