Flange Bolt Tightening Sequence: Why Diagonal Crisscross Matters

You tightened all the flange bolts in order 1 through 8. Pressure test — it leaks at half the joints. The problem was the tightening sequence.

Why Sequential Tightening Fails

If you tighten bolt 1 first, the flange compresses at position 1. When you tighten bolt 2, position 2 compresses — and the flange at position 1 springs back slightly, reducing bolt 1's preload. By the time you reach bolt 8, bolt 1 may have lost 50% of its preload.

Result: wildly uneven preload around the flange. Some spots are crushed, others barely touch — leaks.

Correct Method: Diagonal Crisscross in Passes

Pass 1: Diagonal sequence (1→5→3→7→2→6→4→8) to 30% of target torque

Pass 2: Same diagonal sequence to 60% of target torque

Pass 3: Same diagonal sequence to 100% of target torque

Diagonal crisscross distributes flange compression evenly. Gradual passes let preload build progressively without later bolts pushing out earlier ones.

Special Cases

• Large flanges (DN300+): 3–4 passes, 25–30% increments
• Soft gaskets (graphite, PTFE): add a re-torque after 24 hours — soft gaskets creep
• Titanium/aluminum flanges: significant creep, must re-torque
• More than 12 bolts: use a cross pattern rather than simple diagonal

Verification

After tightening, re-check each bolt with a torque wrench. If any bolt reads more than 10% below target, preload distribution is uneven — redo the sequence.

Toprecision supplies flanged joint fasteners with tightening sequence diagrams and torque values.