Fasteners and Hardware 101
A quick reference on bolt grades, metric and imperial thread pitches, torque specifications, and locking mechanisms.
TL;DR
- Match thread diameter and
thread-pitchprecisely before threading components. - Identify bolt tensile strength using head
grade-markings. - Secure critical fasteners against vibration using anaerobic
thread-locker.
Thread Sizing Standards
Metric Thread CalloutSpecifies nominal diameter in mm followed by thread pitch in mm.
format: M8 x 1.25 // 8mm diameter, 1.25mm pitchImperial UNC vs UNFUnified National Coarse vs Unified National Fine threads per inch.
format: 3/8-16 UNC // 3/8 inch dia, 16 threads/inchThread Engagement RuleMinimum depth of engagement required in tapped blind holes.
steel: 1.0x_diameter | aluminum: 1.5x_diameterFastener Strength Grades
Metric Class 8.8Standard high-strength automotive grade hardware.
tensile_strength: 800_mpa | yield_ratio: 80%Metric Class 10.9High tensile strength hardware for suspension and cylinder heads.
tensile_strength: 1040_mpa // comparable to SAE Grade 8SAE Grade 8Premium carbon alloy steel for extreme shear loads.
head_marking: 6_radial_slashes | strength: 150_ksiRetention and Locking
Nyloc NutNylon insert grips mating male threads to resist vibrational back-off.
reuse_limit: discard_when_nylon_threads_degradeCotter Pin and Castle NutPositive mechanical pin lock through drilled spindle hole.
application: wheel_bearings_and_tie_rod_endsNord-Lock WashersPaired wedge-locking washers that use tension instead of friction.
mechanism: cam_angle_greater_than_thread_pitchTips
- Always lubricate bolt threads when a manual specifies a
wet-torquevalue to prevent severe over-tensioning. - Use hardened flat washers under bolt heads to distribute clamping load evenly across aluminum castings.
Warnings
- Never reuse
torque-to-yieldstretch bolts that have undergone plastic metallurgical deformation. - Avoid mixing metric and imperial hardware; forced threading destroys tapped holes immediately.
In Practice
Calculate required torque setting based on bolt diameter and friction.
- Determine nominal bolt diameter and proof load rating.
- Select friction coefficient for dry vs lubricated threads.
- Calculate target axial clamping force at 75% of yield.
- Compute target wrench torque value in foot-pounds.
def calculate_bolt_torque(dia_in, target_preload_lbs, k_friction=0.20):
# Torque = K * D * Preload
torque_in_lbs = k_friction * dia_in * target_preload_lbs
torque_ft_lbs = torque_in_lbs / 12.0
return round(torque_ft_lbs, 1)FAQ
Radial lines indicate SAE imperial grades (3 lines mean Grade 5; 5 lines mean Grade 8). Metric bolts display embossed numerical ratings like 8.8, 10.9, or 12.9, representing tensile and yield strengths.
Medium-strength blue-threadlocker secures fasteners subject to vibration while still allowing removal with standard hand tools. High-strength red-threadlocker creates a permanent bond requiring high heat for disassembly.
Split lock washers provide spring tension to keep tension on the fastener during slight loosening. However, for severe vibration, nylon-insert lock-nuts or thread-locking fluid provide superior loosening resistance.