The size of the formed head is the visual standard of a proper  rivet installation. The minimum and maximum sizes, as well  as the ideal size, are shown in  Figure 4-76 . Repair layout involves determining the number of rivets  required, the proper size and style of rivets to be used, their  material, temper condition and strength, the size of the holes,  the distances between the holes, and the distance between  the holes and the edges of the patch. Distances are measured  in terms of rivet diameter. To determine the total length of a rivet to be installed, the  combined thickness of the materials to be joined must first  be known. This measurement is known as the grip length.  The total length of the rivet equals the grip length plus the  amount of rivet shank needed to form a proper shop head.  The latter equals one and a half times the diameter of the rivet  shank. Where A is total rivet length, B is grip length, and  C is the length of the material needed to form a shop head,  this formula can be represented as A = B + C.  [Figure 4-76] For structural applications, the strength of the replacement  rivets is of primary importance.  [Figure 4-77]  Rivets made  of material that is lower in strength should not be used as  The 2117-T rivet is used for general repair work, since it  requires no heat treatment, is fairly soft and strong, and is  highly corrosion resistant when used with most types of  alloys. Always consult the maintenance manual for correct  rivet type and material. The type of rivet head to select for  a particular repair job can be determined by referring to the  type used within the surrounding area by the manufacturer.  replacements unless the shortfall is made up by using a larger  rivet. For example, a rivet of 2024-T4 aluminum alloy should  not be replaced with one of 2117-T4 or 2017-T4 aluminum  alloy unless the next larger size is used.  Installation of Rivets Repair Layout Rivet Length  Rivet Strength 4-33 Figure 4-77.  Rivet alloy strength. Figure 4-76.  Rivet formed head dimensions.  Standard Rivet Alloy Code Markings Alloy code—A Alloy—1100 or 3003 aluminum Head marking—None Shear strength—10 kilopounds per square inch (KSI) Nonstructural uses only Alloy code—B Alloy—5056 aluminum Head marking—raised cross Shear strength—28 KSI Alloy code—AD Alloy—2117 aluminum Head marking—Dimple Shear strength—30 KSI Alloy code—D Alloy—2017 aluminum Head marking—Raised dot Shear strength—38 KSI 38 KSI When driven as received 34 KSI When re-heat treated Alloy code—DD Alloy—2024 aluminum Head marking—Two bars (raised) Shear strength—41 KSI Must be driven in “W” condition (Ice-Box) Alloy code—E, [KE*] *Boeing code Alloy—7050 aluminum Head marking—Raised ring Shear strength—43 KSI Replacement for DD rivet to be driven in “T” condition 1.25 d 1.5 d 1.33 d 1.5 d 1.66 d .66 d .5 d .33 d 1.25 d 1.4 d 1.5 d .66 d .6 d .5 d 1.25 d 1.33 d Minimum Maximum Preferred Minimum Maximum Preferred Driven Rivet Standards A, AD, B, DD Rivets D, E, (KE), M Rivets Formed  head  dimension Formed  head  dimension Predrive protrusion Predrive protrusion