How to Adjust Thread Tension on an Overlocker

Tension should be adjusted properly when sewing with an overlocker. If the stitch is not correct, check for proper threading before turning the tension dial.
Even if only one thread has been snagged or has slipped out of a thread guide, it can appear that the tension needs adjusting. If a machine that has been sewing with correct tension after the thread is changed check the threading before adjusting the tension dials.

When learning how to adjust the tension, thread the machine with colours of thread that match the colour coding on the machine. This helps you to visualise and understand each thread path and how the threads interact with each other.

Practice Adjusting Thread Tensions

When the overlocker stitch has poor tension, only one thread may need adjustment. Adjust too tight threads first as they may cause the other threads to seem loose even when they are not. Learning to adjust the correct thread or threads takes practice.

Overlockers have the tensions preset at the factory for basic seaming. If the machine has numbered tension dials, write down the preset numbers before practicing.
Your instruction manual may also give recommended settings for all stitch types. Although factory and recommended settings will not give perfect tension for all fabrics, they do provide a good point of reference, especially for beginners.

Practice adjusting the tensions by sewing on a long strip of fabric. Start sewing, and slowly turn the tension dial for one of the threads to a lower number. As you sew, examine the stitches to see the change in tension. The thread that has been adjusted will be slack or loose. Return the tension dial to the original setting.

Continue to sew, turning the same tension dial to a higher number. Examine the sample again to see how the tension has changed. The fabric puckers when one of the threads it too tight. Return the tension to the original setting. Continue to turn tension dials, one at a time, to see how each thread affects the stitch.

When looper tensions are correctly adjusted for a balanced overedge stitch, two opposing threads lock together at the edge of the fabric. When needle tensions are correctly adjusted, the threads rest smoothly on the top and are slightly visible on the underside. Looper threads form small relaxed loops. Compare your stitches to the pictures below if you are uncertain about which tension dial needs adjusting.

Tips for Analyzing Tension Problems

4-thread Overlock Stitches
  • If fabric puckers lengthwise, one or both needle threads are two tight.
  • If fabric puckers crosswise under stitches, one or both looper threads are too tight.
  • If a looper thread can be moved easily or if it appears uneven, the looper thread is too loose.
  • If a needle thread forms loose loops on the underside of the fabric or if the seam pulls open, the needle thread is too loose.
  • If threads lock on the upper side of the fabric, either the upper looper threads are too tight, pulling the upper side, or the lower looper threads are too loose, spilling over the edge.
  • If threads lock on the underside of the fabric, either the lower looper threads are too tight, pulling to the underside, or the upper looper threads are too loose, spilling over the edge.
2-thread Chainstitch
  • If fabric puckers, one or both threads are too tight. Adjust each dial separately to determine which thread needs to be loosened.
  • If either thread can be moved easily or appears uneven, it is too loose.
2-thread Overedge Stitch
  • If fabric puckers, one or both threads are too tight.
  • If either thread can be moved easily or appears uneven, it is too loose
  • If threads lock on the upper side of the fabric, either the looper thread is too tight, pulling to the upper side, or the needle thread is too loose, spilling over the edge.
  • If threads lock on the underside of the fabric, either the needle thread is too tight, pulling to the underside, or the looper thread is too loose, spilling over the edge.
Overlock Stitches Diagram

3-thread and 4-thread Overlock Stitches

The 3-thread overlock and the mock safety stitches are adjusted the same way. The overlock stitch (a) uses one needle to secure the stitches, and the mock safety stitches use two needles.
There are two types of 4-thread mock safety stitches; on one, the upper looper thread locks at the left needle thread (b), and on the other, at the right needle thread (c), depending on the model. 4-thread mock safety stitches are suitable for seaming wovens. Because these stitches stretch, they are also ideal for knits. The extra needle in in the mock safety stitch make it stronger than the overlock stitch.

Tension Adjustments 4-thread mock safety


Upper Looper Thread too Tight

Upper looper thread
(orange) pulls the lower
looper thread (yellow)
to upper side of fabric 
causing fabric to pucker
or curl under the stitches.
Loosen the upper looper
thread tension dial until
the looper threads lock
at the edge.
upper looper thread too tight
Upper Looper Thread too Loose

Upper looper thread
(orange) spills over cut
edge to underside of the
fabric. Upper looper
threads are slack and
can be moved easily.
Fabric does not pucker
or curl under stitches.
Tighten upper looper
thread tension dial until
looper threads lock
at the edge.
Upper Thread too lose
Lower Looper Thread too Tight

Lower looper thread
(yellow) pulls upper looper
thread (orange) to the
underside of the fabric,
causing fabric to pucker
or curl under stitches. 
Loosen lower looper
thread tension dial until
looper threads lock 
at the edge.

Lower looper too tight
Lower Looper Thread too Loose

Lower looper thread
(yellow) spills over cut
edge to upper side of the
fabric. Lower looper 
threads are slack and
can be moved easily. 
Fabric does not pucker
or curl under stitches.
Tighten lower looper 
thread tension dial until
looper threads lock 
at the edge.
Lower Looper thread too loose
Both Looper Threads too Tight

Both looper threads
(orange and yellow) may
lock at the edge, but fabric
is puckered or bunched 
under the stitches.
Stitches are narrower than
stitch width setting on the
overlocker. Loosen
both looper thread tension
dials until fabric is 
smooth under 
the stitches.
Both looper threads too tight
Both Looper Threads too Loose

Both looper threads
(orange and yellow) extend
over cut edge in loose 
loops. Tighten both looper
thread tension dials until
threads hug the edge.
Both looper threads too loose
Needle Threads too Tight

Fabric puckers lengthwise.
Loosen one or both needle
threads (blue and green)
tension dials until fabric
lies smooth without 
puckers. Test stitches in 
knit fabrics by
stretching; loosen needle
thread tension dials,
if necessary.
Needle-threads-too-tight
Needle Threads too Loose

Needle threads (blue and
green) form loose loops 
on the underside of fabric
(a). Seam pulls
open on the right side, exposing stitches (b).
Tighten one or
both needle thread
tension dials until
seam closes 
without puckers.
Needle threads too loose

Tension Adjustments (Chainstitches)

Lower Looper Thread too Tight

 Lower looper thread
(purple) is tight and drawn
on the underside of the
fabric, causing puckered
seam and skipped 
stitches. Loosen the lower
looper thread tension dial
until even loops are formed. 
If the problem is not 
solved, check for tight
needle thread tension.
Lower looper too tight chainstitch
Lower Looper Thread too Loose

Large, loose loops form in
lower looper thread 
(purple), causing the seam
to pull open, exposing
stitches on the right side
of the fabric. Tighten
the lower looper thread
tension dial until even
loops form on
the underside. 

Lower looper thread too loose chainstitch
Left Needle Thread too Tight

Tight left needle thread
(blue) may cause 
puckered seams and 
skipped stitches. Loosen
left needle thread tension
dial until the fabric does not pucker.
If the problem is not solved,
check for tight lower looper
thread tension.
left-needle tlhread too tight chain stitch

Left Needle Thread too Loose


Left needle thread (blue) forms large loops
on the underside of the fabric. 
Seam may pull open, 
exposing stitches on right
side of the fabric. Tighten left
needle thread tension dial until
close, firm loops form
on the underside.

Left needle thread too lose Chainstich
Information sourced from The New Sewing with a Serger from the Singer Reference Library