within the outer sheet. A combination of countersinks too deep  (creating a knife edge), number of pressurization cycles,  fatigue, deterioration of bonding materials, and working  fasteners caused a high stress concentration that resulted in  skin cracks and fastener failures. In primary structure and  pressurized skin repairs, some manufacturers are currently  recommending the countersink depth be no more than  2 ⁄ 3 the  outer sheet thickness or down to 0.020-inch minimum fastener  shank depth, whichever is greater. Dimple the skin if it is too  thin for machine countersinking.  [Figure 4-91] Keep the rivet high before driving to ensure the force of  riveting is applied to the rivet and not to the skin. If the rivet  is driven while it is flush or too deep, the surrounding skin  is work hardened. While there are many types of countersink tools, the most  commonly used has an included angle of 100°. Sometimes  types of 82° or 120° are used to form countersunk wells.  [Figure 4-84]  A six-fluted countersink works best in  aluminum. There are also four- and three-fluted countersinks,  but those are harder to control from a chatter standpoint. A  single-flute type, such as those manufactured by the Weldon  Tool Company$^{®}$, works best for corrosion-resistant steel.  [Figure 4-92] code. One tap may mean not fully seated, hit it again, while  two taps may mean good rivet, and three taps may mean bad  rivet, remove and drive another. Radio sets are also available  for communication between the technicians. Once the rivet is installed, there should be no evidence of  rotation of rivets or looseness of riveted parts. After the  trimming operation, examine for tightness. Apply a force  of 10 pounds to the trimmed stem. A tight stem is one  indication of an acceptable rivet installation. Any degree of  looseness indicates an oversize hole and requires replacement  of the rivet with an oversize shank diameter rivet. A rivet  installation is assumed satisfactory when the rivet head is  seated snugly against the item to be retained (0.005-inch  feeler gauge should not go under rivet head for more than  one-half the circumference) and the stem is proved tight. An improperly made countersink reduces the strength of a  flush-riveted joint and may even cause failure of the sheet or  the rivet head. The two methods of countersinking commonly  used for flush riveting in aircraft construction and repair are: The proper method for any particular application depends on  the thickness of the parts to be riveted, the height and angle  of the countersunk head, the tools available, and accessibility. When using countersunk rivets, it is necessary to make a  conical recess in the skin for the head. The type of countersink  required depends upon the relation of the thickness of the  sheets to the depth of the rivet head. Use the proper degree  and diameter countersink and cut only deep enough for the  rivet head and metal to form a flush surface. Countersinking is an important factor in the design of fastener  patterns, as the removal of material in the countersinking  process necessitates an increase in the number of fasteners to  assure the required load-transfer strength. If countersinking  is done on metal below a certain thickness, a knife edge with  less than the minimum bearing surface or actual enlarging of  the hole may result. The edge distance required when using  countersunk fasteners is greater than when universal head  fasteners are used. The general rule for countersinking and flush fastener  installation procedures has been reevaluated in recent years  because countersunk holes have been responsible for fatigue  cracks in aircraft pressurized skin. In the past, the general rule  for countersinking held that the fastener head must be contained  •  Machine or drill countersinking. •  Dimpling or press countersinking. Countersinking Countersunk Rivets Countersinking Tools 4-41 Figure 4-91.  Countersinking dimensions. Preferred countersinking Permissible countersinking Unacceptable countersinking