Flange Bolt Torque Chart: PCC-1 Values, the K Factor, and the Three-Pass Method
Most flange bolt torque charts on the internet are a single column of numbers with no stated assumptions. This one is computed in the open: A193 B7 studs, MoS2 lubrication, mid-band target stress per the ASME PCC-1 Appendix O framework — every input disclosed, every value reproducible.
Quick Answer
A representative PCC-1 style torque value: a 4" Class 300 flange (8 × 3/4"-10 studs, A193 B7, MoS2 paste, K = 0.13) tightens to about 215 N·m (157 ft·lb) per bolt; a 6" Class 600 flange (12 × 1-1/8"-8 studs) needs roughly 750 N·m (555 ft·lb) per bolt. The full table is in Section 3. The value moves ±50% with lubrication alone, which is why the assumptions matter more than the table. For your exact joint — different material, lube, gasket or temperature — use theflange bolt torque calculator.
Table of Contents
1. What a Torque Chart Actually Assumes
Every torque chart — printed or online — is the output of one short equation with three inputs:
- →A_t — tensile stress area from ASME B1.1 (0.1419 in² for 1/2-13, 0.606 in² for 1-8). Fixed by the stud size; no controversy.
- →K — empirical nut factor set by lubrication: ~0.12 PTFE-coated to 0.35+ dry and rusty. Charts almost never state theirs.
- →S_target — target bolt stress. ASME PCC-1 Appendix O frames it as a band: roughly 40–70% of the stud material's yield, positioned by gasket seating needs. Charts almost never state theirs either.
Two hidden multiplicative inputs mean two charts can differ by 50% while both look authoritative. The table below removes the ambiguity: it states every assumption and shows how to rescale for yours.
Two more conventions hide inside "the chart" that rarely get stated. First, unit discipline: mixing inch-based diameters with metric torque produces garbage at the fourth significant figure, so serious tools compute in SI and convert only for display (1 ft·lb = 1.3558 N·m exactly). Second, temperature: the table below is ambient. At 400 °C an A193 B7 stud retains roughly 85% of its yield, so either derate the target stress or accept the loss deliberately. Our calculator does this derating automatically; paper charts simply go stale above ambient.
2. PCC-1 Style Reference Torque Table
Computed at build time with disclosed inputs: A193 B7 studs · MoS2 paste (K = 0.13) · spiral-wound gasket (mid-band target stress) · 20 °C · machine-oiled assembly values shown for comparison. Per-bolt torque; tighten all bolts to this value in three passes (Section 5).
| NPS | Class | Bolts × Stud | Torque N·m (K=0.13) | ft·lb | N·m @ K=0.15 (oil) |
|---|---|---|---|---|---|
| 1" | 150 | 4 × 1/2"-13 UNC | 60 | 44 | 69 |
| 2" | 150 | 4 × 5/8"-11 UNC | 120 | 88 | 138 |
| 4" | 150 | 8 × 5/8"-11 UNC | 120 | 88 | 138 |
| 6" | 150 | 8 × 3/4"-10 UNC | 213 | 157 | 246 |
| 8" | 150 | 8 × 3/4"-10 UNC | 213 | 157 | 246 |
| 12" | 150 | 12 × 7/8"-9 UNC | 343 | 253 | 395 |
| 1" | 300 | 4 × 5/8"-11 UNC | 120 | 88 | 138 |
| 2" | 300 | 8 × 5/8"-11 UNC | 120 | 88 | 138 |
| 4" | 300 | 8 × 3/4"-10 UNC | 213 | 157 | 246 |
| 6" | 300 | 12 × 3/4"-10 UNC | 213 | 157 | 246 |
| 8" | 300 | 12 × 7/8"-9 UNC | 343 | 253 | 395 |
| 12" | 300 | 16 × 1-1/8"-8 8UN | 754 | 556 | 870 |
| 2" | 600 | 8 × 5/8"-11 UNC | 120 | 88 | 138 |
| 4" | 600 | 8 × 1"-8 UNC | 514 | 379 | 593 |
| 6" | 600 | 12 × 1-1/8"-8 8UN | 754 | 556 | 870 |
| 8" | 600 | 12 × 1-1/4"-8 8UN | 1060 | 782 | 1223 |
| 12" | 600 | 20 × 1-1/2"-8 8UN | 1898 | 1400 | 2190 |
Stud sizes derive from ASME B16.5 bolt-hole data — see theflange bolt size chart for the full Class 150–2500 matrix. Torque scales linearly with K and with target stress: values outside these assumptions are a multiplication, not a mystery.
3. How the K Factor Rescales the Table
K is empirical — it lumps thread friction and nut-face friction into one number. Only ~10–15% of applied torque becomes preload; the rest is friction. Change the friction and the same setpoint produces a different bolt load:
| Lubrication | K | Torque multiplier vs MoS2 |
|---|---|---|
| Dry (as-received) | 0.20 | × 1.54 |
| Machine oil | 0.15 | × 1.15 |
| MoS2 paste | 0.13 | × 1.00 |
| PTFE / anti-seize coating | 0.12 | × 0.92 |
The dangerous direction is applying a lubricated table value to dry threads: at K = 0.30 (common for dry, as-received studs) the joint receives only 13/30 ≈ 43% of the intended preload. That is a leak waiting for startup. Match the K in the table to the lubricant actually on the bolts.
Worked rescaling: the 4" Class 300 row reads 213 N·m. Your crew applies machine oil (K = 0.15) instead of MoS2? Multiply by 0.15/0.13 → 246 N·m. Your engineer also sets the target at the band top (73.5 ksi) for an RTJ gasket? 213 × (0.15/0.13) × (73.5/57.75) ≈ 312 N·m. Every chart value is just one point on a surface you can move along — once the inputs are named.
4. Target Bolt Stress by Material
Within the PCC-1 Appendix O framework, target stress sits in a band between about 40% and 70% of yield, positioned by gasket seating requirements. The table below lists the approximate bands and defaults used by our calculator:
| Material | Yield (ksi) | Default target (ksi) | Band (ksi) |
|---|---|---|---|
| ASTM A193 B7 | 105 | 52.5 | 42 – 73.5 |
| ASTM A193 B7M | 80 | 40 | 32 – 56 |
| ASTM A193 B8 Cl.2 | 75 | 30 | 30 – 52.5 |
| ASTM A193 B8M Cl.2 | 75 | 30 | 30 – 52.5 |
| ASTM A320 L7 | 105 | 52.5 | 42 – 73.5 |
Disclosure: no authoritative public reprint of the PCC-1 (2013) target-stress table (Table O-3) exists; these are approximate engineering defaults consistent with the published 40–70% framework (the widely cited 73.5 ksi B7 maximum equals 70% of 105 ksi yield). Confirm final targets against the gasket manufacturer or a full Appendix O calculation.
5. The Three-Pass Star Tightening Method
Torque values assume an assembly procedure. PCC-1 specifies a cross (star) pattern in increments, because of elastic interaction — every bolt you tighten relaxes the ones before it:
- Pass 1 — 30%: all bolts in star order; verify flange alignment and gasket position afterwards.
- Pass 2 — 60%: same star order.
- Pass 3 — 100%: full target torque, star order.
- Final circular pass at 100% until no nut rotates — this recovers cross-talk losses.
Skipping passes is how joints end up "torqued to spec" and still leak: documented cases show early bolts losing over half their preload when the rest of the flange is tightened in a single pass.
6. How to Verify a Torque Value Before Trusting It
- 1.Reproduce it. T = K·D·F with F = S·A_t. If you can't tell what K and S the chart used, you can't trust the number. Ours are stated above and adjustable in the calculator.
- 2.Cross-check the magnitude. A 3/4" B7 stud lubricated should land in the 170–210 ft·lb region for mid-band targets. A chart saying 90 or 350 ft·lb for that joint has a hidden assumption problem.
- 3.Check the gasket. The manufacturer's seating stress can move the target up or down within the band — soft PTFE sheet down, RTJ metal up.
- 4.Measure on critical joints. A Skidmore-calibrated sample or ultrasonic bolt length measurement converts torque tables into verified preload — ±5% instead of ±30%.
Get the Torque for Your Exact Joint
Material, lubrication, gasket type and temperature as explicit inputs — with a three-pass table and copyable result for your work pack.
Frequently Asked Questions
Are these torque chart values safe to use directly?
They are a sound starting basis — arguably better sourced than most published charts, because every input is disclosed (A193 B7 studs, MoS2 K = 0.13, mid-band target stress per the PCC-1 Appendix O framework, 20 deg C). But the gasket manufacturer's seating stress requirement always governs: a spiral wound gasket may need more, a soft rubber sheet less. For critical or leak-sensitive service, confirm the target with the full Appendix O calculation or the gasket vendor, and verify actual K on a sample joint.
Why does the same flange appear with different torque in different charts?
Because T = K x D x F. Charts that publish a single number per flange are hiding their assumed K (0.12 to 0.35 depending on lubrication) and their assumed target bolt stress (30 to 73.5 ksi depending on material philosophy). Both appear multiplicatively in the result, so two 'reputable' charts can legitimately differ by 30-50% for the same joint. The fix is not averaging the charts — it is using a calculator that takes K, material and gasket as explicit inputs.
What is the three-pass tightening method in PCC-1?
Tighten all bolts in a cross (star) pattern to about 30% of target torque to seat the gasket evenly, re-check flange alignment, then 60%, then 100% — still in star pattern. Finish with a circular pass at 100% until no nut rotates. The passes exist because of elastic interaction: each bolt you tighten relaxes the previous ones, and a single-pass assembly can leave early bolts at half their intended load.
Can I use this chart for stainless or low-temperature bolting?
The chart itself is computed for A193 B7. For A193 B8/B8M stainless, the target stress drops to about 30 ksi (roughly -43% torque versus this chart at the same K) because strain-hardened stainless is limited by galling, not just yield. A320 L7 matches B7 at ambient. Use the calculator to switch materials, lubrication and temperature — the chart is the illustration, the calculator is the tool.
How do I convert these values for a hydraulic torque wrench?
Hydraulic torque wrenches apply the same T = K x D x F relationship — the chart torque is the tool setpoint per bolt, per pass. Two practical notes: operate in the middle of the wrench's range (not below 20% of capacity, where calibration error grows), and convert precisely — 1 ft-lb = 1.3558 N-m. Apply the 30/60/100% passes as separate tool pressure settings.
Where do the stud sizes in the chart come from?
From ASME B16.5 bolt-hole tables: the standard tabulates holes, and stud diameter follows with 1/8 in clearance up to 2-1/4 in studs and 1/4 in above. A 4 in Class 300 flange has 8 holes of 22.2 mm, mapping to 3/4 in-10 UNC studs. The full mapping for every class and size is on our flange bolt size chart, and the tensile stress area follows ASME B1.1.