TM 9--2520--215--34 0086 00--4 TORQUE LIMITS AND STUD SELECTION – CONTINUED 0086 00 TIGHTNENING METAL FASTENERS When torquing a fastener, select a torque wrench whose range (Table 3) fits the required torque value. A torque wrench is most accurate from 25% to 75% of its stated range. A torque wrench with a stated range of 0 to 100 will be the most accurate from 25 to 75 Pound--Feet. The accuracy of readings will decrease as you approach 0 Pound--Feet or 100 Pound--Feet. The following ranges (Table 3) are based on this principle. Table 3. TORQUE RANGES STATED RANGE MOST EFFECTIVE RANGE 0--2000 lb--in 4--13 lb--ft 0--600lb--ft 50--450 lb--ft 0--170 lb--ft 44--131 lb--ft 15--75 lb--ft 30--60 lb--ft FASTNENER SIZE AND THREAD PATTERN Threaded fasteners are categorized according to diameter of the fastener shank. Thread styles are divided into broad groups, the two most common being coarse (Unified Coarse--UNC) and fine (Unified Fine--UNF). These groups are defined by the number of threads per inch on the bolt shanks. In addition, threads are categorized by thread class (Table 4), which is a measure of degree of fit between the threads of the bolt or screw (external threads) and the threads of the attaching nut or tapped hole (internal threads). The most common thread class for bolts and screws is Class 2. TABLE 4 THREAD CLASSES AND DESCRIPTION EXTERNAL INTERNAL FIT 1A 1B LOOSE FIT 2A 2B MEDIUM FIT 3A 3B CLOSE FIT Thread patterns are designed as follows: NOMINAL SIZE NO. THREADS/INCH THREAD SYMBOL THREAD CLASS 3/4 10 UNC 2A Note: Unless followed with –LH (e.g. ¾--1OUNC--2A--LH), threads are right hand. LENGTH (BOLT SHANK) THICKNESS OR HEIGHT OF HEAD BODY LENGTH THREAD LENGTH NOMINAL DIAMETER THREAD RUNOUT LENGTH GRIP LENGTH HEIGHT OF NUT