with which aluminum is deformed, it is necessary to distribute  screw and nut pressure by means of plain washers. Hardware used to make bonding or grounding connections  should be selected on the basis of mechanical strength,  current to be carried, and ease of installation. If connection  is made by aluminum or copper jumpers to the structure of a dissimilar material, a washer of suitable material should be  installed between the dissimilar metals so that any corrosion  occurs on the washer. Hardware material and finish should be selected on the basis  of the material of the structure to which attachment is made  and on the material of the jumper and terminal specified for  the bonding or grounding connection. Either a screw or bolt  of the proper size for the specified jumper terminal should  be used. When repairing or replacing existing bonding or  grounding connections, the same type of hardware used in  the original connection should always be used. Connectors (plugs and receptacles) facilitate maintenance  when frequent disconnection is required. Since the cable  is soldered to the connector inserts, the joints should be  individually installed and the cable bundle firmly supported to  avoid damage by vibration. Connectors have been particularly  vulnerable to corrosion in the past, due to condensation within  the shell. Special connectors with waterproof features have  been developed that may replace nonwaterproof plugs in  areas where moisture causes a problem. A connector of the  same basic type and design should be used when replacing  a connector. Connectors that are susceptible to corrosion  difficulties may be treated with a chemically inert waterproof  jelly. When replacing connector assemblies, the socket-type  insert should be used on the half that is “ live” or “hot” after the  connector is disconnected to prevent unintentional grounding. Connectors are identified by Air Force Navy (AN) numbers  and are divided into classes with the manufacturer’s  variations in each class. The manufacturer’s variations are  differences in appearance and in the method of meeting a  specification. Some commonly used connectors are shown in  Figure 4-115 . There are five basic classes of AN connectors  used in most aircraft. Each class of connector has slightly  different construction characteristics. Classes A, B, C, and D  are made of aluminum, and class K is made of steel. The following procedures outline one recommended method  of assembling connectors to receptacles: Many special-purpose connectors have been designed for  use in aircraft applications. These include subminiature and  rectangular shell connectors, and connectors with short body  shells, or of split-shell construction. Code letters and numbers are marked on the coupling ring  or shell to identify a connector. This code provides all the  information necessary to obtain the correct replacement for  a defective or damaged part.  [Figure 4-116] Connector Identification Connectors Installation of Connectors Types of Connectors 4-65 1.  Class A—solid, one-piece back shell general-purpose  connector. 4.  Use connector pliers to tighten coupling rings one- sixteenth to one-eighth turn beyond finger tight if  space around the connector is too small to obtain a  good finger grip. 3.  Alternately push in the plug and tighten the coupling  ring until the plug is completely seated. 2.  Start the plug into the receptacle with a slight forward  pressure and engage the threads of the coupling ring  and receptacle. 1.  Locate the proper position of the plug in relation to  the receptacle by aligning the key of one part with the  groove or keyway of the other part. 5.  Class K—a fireproof connector used in areas where it  is vital that the electric current is not interrupted, even  though the connector may be exposed to continuous  open flame. Wires are crimped to the pin or socket  contacts and the shells are made of steel. This class of  connector is normally longer than other connectors. 4.  Class D—moisture and vibration resistant connector  that has a sealing grommet in the back shell. Wires are  threaded through tight fitting holes in the grommet,  sealing against moisture. 3.  Class C—a pressurized connector with inserts that  are not removable. Similar to a class A connector  in appearance, but the inside sealing arrangement is  sometimes different. It is used on walls or bulkheads  of pressurized equipment. 2.  Class B—connector back shell separates into two parts  lengthwise. Used primarily where it is important that  the soldered connectors are readily accessible. The  back shell is held together by a threaded ring or by  screws.