Torque Specs
A tightening torque for one specific bolted joint. Describe the fastener, what it clamps and threads into, and the thread finish; the page gives the torque in six units, the preload it makes and what limits that preload. Designers use it to put a torque on a drawing or work instruction.
What the page covers
- Torque Spec: SAE #0 to 1-1/2″, metric M1.6 to M36, or a custom thread. The target preload is 75 % of proof for a reusable joint and 90 % for a permanent one [1]. It is cut back if the clamped part would crush [2], [11], the mating thread would strip [15], [16], or tightening would pass 90 % of yield [2].
- Quick Torque Reference: preload and torque at K = 0.20 and 75 % of proof for common inch and metric grades.
- Nut Factor & Friction Classes: K by bolt condition [1] and VDI 2230 friction classes [2].
- Fastener Strength by Grade: proof, yield and tensile strength and head marking for a grade and size [8].
- Tightening Method Accuracy and Limiting Surface Pressure (pG) tables from VDI 2230 [2], [11].
Reading the results
K is the nut factor in T = K·Fi·d, with Fi the preload and d the nominal diameter. Limited by names the cap that set the preload. Preload range is the torque-control scatter: ±35 % dry or as received, ±25 % lubricated [4]. Where the Torque Goes splits the torque into bolt stretch, thread friction and under-head friction. A ! note means preload was held below target.
Limits and common mistakes
- K is an estimate; only a torque-tension test on the real joint removes the scatter.
- A locknut’s running torque makes no preload [5], [6]. Measure it and add it to the torque shown.
- Pick the finish as assembled. A dry torque on an oiled bolt overloads it.
Worked example
An aluminum cover is held to a 6061-T6 aluminum housing by 1/4-20 socket head cap screws in tapped holes 0.375″ deep. The screws are zinc plated, with flat washers. The cover comes off for service.
- Choose Thread system SAE (Inch), Size 1/4″, Thread 1/4-20 UNC, Head style Cap Head and Grade / property class Alloy Steel, 180 ksi (ASTM A574, NAS1352).
- Choose Joint type Reusable, Clamped material Aluminum 6061-T6 / 6082-T6 and Nut / mating thread Tapped hole — no nut. Leave Tapped material Same as clamped part and choose Thread engagement 1.5 D — 0.375 in.
- Leave Washer Flat washer (ASTM F436), Fastener finish / lubricant Zinc plated — K 0.20 and Threadlocker None. The Torque Spec shows:
Tightening torque: 124 in·lb (10.3 ft·lb, 14.0 N·m)
Target preload: 2,477 lbf · 56 % of proof
Limited by: Thread strip: tapped threads (FED-STD-H28/2B)
Thread strip load: 4,247 lbf
Engagement for full bolt strength: 0.506 in (2.02 D)
Check by hand: 0.20 × 2,477 lbf × 0.250″ = 124 in·lb. The aluminum threads set the torque. At 2.5 D engagement, preload returns to 75 % of proof: 3,341 lbf at 167 in·lb.
How the Torque Is Calculated
- Fi
- Target preload (clamping force)
- P
- Thread pitch
- μ
- Coefficient of friction
- d2
- Thread pitch diameter
- Db
- Mean diameter of the bearing face under the head
- kc
- Countersink wedge factor (1 for a flat face)
- Trun
- Running torque from a prevailing-torque locknut
T = Fi · ( P2π + μ d22 cos 30° + μ kc Db2 ) + Trun
| Input | What it changes |
|---|---|
| Size & thread | Sets diameter d, pitch P, pitch diameter d2, and tensile stress area At |
| Grade | Sets proof strength Sp (target preload) and yield Sy; the dropdown shows each grade's minimum tensile strength at this size |
| Joint type | Target preload Fi: 75 % of proof for a reusable joint, 90 % for permanent |
| Head style | Sets the turning-face diameter Db and, for a countersunk head, the wedge factor kc; button and countersunk socket heads are rated at 80 % of thread load |
| Washer | Moves the turning face onto the washer and spreads the clamping load through the part at 45° |
| Clamped material | Caps preload so bearing pressure under the head, nut, or washer stays at or below the material's limiting surface pressure pG |
| Nut / mating thread | Checks the mating thread for stripping — a nut is matched to the bolt's grade; a tapped hole or insert must carry the bolt's full breaking load, or preload is scaled down to match its strength |
| Fastener finish / lubricant (K) | Sets the torque factor K, or the friction coefficient μ used in the torque equation |
| Friction (from K) | Caps preload so tension plus thread torsion during tightening stays at or below 90 % of yield — the same K or friction coefficient set above, whichever source it came from |
| Locknut | Adds running torque (makes no preload); ISO 2320 halves the metric limit for a nylon insert |
| Threadlocker | Changes K by −10 to +10 %, so torque is unchanged and preload scatter widens by 10 points |
| Lubricated vs. dry | Preload scatter under torque control: ±25 % lubricated at assembly, ±35 % dry or as received |
Quick Torque Reference — K = 0.20, 75 % of Proof
| Thread | SAE Grade 5 | SAE Grade 8 | MS16995 | |||
|---|---|---|---|---|---|---|
| Preload (lbf) | Torque (ft·lb) | Preload (lbf) | Torque (ft·lb) | Preload (lbf) | Torque (ft·lb) | |
| Thread | Class 8.8 | Class 10.9 | Class 12.9 | |||
|---|---|---|---|---|---|---|
| Preload (kN) | Torque (N·m) | Preload (kN) | Torque (N·m) | Preload (kN) | Torque (N·m) | |
Nut Factor & Friction Classes
| Class | K | Typical surface + lubricant |
|---|---|---|
| A | 0.04–0.10 | Bright/black-oxide/phosphated/zinc steel, solid lubricant (MoS2, graphite, PTFE) or wax |
| B | 0.08–0.16 | Same surfaces with grease, oil, or as delivered; lubricated Al/Mg; galvanized or stainless with solid lubricant or paste |
| C | 0.14–0.24 | Bright/phosphated steel lightly oiled; dry zinc; threadlocker; stainless with wax or paste |
| D | 0.20–0.35 | Stainless with oil; dry zinc or zinc-iron; dry galvanized |
| E | ≥ 0.30 | Dry zinc-iron or zinc-nickel; dry stainless; dry Al/Mg |
| Bolt condition | K |
|---|---|
| Nonplated, black finish | 0.30 |
| Zinc plated | 0.20 |
| Lubricated | 0.18 |
| Cadmium plated | 0.16 |
| Anti-seize compound | 0.12 |
| Proprietary low-friction nuts | 0.09 |
| Condition not stated | 0.20 |
Fastener Strength by Grade
Tightening Method Accuracy
| Tightening method | Tightening Factor | Preload scatter |
|---|
Limiting Surface Pressure Under the Head
| Group | Material | Tensile Strength (MPa) | Limiting Pressure (MPa) | Limiting Pressure (ksi) |
|---|