Shear Stress Surfaces  
scene 3 of 6
The figure on the left shows a shaft with a flange at one end. The axis of the shaft is along the x axis. The torque at the end of the shaft has a magnitude T and is directed counterclockwise. The figure on the right shows the front view of the flange. The bolt circle has a diameter of 250 millimeters.
 

Consider the surfaces that would be exposed if the external torque was large enough to break the bolts.

The cross-sectional area of all four bolts would be exposed. It is these surfaces upon which shear stress acts.