Hardware, Fasteners, Drills, and Thread Repair

1. Hardware, Fasteners, Drills, and Thread Repair Chapter 7

2. Objectives • List fastener grades and describe clamping force • Describe the various methods of thread repair • Describe characteristics of hand drills

3. Introduction • Good automotive technicians • Know many things about fasteners (e.g., nuts and bolts) • Different methods of repairing them • How not to break them

4. 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

5. 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

8. 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

9. 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

11. 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

12. 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

14. 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

15. 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

16. Dies • Make external threads on a round rod • Advance the die and then turn backward as with a tap • Useful for chasing burred threads

17. 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

19. 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

20. 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