4-Needle 6-Thread Boxer Briefs: Specification, Verification and MOQ 3,000 (2026)

Quick answer. Four-needle six-thread is the ISO 4915 class 607 cover stitch: four needles and six threads forming a flat seam that lies level on both faces of the cloth. On a men’s boxer brief it goes where a raised seam would otherwise sit against the skin — the front panel, the side seam, the crotch seam and the leg opening. This programme runs that stitch on cotton-spandex, nylon-spandex and polyester-spandex knits, at 3,000 pieces per style and colour, in sizes S–XXXXL.

1. What the stitch actually is

A four-needle six-thread flatlock is not an overlock with loosened tension, although the two are often described as if they were the same. It is a cover stitch in the 600 class, formed by four needle threads crossing the join and two looper threads — one above, one below — that cover both faces. That is why the seam reads as a flat ladder on the inside and as a flat surface on the outside, with no raised ridge in either direction.

• The needle threads carry the load across the join and are what you see as four parallel rows.
• The top looper covers the upper face and is what produces the ladder on the inside of the garment.
• The bottom looper covers the lower face, so the seam is finished on both sides without a fold.
• The edges are butted, not overlapped. There is no seam allowance folded into the join, which is the source of the flat profile — and the reason the seam cannot be unpicked and resewn the way a lockstitch can.

The practical consequence: the seam carries stretch in both directions, sits at the thickness of the fabric rather than twice it, and has nothing to abrade against skin under movement. Those three properties, not the thread count, are what make the construction worth specifying.

2. How it differs from three-needle five-thread and four-thread overlock

Three constructions get confused in tech packs, and the difference is not cosmetic — it changes the seam thickness, the failure mode and the cost of the garment.

• Four-thread overlock (ISO 514). The workhorse for assembly. It wraps the edge, leaves a raised ridge on the inside face and adds a fold of fabric to the join. Fast, cheap, and entirely adequate where the seam does not touch moving skin. It fails by thread break.
• Three-needle five-thread flatlock (ISO 605). Also a cover stitch, also flat, but narrower. Common on lighter knits and on panels where a 607 seam would add unnecessary width. Many factories quote “flatlock” and deliver this.
• Four-needle six-thread flatlock (ISO 607). The widest of the three, with one more needle row and one more thread than the 605. It spreads the load over a broader band, which is what a boxer brief needs at the crotch and the leg opening. Costs more in machine time and operator skill, and is the reason a 607 seam is a signal of construction intent rather than of marketing.

If a tech pack says “flatlock” without a class number, it has not specified anything. The number is the specification.

3. Three fabric families, one line

The same stitch runs on three different knits, and each one changes what the seam has to do. This is the part most product pages leave out, because most pages are built around a single fabric.

• Cotton-spandex, typically 95/5 or 92/8, at 160–180 g/m². The everyday and retail-own-brand base. Absorbent, matte, familiar against the skin. It has less snap-back than the synthetics, so the seam has to do more of the recovery work at the leg opening. A brushed finish is available within this family and changes the hand, not the construction.
• Nylon (polyamide)-spandex, typically 87/13, 90/10 or 85/15, at 160–200 g/m². The performance and premium base. Cool to the touch, high recovery, holds colour well. It is also the thinnest of the three at a given weight, which is exactly where a flatlock seam has the least margin for error.
• Polyester-spandex, typically 85/15 or 90/10, at 140–180 g/m². The quick-dry and print base. Dries fastest, takes sublimation print well, holds colour under UV. It is the most common base for high-volume programmes in hot climates.

Other compositions and weights are open to discussion; the ranges above are where the line normally sits rather than the boundary of what can be quoted.

4. What changes when the line switches fabric

Running three families on one line is normal. Running them on one setting is not. Four parameters move when the fabric changes, and each one is a place where an under-specified order goes wrong.

• Needle size. A thinner needle on a thin nylon knit reduces the hole it punches; too thin on a heavier cotton and the needle deflects. The needle has to match the fabric, not the machine’s default.
• Stitch density. The 12–16 SPI band is industry practice, but the number inside that band is chosen per fabric: denser on a thin, high-recovery knit, slightly more open on a heavier cotton where dense stitching can perforate the surface.
• Thread tension. Nylon and polyester need different looper tension from cotton to keep the ladder flat rather than puckered. A line that carries one tension setting across all three produces a seam that looks correct on the cotton and cups on the nylon.
• Presser-foot pressure and feed. The butted-edge feed is the hardest part of the operation. Higher pressure helps on a stable cotton and distorts a thin nylon.

This is why “we can do all three” and “we can do all three to the same standard” are different claims. Ask for the setting sheet per fabric, not only the stitch name.

5. Where fabric and stitch conflict

Three conflicts recur, and all three show up first in the wash rather than in the sample room.

• Thin nylon and needle holes. On the lightest nylons the limiting factor is not thread strength but the fabric tearing at the needle line. Dense stitching on a thin knit shortens seam life instead of extending it. The counter is a finer needle, a slightly more open stitch and a fabric weight chosen for the seam, not only for the hand.
• Polyester and glazing. Polyester is thermoplastic. Too much heat at the presser foot or the finishing press leaves a shine along the seam line that no amount of trimming removes. It is a machine-setting problem that appears as a cosmetic defect.
• Brushed cotton and hidden pinholes. A brushed surface lifts fibre over the stitch line, which hides exactly the defect a buyer would otherwise catch at inspection. On brushed styles the check has to be a physical pull at the seam, not a visual pass.

6. Which seam goes where on a boxer brief

Not every seam on the garment should carry the same stitch, and the allocation is a specification decision rather than a factory preference.

• Front panel and pouch. Four-needle six-thread. This is the highest-visibility joint and the one that sits against the most sensitive skin; a flat seam is the whole point of the construction.
• Side seam. Four-needle six-thread where the style is close-fitting; four-thread overlock is acceptable on loose cuts and cheaper, but it puts a ridge on the inner thigh.
• Crotch and gusset. The load-bearing joint. Four-needle six-thread, with the gusset on its own specification line because it is often a different fibre from the body.
• Leg opening. Four-needle six-thread or a bonded hem — never a plain overlocked edge on a close-fitting style, because that is the edge that rides and rubs.
• Waistband attachment. Depends on the band: a sewn elastic band takes a cover stitch, a bonded band takes tape. The two are not interchangeable within one style.

7. Six lines for the tech pack

Six lines remove most of the room for substitution. They are short enough to paste into an existing spec sheet.

• Stitch type: ISO 4915 class 607, four needles six threads, cover stitch, flat seam.
• Stitch density: 12–16 SPI, with the chosen value stated per seam.
• Needle span: 5.5–5.6 mm class, set by the machine needle positions.
• Thread: polyester or nylon, colour-matched, with the ticket number stated.
• Seam strength: tested to ISO 13935-2, acceptance at AQL 2.5 to ISO 2859-1.
• Stretch acceptance: no grinning at 150% extension of the seam.

Add the fabric as fibre, blend ratio and weight in g/m², and the leg-opening recovery test, and the pack describes a garment rather than a style.

8. How to verify it in three steps

Verification is cheap and does not need a laboratory for the first pass. The laboratory is for the second pass.

• Turn the garment inside out. A true 607 seam shows four parallel needle rows on one face and a continuous ladder on the other, running without breaks through the whole seam. A mock flatlock shows a lattice on one side and a single loose row on the other, and the ladder stops where the tension changed.
• Pull the seam to 150%. No pinholes should open along the stitch line. If they do, either the needle is too coarse or the stitch is too dense for the fabric.
• Test the seam and the leg opening after washing. Seam strength to ISO 13935-2; leg opening pulled to 1.5× relaxed circumference for 20 cycles, with permanent growth over 5% treated as a fail (an acceptance protocol, not a standard limit). Thirty wash cycles to ISO 6330 before re-measuring is the industry baseline.

9. What mock flatlock looks like

A mock flatlock is a three-thread overlock run at almost zero needle tension. It looks convincing in a photograph and fails the three checks above. Four variants turn up in buyer claims.

• The loosened overlock. One thread path, pulled open after sewing. The ladder is on one face only and there is no top cover thread.
• The 605 sold as 607. A real cover stitch, one needle row narrower than specified. Only a needle count settles it.
• The open stitch. Correct class, SPI run at eight or nine to raise throughput, which is where pinholes and seam grin come from.
• The wrong thread. Cotton thread on a synthetic knit. The seam passes at shipment and the thread abrades through within the first quarter of wear.

Each of these is a machine setting, not a material shortage — which is why the check belongs in the pre-production approval rather than in final inspection.

10. Why the leg opening rides up

Ride-up is usually read as a fit problem and is more often a construction and recovery problem. Four causes account for most of it.

• Recovery, not grip. The hem needs to return to its relaxed circumference after stretching. A leg opening that has grown 5% will not grip by mid-season, whatever the elastic.
• Differential shrinkage. Body fabric and elastic shrink at different rates in the wash, and the hem loses the fight. Pre-shrinking belongs in the fabric specification.
• Rise and back pitch. A short rise on a longer torso pulls the hem up from the crotch side. This is a pattern decision, and it is cheap before cutting.
• Hem angle. A hem cut square across a tapering thigh fights geometry. An angled or curved hem holds better on athletic builds.

The acceptance protocol is the same one used for the seam: 1.5× stretch for 20 cycles, permanent growth over 5% fails, then thirty washes and re-measure.

11. The brand-level specification, and what 3,000 pieces buys

The reason this construction matters commercially is that it is the one used by performance underwear brands selling at a multiple of commodity pricing. A brand-level boxer brief, read as a specification rather than as a brand, comes down to six decisions.

• Seam: four-needle six-thread flatlock, ISO 4915 class 607, by seam rather than by garment.
• Fabric: a spandex knit at 160–200 g/m² — nylon-spandex for performance, cotton-spandex for everyday, polyester-spandex for quick-dry and print.
• Weight and recovery: enough body to hold shape, enough recovery to bring the leg opening back.
• Leg length: 5–8 inches depending on the end use; longer for running and cycling, mid for daily wear.
• Waistband: a wide elastic band, roughly 1.5–1.75 inches, that does not roll.
• Panel construction: a multi-panel body rather than a flat front, so the pouch has volume without a seam down the centre.

This is the same stitch that appears in the products of SAXX, DAVID ARCHY and ExOfficio — DAVID ARCHY names the four-needle six-thread construction directly in its product copy. Brand names are cited here to show where the construction sits in the market; they are not customers of Unitex.

On volume: this programme runs at 3,000 pieces per style and colour as a benchmark, with the final figure confirmed against the construction, because a brushed finish, a print or a bonded panel each change the setup base. That is the band in which the machine setting, the fabric lot and the sampling all become worth running — which is the difference between a promotional brief and one that can carry a brand’s name. Sizes run S–XXXXL graded to the buyer’s requirement, and the colour count is set by the buyer, subject to dye-lot minimums.

12. Custom logo, labels and packaging

Branding on this construction is straightforward, with one constraint worth knowing before the first sample.

• The logo goes on as a heat transfer or a printed label inside the centre-back waistband, which is the standard position on a flatlock brief.
• A second placement on the outer waist edge or the leg is possible where the panel allows it.
• The label format can be a printed transfer, a silicone print, or a woven label on a stitched tab where the style keeps one.
• Packaging — polybag print, hangtag and carton marks — is specified separately; the quotation is normally issued without packing unless asked for.
• Limit: the logo does not change the pattern. A block change is a development project and re-opens the seam trials; the logo setup itself is a one-off charge per artwork, and who carries it is stated on the quotation.

13. What drives the quotation and the lead time

No prices here, but the cost structure is worth stating because it explains why two quotations for the same style can differ by a third.

• Seam count, not garment count. The cost sits in the number of 607 seams and the machine time they take. A style with four flat seams is not priced like one with two.
• Fabric family. Nylon costs more than polyester at the same weight; a brushed finish adds a process; a print adds a plate or a sublimation step.
• Colour count. Each colour is a dye lot with its own minimum. Thirty colours is not ten colours times three.
• Size grading. A wider size range adds grading and marker work, and S–XXXXL is wider than the usual S–XXL.
• Packing. Quoted separately because it is a different supplier chain.

Lead time follows the same logic: 45–60 days for a new style with sampling, 30–40 days on a repeat where the settings and the fabric lot are already approved.

14. Where this connects

If the question is what the stitch is and how it evolved, the guide to four-needle six-thread flatlock goes into the machine and the seam class in more detail. If the question is whether to sew at all, sewing versus bonding sets the two constructions side by side — bonding needs a thermoplastic face, so cotton stays on the sewn route. For the wider men’s programme, the boxer brief OEM guide covers the categories around this one.

Published: 2026-10-20 · Xiamen Unitex Trade Co., Ltd. — OEM & ODM underwear sourcing partner

Unitex Underwear (Xiamen Unitex Trade Co., Ltd.) — OEM/ODM underwear sourcing and supply-chain partner in Xiamen, China; in the trade since 2009, incorporated in 2013. Based in the Xiamen–Quanzhou–Jinjiang knitwear cluster, Fujian Province, south-east China.