ISO 965-1 UNDERSTANDING THREAD CALLOUTS ON DRAWINGS

BASIC FORMAT

M [Diameter] × [Pitch] – [Tolerance Class] – [Options]

Example: M10 × 1.5 – 6H/6g – L

SYMBOL MEANINGS

Symbol	Meaning	Example
M	Metric thread	M (always first)
Number (first)	Nominal diameter in mm	M10 = 10mm diameter
× Number	Pitch in mm (distance between threads)	M10 × 1.5 = 1.5mm pitch
Number + LETTER	Tolerance class (quality + fit)	6H (internal), 6g (external)
/	Separates internal/external tolerance	6H/6g (nut/bolt fit)
S, N, L	Engagement length: Short, Normal, Long	– L (long engagement)
LH	Left-hand thread	– LH (turns counterclockwise)

TOLERANCE CLASS EXPLAINED

Part	Meaning	Example
Number (3-9)	Tolerance grade (precision level)	6 = medium quality (standard)
UPPERCASE letter	Internal thread position (nut/hole)	H = basic, G = allowance for coating
lowercase letter	External thread position (bolt/screw)	h = basic, g,f,e = allowance for coating

TOLERANCE GRADES TABLE

ALL TOLERANCE GRADES

Grade	Quality Level	Description	Typical Applications
3	Extra Fine	Highest precision, tightest tolerances	Precision instruments, aerospace, special applications
4	Fine	High precision	Precision machinery, measuring equipment, aerospace
5	Fine	Precision threads	Quality machinery, precision assemblies
6	Medium (STANDARD)	General commercial quality	Standard bolts/nuts, automotive, general engineering (MOST COMMON)
7	Medium-Coarse	Relaxed tolerances	General assemblies, non-critical applications
8	Coarse	Large tolerances	Hot-rolled bars, rough machining, long blind holes
9	Extra Coarse	Largest tolerances, easiest manufacturing	Very rough conditions, difficult manufacturing situations

Note: Higher numbers = Larger tolerances = Easier to manufacture but less precise fit

COATED THREADS

Position	Thread Type	Purpose	When to Use
H	Internal (nut/hole)	No coating allowance	Uncoated threads, standard fit
G	Internal (nut/hole)	Extra clearance for coated bolts	Mating with plated/galvanized bolts
h	External (bolt/screw)	No coating allowance, maximum metal	Uncoated precision bolts
g	External (bolt/screw)	Small allowance	Thin coatings (zinc, cadmium), standard bolts
f	External (bolt/screw)	Medium allowance	Medium thickness coatings
e	External (bolt/screw)	Large allowance	Thick coatings (hot-dip galvanizing)

Key Points:

G position = Internal thread with extra clearance to accept coated external threads
g, f, e positions = External threads before coating (undersized to accommodate coating thickness)
Common coated combinations: 6G/6g, 6H/6e, 7G/6h

COMMON EXAMPLES ON DRAWINGS

Designation	Read As
M8	8mm metric thread, coarse pitch (pitch not shown = standard)
M10 × 1.5	10mm diameter, 1.5mm pitch (fine pitch, must be shown)
M12 – 6H	12mm internal thread, grade 6, position H (tapped hole/nut)
M12 – 6g	12mm external thread, grade 6, position g (bolt/screw)
M8 – 6H/6g	8mm thread fit: 6H internal with 6g external (assembly spec)
M12 – 6G	12mm internal thread with allowance for coated bolts
M10 – 6e	10mm external thread before coating (large allowance for thick coating)
M16 – 6G/6g	16mm fit for coated threads (G allows for coating on bolt)
M20 × 2 – 6H – L	20mm dia., 2mm pitch, internal 6H, long engagement
M10 × 1 – 6g – LH	10mm dia., 1mm pitch, external 6g, left-hand thread
M16 × Ph3P1.5	16mm dia., 3mm lead, 1.5mm pitch (2-start thread)

QUICK TIPS

No pitch shown? = Coarse pitch (standard for that diameter)
No tolerance class? = 6H for internal (≥M1.6), 6g for external (≥M1.6)
No S/N/L shown? = Normal engagement (most common)
Uppercase letters (H, G)? = Internal threads (nuts, tapped holes)
Lowercase letters (h, g, f, e)? = External threads (bolts, screws)
Position G? = Extra clearance in internal thread for coated external threads
Position g, f, or e? = External thread designed for coating (measured before coating)
6H/6g is most common = Standard fit for uncoated general purpose threads
6G/6g or 6H/6e = Common for galvanized/plated bolts

MOST COMMON DESIGNATIONS

For Internal Threads (Holes/Nuts)	For External Threads (Bolts/Screws)
M8 – 6H	M8 – 6g
M10 – 6H	M10 – 6g
M12 – 6H	M12 – 6g
M16 – 6H	M16 – 6g

Remember: If you see just “M8” or “M10” on a drawing without tolerance class, it typically means standard medium quality (6H for holes, 6g for bolts).