7. Thread Terminology
External (male) thread: Thread cuts outside of
the cylindrical body.
Internal (female) thread: Thread cuts inside of
the cylindrical body.
Internal thread
External thread
7
8. Thread Terminology
Crest – Peak edge of thread
Root – Bottom of thread cut into a cylindrical
body
Thread angle – Angle between threads faces
External Thread
CrestCrestRootRoot
Thread angleThread angle
Internal Thread
CrestCrestRootRoot
8
9. Thread Terminology
Major diameter – Largest diameter on an
external or internal thread
Minor diameter – Smallest diameter on an
external or internal thread
Internal ThreadExternal Thread
Minordia.Minordia.Minordia.
Majordia.Majordia.Majordia.
Minordia.Minordia.
Majordia.Majordia.
9
10. Thread Terminology
Pitch – Distance between crests of
threads
Internal ThreadExternal Thread
PitchPitch
PitchPitch
10
11. Types of Thread Form
Profile shape of the thread
Example :
“knuckle thread form”
Example :
“knuckle thread form”
11
12. Thread Standard
A metric ISO screw thread is designated by the letter M
followed by the value of the nominal diameter D and
the pitch P, both expressed in millimeters and
separated by the multiplication sign “×”.
Example: M8×1.25
10.0010.111.7512.00M12
8.508.381.5010.00M10
6.756.651.258.00M8
5.004.921.006.00M6
Tap drill sizeMinor
diameter
PitchMajor
diameter
Nominal
size
10.0010.111.7512.00M12
8.508.381.5010.00M10
6.756.651.258.00M8
5.004.921.006.00M6
Tap drill sizeMinor
diameter
PitchMajor
diameter
Nominal
size
Minor diameter ≈ Tap drill size
Metric thread
12
13. Thread Drawings
For simplified representation, use thick and thin
continuous lines for representing crests and
roots respectively:
External thread Internal thread
Pitch/2Pitch/2
RootRoot
CrestCrest
Thread runout
13
14. Bolt Terminology
Bolt is a threaded cylinder with a
head
Hexagonal head
bolt and nut
Head
thickness
Head
thickness
Thread lengthThread length
LengthLength
Width
across flat
Width
across flat
14
16. Bolt Application - Engine
16
• Engine case bolt
and nut on a
Lycoming IO-320
engine
• Nut is on the left
hand side, bolt
head is on the right
17. Screw Terminology
Screw is similar to bolt, but has a longer
thread than a bolt
Bolted joint is usually created by turning
a nut; screw is installed by turning it.
17
18. Screws Application
x drill,
CSK to φy
x drill, w deep,
CSK to φy
yy
Draw 90oDraw 90o
1. Drill a hole.
2. Place the screw.
18
19. Screws Application - Fuselage
19
• Countersunk screws
are flush with the skin
on this Cessna 310R