Fasteners, Sealants, and Gaskets

2. 3 Fasteners, Sealants, and Gaskets

3. Learning Objectives • Identify fasteners used on small gas engines and implements. • Remove and install various fasteners correctly. • Repair or produce internal and external threads. • Properly select and install fasteners. • Remove, select, and install gaskets correctly.

4. Threaded Fasteners • Thread • Inclined plane that circles the cylindrical bolt or hole • Incline of bolt or screw thread must be the same as incline of thread in nut or threaded hole into which it is placed

5. Bolts • Hold parts together by squeezing them between the head and a nut • Holes bolt passes through is not threaded • Shank is unthreaded portion of bolt near head • Shank should pass all the way through top part and partially through second component

6. Bolts • Should be long enough to pass through parts, washers and nut, and protrude 1 ½ to 2 threads beyond nut

7. Nuts

8. Nuts • Hexagon nut • Loosened or tightened with standard sockets or box-end wrenches • Square nut • Found on older implements • Jam nut • Used with hexagon nut to produce locking condition

9. Nuts • Castle nut • Used when axial clearance is required • Used on bolts or shafts when a component turns or pivots on it • Cotter pin used to prevent nut from turning

10. Nuts • Acorn nut • Provides smooth, neat appearance • Wing nut • Can be tightened or loosened by hand • Lock nut • Reduces tendency for vibration or motion to loosen nut

11. Bolt and Nut Terminology • Bolt size • Bolt head size • Bolt length • Thread pitch • Thread length

12. Bolt Grades • Minimum tensile strength of bolt • Bolt head markings indicate bolt grade • On SAE bolts, number of marks is two less than grade number • Grade 1 and 2 bolts have no markings • Metric bolts are marked with 5D, 8G, 10K, or 12K

13. Bolt Grades • Load applied to bolt during tightening coupled with external load could exceed tensile limit of bolt • When bolt reaches its load limit, it exceeds its elastic limit and begins to stretch plastically • A bolt that stretches plastically will not return to its original length or shape when load is released

14. Thread Terminology

15. Thread Types • Unified National Coarse (UNC) series • For general use • Should not be subjected to vibration • Unified National Fine (UNF) series • For work where vibration is a factor • Metric (M) series • Formed with a 60° angle

16. Thread Types • Right-handed thread • Turned clockwise to tighten • Left-handed thread • Turned counterclockwise to tighten • The letter L may be stamped on the fastener

17. Thread Fit • Class 1 fit • Largest manufacturing tolerance • Used where ease of assembly is desirable • Class 2 fit • Used on largest percentage of threaded fasteners • Class 3 fit • Will be tight when assembled

18. Thread Designations • Series of numbers and letters used to describe a bolt or thread • Standard designation ½-13 UNC-2A  1 • Metric designation M-10  1.5  25

19. Screws • Hold parts together by passing through one part and threading into another

20. Screw Types • Machine screws • Used in threaded holes • Cap screws • Similar to bolts but smaller in diameter • Set screws • Secure rotating components • Self-tapping screws • Cut threads

21. Screw Head Types

22. Tightening and Loosening Threaded Fasteners • Right-handed threads • Turned clockwise for tightening and counterclockwise for loosening • Left-handed threads • Turned counterclockwise for tightening and clockwise for loosening • Bolts, nuts, and screws on engines should be tightened to specific torque

23. Removing Broken Fasteners • If fastener protrudes from hole, use vise grips or pipe wrench • If fastener breaks off flush with hole, it can be driven out with hammer and small punch • If fastener breaks off even with or below surface of hole, it can be removed using a screw extractor

24. Using a Screw Extractor

25. Chasing Threads • Threading tap used to recut damaged threads • A taper tapshould be used to chase a through hole • Damaged bolt or screw threads can be chased with a threading die

26. Internal Thread Repair

27. Tapping New Threads • Before tapping, a new hole must be drilled • Proper hole diameter obtained from tap drill chart • Start tap straight in hole • Use cutting fluid • Tap wrench should be turned counterclockwise two-thirds of a turn and then reversed one-third of a turn to break and clear chips. • Continue through or to bottom of hole

28. Tapping New Threads

29. Threading • Threading is done with a die. • Die is mounted in die stock • Threading procedure is similar to tapping procedure • Diameter of rod being threaded must be same as major diameter of thread

30. Washers • Flat washers • Provide wide bearing surface • Lock washers • Prevent loosening of bolts, screws, or nuts • Kantlink washer • Wide bearing lock washer • Toothed washer

31. Common Washers

32. Pins • Used to retain parts in fixed position or preserve part alignment

33. Retaining Rings • Spring steel fasteners that fit into a groove in part • Prevent lateral movement of shafts in hole • Do not prevent the shaft from rotating in hole • Removed and installed with snap ring pliers

34. Retaining Rings • External retaining is placed in groove in surface of shaft • Internal retaining ring fits into groove in hole

35. Keys • Used on shafts that have a component that fits and rotates with the shaft • Recess in shaft is the keyseat • Groove in component is the keyway

36. Adhesives and Sealants • Liquid or Semiliquid • Can be sprayed, brushed, or spread on • Some set up hard, while others remain pliable

37. Thread Adhesives • Prevent nuts, bolts, or screws from loosening during service • Strengths vary from light (removable) to high strength (require applied heat for removal) • Place one drop of adhesive on thread prior to fastening • Cure time varies from 30 minutes to 24 hours

38. Sealants • Resistant to oil, water, gas, grease, and salt solutions • Form-in-place sealants • Used in place of conventional gaskets • Room temperature vulcanizing (RTV) sealant • Form-in-place sealant that can be used on both rigid and flexible mating surfaces • Anaerobic sealants • Can be used as a thread locking material or between two machined surfaces

39. Antiseize Compounds • Applied to threaded fasteners and metal components exposed to constant heat • Prevents metal material from being cold welded together • Can be used for exhaust system connections

40. Gaskets • Soft pliable material • Used between engine parts to seal and prevent leakage • Type of gasket material depends on functions and conditions of parts being joined

41. Gasket Rules • Inspect for leaks before disassembling engine • Do not damage mating surfaces during disassembly • Remove old gasket material carefully • Wash and dry parts thoroughly • Check new gasket fit

42. Gasket Rules • Use gasket sealant as directed • Some gaskets do not use sealant • Start all fasteners by hand • Tighten fasteners in small steps • Use a crisscross tightening pattern • Do not overtighten fasteners

43. Bolt Tightening Pattern

44. List five conditions that fasteners are exposed to during engine operation. Heating and cooling, cyclic loading, tensile and shearing loads, corrosion, and vibration

45. What is the most common type of nut used in small engine applications? Hexagon nut

46. What is tensile strength? Tensile strength is the amount of tension force a fastener can withstand before failing.

47. What is the difference between a machine screw and a cap screw? Machine screws have round heads that accept a screwdriver or Allen wrench. Cap screws have hexagonal heads.

48. What is the difference between tapping and threading? Tapping is the process of cutting threads in a hole. Threading is the process of cutting threads on an external cylindrical surface.

49. What type of tool should be used to remove a fastener that breaks off below the surface of a hole? A screw extractor

50. How should parts sealed with a gasket be tightened? They should be tightened in small steps using a crisscross pattern.