🔩 Torque & Bolt Pattern Reference

Bolt Grade Identification, Torque Specifications & Tightening Sequences

Bolt Grades

SAE Torque

Metric Torque

Tightening Patterns

Quick Reference

🇺🇸 SAE Grade Identification (Imperial)

SAE bolts use radial lines on the head. Number of lines = Grade - 2

Grade 2 (Low Carbon Steel)

Tensile: 74,000 PSI (up to 3/4") / 60,000 PSI (over 3/4")

Use: General purpose, non-critical applications

Grade 5 (Medium Carbon Steel, Quenched & Tempered)

Tensile: 120,000 PSI (up to 1") / 105,000 PSI (over 1")

Use: Automotive, machinery, most industrial applications

Grade 8 (Alloy Steel, Quenched & Tempered)

Tensile: 150,000 PSI

Use: High-stress, heavy equipment, demanding applications

SAE Grade Quick Formula

Number of radial lines + 2 = Grade number

0 lines = Grade 2
3 lines = Grade 5
6 lines = Grade 8

🌍 Metric Property Class (ISO 898-1)

Metric bolts have the class number stamped directly on the head (e.g., 8.8, 10.9)

Class 8.8 (Medium Carbon Steel)

Tensile: 800 MPa (116,000 PSI)

Equivalent: Similar to SAE Grade 5

Use: General industrial, automotive

Class 10.9 (Alloy Steel, Quenched & Tempered)

Tensile: 1040 MPa (151,000 PSI)

Equivalent: Similar to SAE Grade 8

Use: High-strength applications, automotive

Class 12.9 (Alloy Steel, Highest Strength)

Tensile: 1220 MPa (177,000 PSI)

Equivalent: Exceeds SAE Grade 8

Use: Critical, highest stress applications

Metric Class Meaning

The property class has two numbers:

First number × 100 = Tensile strength (MPa)
Second number × 10 = Yield/Tensile ratio (%)

Example: 10.9 = 1000 MPa tensile, 90% yield ratio

🏗️ ASTM Grade Identification (Structural/Industrial)

ASTM A325

Tensile: 120,000-150,000 PSI

Use: Structural steel connections

ASTM A490

Tensile: 150,000-173,000 PSI

Use: High-strength structural

ASTM A193 B7

Tensile: 125,000 PSI

Use: High temp/pressure flanges

📊 Grade Comparison Chart

SAE Grade	Metric Class	Head Marking	Tensile Strength	Yield Strength	Proof Load	Typical Use
Grade 2	4.6 / 4.8	No lines / 4.6	74,000 PSI	57,000 PSI	55,000 PSI	Non-critical, general purpose
Grade 5	8.8	3 lines / 8.8	120,000 PSI	92,000 PSI	85,000 PSI	Automotive, machinery
Grade 8	10.9	6 lines / 10.9	150,000 PSI	130,000 PSI	120,000 PSI	Heavy equipment, high stress
—	12.9	12.9	177,000 PSI	159,000 PSI	140,000 PSI	Socket head cap screws

🔧 SAE Bolt Torque Specifications (Coarse Thread - UNC)

Values are for dry, unlubricated bolts. DRY For lubricated bolts, reduce by 15-25%. Based on 75% of proof load.

Bolt Size	TPI	Grade 2		Grade 5		Grade 8
Bolt Size	TPI	ft-lbs	N-m	ft-lbs	N-m	ft-lbs	N-m
1/4"	20	4	5	8	11	12	16
5/16"	18	8	11	17	23	24	33
3/8"	16	15	20	31	42	44	60
7/16"	14	24	33	49	66	70	95
1/2"	13	37	50	75	102	105	142
9/16"	12	53	72	110	149	155	210
5/8"	11	74	100	150	203	210	285
3/4"	10	120	163	260	353	375	509
7/8"	9	140	190	430	583	605	821
1"	8	190	258	640	868	910	1234
1-1/8"	7	-	-	800	1085	1280	1736
1-1/4"	7	-	-	1120	1519	1820	2468
1-1/2"	6	-	-	1940	2631	3160	4285

💧 Lubrication Factors

Multiply DRY torque values by these factors when using lubricants:

Condition	Factor	K Value
Dry (no lube)	1.00	0.20
Zinc plated, dry	0.90	0.17
Lightly oiled	0.75	0.15
Engine oil	0.65	0.13
Anti-seize compound	0.60	0.12
Moly paste	0.55	0.11

⚠️ Critical Warning

Using DRY torque values on lubricated bolts will result in over-tightening, potentially causing bolt stretch, thread damage, or breakage!

📐 Torque Formula

                        T = K × D × F
                    

T = Torque (in-lbs)

K = Friction coefficient (0.15-0.20 typical)

D = Nominal bolt diameter (inches)

F = Desired clamp load (lbs)

Clamp Load Target

Standard practice: Clamp load = 75% of proof load. This provides adequate clamping while maintaining a safety margin below yield.

🔧 Metric Bolt Torque Specifications (Coarse Thread)

Values are for dry, unlubricated bolts. DRY For lubricated bolts, reduce by 15-25%. Based on 75% of proof load.

Size	Pitch	Class 8.8		Class 10.9		Class 12.9
Size	Pitch	N-m	ft-lbs	N-m	ft-lbs	N-m	ft-lbs
M5	0.8	5.5	4	8	6	9	7
M6	1.0	9.5	7	14	10	16	12
M8	1.25	23	17	33	24	39	29
M10	1.5	46	34	66	49	77	57
M12	1.75	80	59	115	85	135	100
M14	2.0	127	94	183	135	215	159
M16	2.0	198	146	285	210	335	247
M18	2.5	273	201	393	290	460	339
M20	2.5	385	284	555	409	650	479
M22	2.5	530	391	760	561	890	657
M24	3.0	670	494	960	708	1125	830
M27	3.0	980	723	1410	1040	1650	1217
M30	3.5	1330	981	1910	1409	2240	1652
M36	4.0	2320	1711	3340	2464	3910	2884

🔄 SAE to Metric Conversion

SAE Size	Nearest Metric	Notes
1/4"	M6	M6 slightly smaller
5/16"	M8	Close match
3/8"	M10	M10 slightly smaller
7/16"	M11	Not common metric
1/2"	M12	Close match
5/8"	M16	M16 slightly smaller
3/4"	M20	M20 slightly larger
1"	M24/M27	Between sizes

⚠️ Never Interchange

SAE and metric threads are NOT interchangeable. Attempting to mate them will damage threads and create an unsafe joint.

📏 Fine Thread Adjustment

Fine thread bolts (UNF/metric fine) have approximately 10-15% higher torque values than coarse thread due to increased tensile stress area.

Thread Type	Factor	Application
Coarse (UNC)	1.00	General purpose, easier assembly
Fine (UNF)	1.10-1.15	Vibration resistance, precision
Metric Coarse	1.00	Standard metric
Metric Fine	1.10-1.15	Automotive, precision

⭐ Star (Cross) Pattern

8-Bolt Star Pattern

Tighten opposite bolts in sequence: 1→2→3→4→5→6→7→8

Best For

Standard flanges (12 bolts or fewer)
All gasket types including RTJ
Industry standard per ASME PCC-1

🔢 Tightening Sequence Steps

Hand Tight SNUG

Install all bolts finger-tight, ensure gasket is centered

Pass 1 30%

Torque all bolts to 30% of final value in star pattern

Pass 2 60%

Torque all bolts to 60% of final value in star pattern

Pass 3 100%

Torque all bolts to final value in star pattern

Circular Pass VERIFY

Go around clockwise at 100% until no nut movement

⚠️ Common Mistakes

Tightening adjacent bolts in sequence
Applying full torque in one pass
Skipping the final circular pass
Not verifying torque wrench calibration

4️⃣ 4-Bolt Pattern

Criss-cross: 1→2→3→4

🔘 12-Bolt Pattern

Follow numbered sequence

📋 Bolt Pattern by Number of Bolts

# Bolts	Sequence	Notes
4	1-2-3-4 (opposite pairs)	Simple criss-cross pattern
6	1-4-2-5-3-6	Skip 2 bolts, tighten opposite
8	1-5-3-7-2-6-4-8	Standard star pattern
12	1-7-4-10-2-8-5-11-3-9-6-12	Skip 5, then work around
16	1-9-5-13-3-11-7-15-2-10-6-14-4-12-8-16	Consider quadrant method
20+	Quadrant or Modified Star	Use ASME PCC-1 alternative patterns

✓ Key Principle

Always tighten opposite bolts to bring the joint together evenly. Never tighten adjacent bolts in sequence - this causes uneven gasket compression and potential leaks.

👁️ Visual Grade ID Summary

✅ Pre-Torque Checklist

☐ Verify bolt grade matches specification
☐ Inspect threads for damage/corrosion
☐ Clean mating surfaces
☐ Check gasket condition and alignment
☐ Apply lubricant if specified
☐ Verify torque wrench is calibrated
☐ Know if spec is DRY or LUBRICATED
☐ Use correct tightening pattern
☐ Tighten in multiple passes (30-60-100%)
☐ Perform final circular verification pass

📊 Common Application Torque Quick Reference

Application	Typical Bolt	Grade	Torque (ft-lbs)	Notes
Wheel lug nuts (car)	1/2"-20	Grade 8	80-100	Check vehicle spec
Wheel lug nuts (truck)	9/16"-18	Grade 8	120-140	Check vehicle spec
Head bolts (typical)	Various	TTY/Grade 8	Varies	Always use OEM spec!
Connecting rod	3/8"-24	ARP/Grade 8	45-50	Use OEM spec
Flywheel bolts	7/16"-20	Grade 8	60-75	Use thread locker
Valve cover	1/4"-20	Grade 5	6-8	Don't overtighten
Flange bolts (150#)	5/8"-11	B7	60-90	Per gasket spec
Flange bolts (300#)	3/4"-10	B7	115-175	Per gasket spec
Bearing housing	1/2"-13	Grade 8	75-90	Precision important
Coupling bolts	Various	Grade 8	Per OEM	Check alignment first

⚠️ Always Use OEM Specifications

This table is for general reference only. Critical applications (engines, structural, pressure) require manufacturer specifications. Torque values vary based on bolt material, lubrication, and joint design.

📚 Standards Reference

Standard	Scope
SAE J429	Mechanical properties of externally threaded fasteners
ISO 898-1	Metric bolt property classes
ASTM A193	Alloy steel bolts for high temp service
ASTM A325/A490	Structural bolts for steel construction
ASME PCC-1	Bolted flange joint assembly guidelines
ASME B16.5	Pipe flanges and flanged fittings

🔧 Torque Wrench Tips

Best Practices

Calibrate at least annually (or per mfr spec)
Store at lowest setting to preserve spring
Pull smoothly, don't jerk
Stop when click is felt
Keep perpendicular to fastener axis
Don't use as a breaker bar
Don't exceed rated capacity

Torque Wrench Types

Click type: Most common, audible/tactile feedback
Beam type: Simple, reliable, needs visual reading
Digital: Precise readout, good for documentation
Dial type: Visual indicator, good for inspection