hardware, fasteners, drills, and thread repair

© 2012 Delmar, Cengage Learning

Hardware, Fasteners, Drills, and Thread Repair

Chapter 7


Objectives• List fastener grades and describe clamping

force• Describe the various methods of thread repair• Describe characteristics of hand drills


Introduction• Good automotive technicians

– Know many things about fasteners (e.g., nuts and bolts)

• Different methods of repairing them • How not to break them


Characteristics of Fasteners• Key concepts

– Bolt • Externally threaded fastener used with a nut• When used without a nut it is a capscrew

– Fastener size • Determined by screw thread

– Tempering • Toughens a metal by heating and quenching

– Annealing • Softens a metal by heating and slowly cooling


Bolt Stretch and Fastener Grades• Bolt stretch

– Typical fastener is stretched to 70% of elastic limit

• Fastener grades– SAE bolt grades measure tensile strength

• Higher number means greater strength– ISO bolt grades

• First number is tensile strength• Second number is yield strength

– Strongest bolts are not always the best choice


Thread Lubricants• Antiseize compounds

– Used where a bolt might become difficult to remove over time

• Example: exhaust manifold bolts• Lubricants may cause hydrostatic lock

– Oil trapped in a blind hole– Bolt cannot compress the oil

• Cannot be properly tightened• Part may crack


Nuts, Washers, and Removing a Stud

• Nut grade must match bolt grade• Nut must be slightly softer than bolt

– Distortion is permanent• Nuts lose effectiveness after repeated use

– Each thread carries progressively less load• Washers are necessary for correct load on a bolt• Special stud puller may be used to remove studs

– If not available, two nuts with a lock washer between them may be used


Fastener Failures• Fatigue breaks account for 75% of fastener

problems– Bolts become fatigued from working back and

forth when too loose• Shear or torsion breaks

– Result from:• Poor grade fastener• Too much friction• Improper thread fit

• Bolts can be broken when bottomed out


Drill Bits• Used to make or enlarge holes in metal and

remove broken fasteners• Tip must be sharp

– Flatter for hard metals and steeper for softer metals

• Hand sharpening requires practice• Prevent drill bit wear by keeping to

recommended speed• Drill size is printed on the shank of the drill bit

– A micrometer can determine size of a drill bit


Taps and Threads• Tap: used to cut internal threads in a previously

drilled hole and cleans existing threads– Tapered, plug, and bottom tap

• Pipe threads– Have tapered threads designed to wedge against

each other• Tap drill

– Usually provides about 75% of a full thread


Taps and Threads (cont'd.)• Tapping a hole

– Advance the tap clockwise– Back off ¼ turn and break off any metal chips– Thread lubricants required when tapping steel

and nonferrous metals• Cast iron can be tapped dry

• Removing a broken tap– Drive a broken tap counterclockwise with a

centerpunch– EDM erodes the fastener, leaving the thread


Dies• Make external

threads on a round rod– Advance the die

and then turn backward as with a tap

• Useful for chasing burred threads


Repairing Broken Fasteners• Broken bolts may be removable with a chisel,

pliers, or stud extractor– Left-hand drill bit with reversible drill motor may

remove broken bolt that is not bottomed out• Screw extractors

– Easy out screw extractor– Extractor with flutes

• Nut may be welded to the fastener and a wrench unscrews it


Repairing Broken Fasteners (cont'd.)

• Broken fasteners can be removed by drilling and tapping the hole

• Stripped screw threads can be repaired with thread inserts– Locking inserts

• Solid-Threaded Inserts

– Spark plug inserts


Repairing Broken Fasteners (cont'd.)

• Pop rivets – Fasten sheet metal parts to engine parts

• Inexpensive and easy to use• Retaining rings

– Prevent a pin from sliding out of position

hardware, fasteners, drills, and thread repair

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