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Hardware, Fasteners, Drills, and Thread Repair
Chapter 7
© 2012 Delmar, Cengage Learning
Objectives• List fastener grades and describe clamping
force• Describe the various methods of thread repair• Describe characteristics of hand drills
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Introduction• Good automotive technicians
– Know many things about fasteners (e.g., nuts and bolts)
• Different methods of repairing them • How not to break them
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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
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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
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
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
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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
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
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
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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
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Dies• Make external
threads on a round rod– Advance the die
and then turn backward as with a tap
• Useful for chasing burred threads
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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
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
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
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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
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning