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Flange Bolt Tightening Sequence Calculator (Star Pattern Chart + Pass Torques)

Generates the star (cross) bolt-tightening sequence for any flange bolt count — a numbered phone-sized chart plus the written order — and splits a target torque into the standard multi-pass schedule: snug, 30% / 70% / 100% cross passes (editable), then the final circular pass that catches elastic interaction.

Method last updated (calculation changelog) · fixture-verified on every build — most recently 2026-09-03.

Built and fixture-verified by Matthew Norris, P.E. — active P.E. licensure in Arizona, California, Kansas, Missouri, North Carolina, Texas.

This tool came out of a working thread with field crews: the leaks that get called in at the end of the day trace back to tightening patterns far more often than to torque values — the sequence matters more than the number. So this is the sequence, by bolt count, as a chart that fits a phone screen: every bolt hole numbered with its tightening step, the written order underneath for paint-marking the studs, and — for the crews that torque — the pass percentages that take the joint to target in stages.

Enter the bolt count (any even count from 4 to 68 — ASME drillings are multiples of four) and, if you torque, the target from your job spec or gasket sheet: the tool splits it into per-pass wrench settings and always finishes with the final circular pass — around the clock in position order until no nut turns. That last pass is the one most crews skip, and it is the one that fixes the joint that “won't stop leaking”: every bolt you tighten partially unloads its neighbours (elastic interaction), so the bolts done early in a pass end up below target unless something evens them out.

One thing it deliberately does not do: calculate the torque value. Nut factor, thread lubrication, bolt spec and gasket stress limits are joint design, not a field chart — the target belongs to your project spec or the gasket data sheet, and this tool only sequences it.

Star tightening pattern on an 8-bolt flange A flange face with eight bolt holes on a dashed bolt circle. Each hole is numbered with its tightening step, and a red path crosses the flange from each bolt to its diametric opposite, rotating a quarter turn between pairs. A legend lists the written order and the pass schedule. 1 5 3 7 2 6 4 8 star pattern — 8-bolt flange 1 → 5 → 3 → 7 → 2 → 6 → 4 → 8 each bolt, then its diametric opposite; consecutive pairs rotate a quarter turn snug — hand-tight, flange gap even pass 1 · 30% — pass 2 · 70% — pass 3 · 100% final — circular, until nuts stop turning numbers = tightening step at each hole
Star (cross) tightening pattern on an 8-bolt flange: the number at each hole is its tightening step — every bolt is followed by its diametric opposite, and consecutive pairs rotate a quarter turn. The calculator draws this chart for any bolt count and appends the pass schedule (snug, staged cross passes, final circular pass).
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Method

The sequence is built by two rules, applied to studs numbered clockwise from any starting bolt:

1. every bolt is followed immediately by its diametric opposite (bolt i → bolt i + N/2)

2. consecutive pairs rotate quadrants — the pair order is the same crossing applied to the half-count, recursively

For 4, 8, 12, 16, 24 and 32 bolts this reproduces, digit for digit, the legacy star charts published on gasket manufacturers' torque cards (the pattern ASME PCC-1's legacy tightening guidance describes). Crossing seats the gasket flat and keeps the flanges parallel; the quadrant rotation stops compression from walking around the joint one side at a time.

schedule: snug → 30% → 70% → 100% (cross pattern each pass) → final pass at 100%, circular

The percentages are editable (up to six cross passes); the final circular pass is always appended because it is the anti-leak pass: it catches the preload lost to elastic interaction during every cross pass. With a target torque entered, each pass row becomes a wrench setting, rounded to a tenth.

Inputs
boltCountBolts on the flange — even, 4 to 68 (ASME drillings are multiples of 4)—
targetTorqueTarget torque from the job spec or gasket sheet; 0 or blank for a sequence-only chartft·lb or N·m
passPercentsCross-pass percentages of target, ascending — default 30 / 70 / 100%
Outputs
chartThe numbered flange diagram — each bolt hole carries its tightening step; the red path traces the star—
sequenceThe written tightening order by clockwise stud number, e.g. 1 → 7 → 4 → 10 → …—
passesThe full schedule: snug, each cross pass (percent and wrench setting), final circular pass—
warningsNon-standard counts, sub-100% finishes, and large-joint pattern advisories—

Limitations — what this calculator is not

Quick reference — star sequence chart, 4 to 32 bolts

Every row below is read at build time from the calculator's CI fixture file — the same cases the release gate re-runs on every commit — so this table cannot drift from the engine. The rule the fixtures lock: every bolt is followed by its diametric opposite, and consecutive pairs rotate quadrants; for the counts below the result is digit-for-digit the chart printed on gasket manufacturers' assembly cards. Number the studs clockwise from any starting bolt, then pull them in this order on every pass:

BoltsTightening order (studs numbered clockwise)
41 – 3 – 2 – 4
81 – 5 – 3 – 7 – 2 – 6 – 4 – 8
121 – 7 – 4 – 10 – 2 – 8 – 5 – 11 – 3 – 9 – 6 – 12
161 – 9 – 5 – 13 – 3 – 11 – 7 – 15 – 2 – 10 – 6 – 14 – 4 – 12 – 8 – 16
201 – 11 – 6 – 16 – 2 – 12 – 7 – 17 – 3 – 13 – 8 – 18 – 4 – 14 – 9 – 19 – 5 – 15 – 10 – 20
241 – 13 – 7 – 19 – 4 – 16 – 10 – 22 – 2 – 14 – 8 – 20 – 5 – 17 – 11 – 23 – 3 – 15 – 9 – 21 – 6 – 18 – 12 – 24
281 – 15 – 8 – 22 – 2 – 16 – 9 – 23 – 3 – 17 – 10 – 24 – 4 – 18 – 11 – 25 – 5 – 19 – 12 – 26 – 6 – 20 – 13 – 27 – 7 – 21 – 14 – 28
321 – 17 – 9 – 25 – 5 – 21 – 13 – 29 – 3 – 19 – 11 – 27 – 7 – 23 – 15 – 31 – 2 – 18 – 10 – 26 – 6 – 22 – 14 – 30 – 4 – 20 – 12 – 28 – 8 – 24 – 16 – 32

Larger counts (big B16.47 and girth flanges) work the same way in the calculator, up to 68 bolts. Past roughly 48 bolts the legacy pattern gets slow and alternative assembly patterns start earning their keep on pass count — the order above stays valid either way.

Worked example — fixture-verified

The chart case from the thread: a 12-bolt flange, crew running impacts — no torque numbers anywhere on the job. The order is the whole product.

Given
Bolts on the flange12—
Target torquenot entered — sequence-only chart—
Passes30% / 70% / 100% + final circular (defaults)—

Step by step

  1. Pick any stud, call it bolt 1, and number the rest clockwise — the chart draws bolt 1 at 12 o'clock.
  2. Cross the flange: bolt 7 is diametrically opposite, so it is tightened second. Every bolt in the order is followed by its opposite — that is what pulls the flanges up parallel instead of cocking them.
  3. Rotate a quarter turn for the next pair: 4 and 10. Then 2–8, 5–11, 3–9, 6–12 — consecutive pairs keep rotating quadrants so gasket compression walks around the joint evenly.
  4. Full order: 1 → 7 → 4 → 10 → 2 → 8 → 5 → 11 → 3 → 9 → 6 → 12 — digit for digit the 12-bolt chart on every gasket manufacturer's torque card.
  5. Run that order at snug first and check the flange gap is even all around; run it again at each pass stage even without a wrench — the bolts hit early in a pass relax as their neighbours come up; finish around the clock 1, 2, 3… until nothing moves.
Result COMPUTED
Tightening order1 – 7 – 4 – 10 – 2 – 8 – 5 – 11 – 3 – 9 – 6 – 12—
Chart numbering, clockwise from bolt 11, 5, 9, 3, 7, 11, 2, 6, 10, 4, 8, 12—
Schedulesnug → 30% → 70% → 100% cross → final circular—

Twelve numbers a fitter can paint-mark on a flange in thirty seconds. The final circular pass is the one most crews skip — and it is the pass that fixes the joint that leaks at the end of the day no matter how hard the last bolts got hit.

Why you can trust these numbers: this exact case is fixture bolt-sequence.json — case “12-bolt flange, sequence only -> the universal published cross order” (tolerance 1e-9) — in the calc-core release gate. It re-runs on every commit; a red fixture blocks deployment. See the validation methodology.

Worked example 2 — the same flange with a 200 ft·lb torque spec

Now the crew torques: the job spec calls for 200 ft·lb on these studs, lubricated, from the gasket sheet. Same 12-bolt flange — the tool splits the target into wrench settings.

Given
Bolts on the flange12—
Target torque200 ft·lb (from the job spec — not computed here)—
Passes30% / 70% / 100% (defaults)—

Step by step

  1. Pass 1 at 30%: set the wrench to 60 ft·lb and run the full cross order — the gasket seats at low load while the flanges are still free to pull up parallel.
  2. Pass 2 at 70%: 140 ft·lb, same order. Pass 3 at 100%: 200 ft·lb, same order.
  3. Final circular pass at 200 ft·lb: around the clock in position order — 1, 2, 3… — until no nut turns. This is where the preload lost to elastic interaction during the cross passes gets put back.
  4. Where the spec calls for it, retorque after initial service or the first thermal cycle — soft gaskets relax and the joint that passed at noon reads low by morning.
Result COMPUTED
Pass 1 / 2 / 3 wrench settings60 / 140 / 200 ft·lb—
Final passcircular at 200 ft·lb until nuts stop turning—

Sites tune the percentages — large or sensitive joints often run more passes at tighter increments, and the tool takes up to six — but the shape never changes: cross pattern up to target, circular pass to finish.

Fixture case “12-bolt flange at a 200 ft-lb target -> 60 / 140 / 200 wrench settings + final circular at 200” (tolerance 1e-9) — locked in the same release gate as the example above.

Additional verified cases in this fixture

8-bolt flange -> 1-5-3-7-2-6-4-8 COMPUTED 
input:  {"boltCount":8}
expect: {"sequenceText":"1-5-3-7-2-6-4-8"}
tol:    1e-9
4-bolt flange at 25 ft-lb in two passes -> 1-3-2-4, 12.5 / 25 COMPUTED 
input:  {"boltCount":4,"targetTorque":25,"passPercents":[50,100]}
expect: {"sequenceText":"1-3-2-4","crossPassCount":2,"passTorquesText":"12.5 / 25 ft·lb"}
tol:    1e-9
16-bolt flange -> published chart order COMPUTED 
input:  {"boltCount":16}
expect: {"sequenceText":"1-9-5-13-3-11-7-15-2-10-6-14-4-12-8-16"}
tol:    1e-9
20-bolt flange -> opposite-pair order with quadrant rotation COMPUTED 
input:  {"boltCount":20}
expect: {"sequenceText":"1-11-6-16-2-12-7-17-3-13-8-18-4-14-9-19-5-15-10-20"}
tol:    1e-9
24-bolt flange -> published chart order COMPUTED 
input:  {"boltCount":24}
expect: {"sequenceText":"1-13-7-19-4-16-10-22-2-14-8-20-5-17-11-23-3-15-9-21-6-18-12-24"}
tol:    1e-9
28-bolt flange -> opposite-pair order with quadrant rotation COMPUTED 
input:  {"boltCount":28}
expect: {"sequenceText":"1-15-8-22-2-16-9-23-3-17-10-24-4-18-11-25-5-19-12-26-6-20-13-27-7-21-14-28"}
tol:    1e-9
32-bolt flange -> published chart order COMPUTED 
input:  {"boltCount":32}
expect: {"sequenceText":"1-17-9-25-5-21-13-29-3-19-11-27-7-23-15-31-2-18-10-26-6-22-14-30-4-20-12-28-8-24-16-32"}
tol:    1e-9
6-bolt (even, not a multiple of 4) -> still crosses opposites: 1-4-2-5-3-6 COMPUTED 
input:  {"boltCount":6}
expect: {"sequenceText":"1-4-2-5-3-6"}
tol:    1e-9
16-bolt flange at a 140 ft-lb target -> 42 / 98 / 140 wrench settings COMPUTED 
input:  {"boltCount":16,"targetTorque":140}
expect: {"sequenceText":"1-9-5-13-3-11-7-15-2-10-6-14-4-12-8-16","passTorquesText":"42 / 98 / 140 ft·lb"}
tol:    1e-9

Sources & citations

Per the source & citation policy, allowable-stress and factor table values are user-supplied. Where a page does reproduce specific ASME data (the B16.5 ratings, the quick-reference tables), it states the source table and conditions inline.

FAQ

Why does the sequence matter more than the torque number?

Because bolted joints are elastic systems: tightening one bolt compresses the gasket and flange locally, and that compression partially unloads the bolts near it. Go once around the clock at full torque and the first bolts finish well below target while the gasket seats cocked — a joint that leaks even though every bolt “got the number.” The cross pattern loads the joint symmetrically so the flanges stay parallel, the staged passes keep any one bolt from doing all the crushing, and the final circular pass puts back the preload the earlier bolts lost. Torque scatter from a decent wrench is small compared to the scatter a bad pattern creates.

We run impacts, not torque wrenches — does any of this still apply?

It applies more, not less. An impact crew controls exactly two things: the pattern and the passes — and both are free. Snug the joint in the cross order and check the flange gap is even all around; run the pattern in stages rather than burying each stud in one hit; finish with the circular pass until nothing moves. That discipline is what separates an impact-tightened joint that seals from one that gets the “it won't stop leaking” call — with no torque number involved anywhere.

Why won't it give me a torque value?

Because a torque value without its assumptions is how studs get yielded and gaskets get crushed. The right target depends on the bolt spec and condition, the nut factor (dry versus lubricated threads move the number by half), and the gasket's seating and crush limits — that is joint design, documented in the project bolting spec or the gasket data sheet. This tool takes that number and turns it into a field schedule. For the design side of the same joint — gasket loads, bolt area, flange stresses — that is what PiperFLG's bolted flange analysis is for.

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