Flatlock vs Overlock, Explained

Flatlock butt-joins two fabric edges: they meet without overlapping and are locked by looper threads that pass through both fabrics from both sides. A 2-needle 4-thread flatlock produces two parallel needle lines on the face and a zig-zag looper thread on the reverse. A 3-needle 5-thread flatlock — the industry standard for compression baselayer — spreads the load across three needle lines.

• Seam thickness — 0.6–0.9 mm (essentially flat).
• Elongation before break — 45–60%, matches most poly-spandex fabrics.
• Machines — Yamato, Pegasus, Kansai flatlockers.
• Downside — the butt joint has no safety wrap, so a needle-thread break can propagate.

Overlock (also called serging) overlaps two fabric layers, wraps the raw edge in a chain of needle and looper threads, and trims excess fabric with an on-machine blade in one pass. A 4-thread overlock uses two needle threads and two looper threads; a 5-thread overlock adds a safety stitch for critical seams.

• Seam thickness — 2.5–3.5 mm ridge on the reverse.
• Tensile strength — 40–60% higher than flatlock on the same fabric.
• Machines — Juki MO series, Pegasus M900 series.
• Downside — the ridge is felt against skin and can chafe on long training sessions.

• Stitch density — 8–10 stitches per cm on match kits, 10–12 on baselayer. Below 7/cm and the seam waves under load.
• Needle size — 65/9 for baselayer, 75/11 for jerseys, 90/14 for tracksuit fleece. Wrong needle bruises polyester and shortens seam life.
• Thread — spun polyester (Amann Serafil, Coats Astra) at Tex 27 for looper, Tex 40 for needle. Cheaper filament thread saves 3 cents per garment and fails wash-fastness inside 20 cycles.

Flatlock is roughly 8–12% slower per seam than overlock because the machine setup is finicky and operators need higher skill. On a 1,000-piece run this typically adds $0.35–$0.55 per garment. For teamwear and match kits the payoff is chafe-free performance the buyer can feel — for training tees and promotional apparel it is rarely worth the premium.