A serger has two knives: the upper knife moves and does the cutting; the lower knife is fixed and acts as the anvil. The upper knife wears first and is the one to replace when the cut edge turns ragged, fabric pushes or chews instead of slicing, or threads fray before the loopers catch them — usually after one to two years of regular use, sooner on polyester fleece, knits and anything with glue. Replace the lower knife only if it is chipped. Knives are model-specific, so buy from the page that names your serger, and choose a carbide upper knife if you serge often.
How the knives work
The upper knife is mounted on an arm driven by the main shaft and swings down past the lower knife on every stitch, shearing the fabric edge a few millimeters before the needles reach it, so the loopers wrap a clean edge. The lower knife is a flat blade clamped at the fabric level; on most machines it can be moved sideways to set the cutting width. The upper knife is the hard, exposed, moving part and takes all the wear.
Signs the upper knife is worn
- The cut edge is fuzzy or ragged instead of clean
- The fabric bunches ahead of the knife, or the serger pushes thick fabric instead of cutting it
- You hear a chewing sound on knits or fleece
- Loose threads from the fabric edge get caught in the stitch
- Cutting width has to be adjusted wider and wider to get a clean cut
If the edge is clean on cotton but ragged on knits, the knife is on its way; if it will not cut a single layer of cotton, it is overdue.
Signs it is the lower knife
Rare. A visible chip or a flat spot on the lower blade, or a fine uncut thread left along the edge after a new upper knife is fitted. Lower knives last through several upper knives.
Steel or carbide
Standard upper knives are hardened steel. Carbide (tungsten-carbide-tipped) upper knives cost more and hold their edge several times longer, especially on synthetics; for a serger used every week they are the better buy. Not every model has a carbide option — the knives sub-category lists what the distributor supplies.
Replacing the upper knife
- Unplug the machine. Open the front and side covers and remove the presser foot if it is in the way.
- Turn the hand wheel toward you until the upper knife is at the top of its travel.
- Loosen the upper knife clamp screw (on most sergers one screw, from the front or the left); note which way the bevel faces.
- Lift the old knife out and fit the new one with the bevel facing the same way, pushed fully home.
- Set the height: the cutting edge of the upper knife should sit about 0.5–1 mm below the cutting edge of the lower knife at the bottom of its stroke, so the two blades shear like scissors. The manual gives the exact figure.
- Tighten the screw, hand-turn a full revolution to check the knife clears the loopers and the foot, then test on two layers of cotton and one of knit.
You do not need to touch the loopers or the needles to change a knife. If the stitch changes after the swap, the knife is too high or too low — re-set the height before anything else.
Replacing the lower knife
Loosen its clamp screw, slide it out, fit the new one flush with the needle plate surface, and re-set the cutting width. Then check the upper knife height against it as above.
Knives by serger brand
Brother (925D, 929D, 935D, 1034D, 1134DW, 3034D, DZ1234), Baby Lock, Singer (S0230/S0235, 14T948DS), White (534D, 734DW, 1934D, 2000ATS, S34), Elna (604E, 614DE, 624DSE), Bernina (L450, L460), Husqvarna Viking Huskylock, Juki MO, Pfaff and Simplicity knives are listed under the serger knives sub-category and on each serger’s machine hub, each naming the models it fits.
Shop the parts
- Serger knives by model — /shop/machine-parts/knives/
- Loopers — /shop/machine-parts/loopers/
- Serger needles — /shop/needles/serger/
- Serger not cutting cleanly — /help/serger-not-cutting-cleanly/
Frequently asked questions
Upper or lower knife?
Upper, almost always. It moves and does the cutting; the lower is the fixed anvil and outlasts several uppers.
How often should I replace the serger knife?
When the cut goes ragged — typically one to two years of regular use, sooner on synthetics. Carbide lasts several times longer.
Do I need to re-time anything after changing a knife?
No. Knives do not affect looper or needle timing. Only the knife height needs setting.
Can I sharpen a serger knife?
Upper knives can be professionally sharpened once or twice, but a new knife costs about the same as sharpening.
Why is my new knife not cutting?
Height. The upper knife must pass just below the lower knife’s edge; if it stops above, it will not shear. Re-set per the manual.
Is a carbide knife worth it?
If you serge weekly or cut fleece, knits and laminates, yes.
Does the knife fit my serger?
Only if your model is named on the knife’s page. Sibling models often take different knives.