How to Restring a Kobalt 40V Weed Trimmer

Use 0.08-inch twisted nylon line. In most cases, you remove the battery, release the spool cap, rewind or replace the spool, thread the line through the outlet eyelets, and snap the cap back on.

Use this quick guide to choose the right refill approach before opening the trimmer head.

Trimmer-Head Situation	Line or Refill Choice	What to Check
Trimmer head	0.08-inch twisted nylon line	Confirm your model’s specified line type before purchasing
Head uses a prewound replacement spool	Model-matched prewound spool	Match the spool and cap shape to your trimmer
Head has line-loading holes and a winding knob	Bulk line loaded through the head	Follow the arrows molded into the head

How a Kobalt 40V Bump-Feed Head Works

A bump-feed trimmer stores line inside a spool at the bottom of the cutting head. Two short ends extend through outlet eyelets. When the trimmer is running, tapping the bump head against the ground releases a little more line.

The guard-mounted cutter trims excess line to the working length as the line advances. If line will not feed, the usual causes are crossed wraps, line fused together inside the spool, a broken line end, or a cap that is not fully latched.

You usually do not need to remove the entire trimmer head to restring it. The spool cap is the part designed to come off for routine line replacement.

How to Restring a Kobalt 40V Weed Trimmer

Before starting, work on a clean, flat surface where you can keep track of the spool, cap, and any loose line ends.

Step 1: Remove the Battery and Clean the Head

Switch the trimmer off and remove the battery.
Lay the trimmer down with the cutting head facing up.
Brush away grass, dirt, and packed debris around the spool cap and line outlets.
Look for the cap’s release tabs. They are commonly positioned on opposite sides of the bump-head housing.

Removing debris first matters: packed grass can hide the tabs and make a properly working cap feel stuck.

Step 2: Remove the Spool Cap and Inspect the Parts

Press the cap’s release tabs inward.
Lift the cap straight away from the cutting head.
Remove the spool and set the cap aside.
Pull out any remaining short pieces of old line.
Check the spool, cap tabs, and outlet eyelets for cracks, deep wear, or melted plastic.

A spool with damaged flanges cannot guide line evenly, and a cracked cap may pop off while the trimmer is running. Replace damaged parts rather than forcing a replacement into the head.

Step 3: Choose and Prepare the Trimmer Line

Use 0.08-inch twisted nylon trimmer line.
Check the product specifications for your particular trimmer before loading line, especially if the old spool or trimmer head is marked with a different line size.
Do not substitute wire, metal cable, or other non-trimmer material for nylon line.
Cut a manageable piece from the bulk line only after you understand how your spool is arranged.

Some spools have one winding channel; others have two separate channels, one for each line end. Look at the empty spool before winding so you can reproduce its intended layout.

Step 4: Wind the Line Onto the Removable Spool

Find the directional arrow molded into the spool. Wind in that direction.
Insert one end of the line into the spool’s starter hole or anchoring slot.
Wind the line neatly in the channel, keeping each wrap snug and beside the previous wrap rather than crossing over it.
If the spool has two channels, load one line end in each channel as the spool design requires.
Keep the line below the spool’s outer rim. Line piled above the edge can jam when the spool is reinstalled.
Leave short free ends outside the wound section and secure them in the spool’s temporary holding notches, if your spool has them.

Correct winding direction matters more than how tightly you pull the line. A spool wound against its arrow can unravel, bind, or refuse to advance when you bump the head.

If you bought a prewound spool instead of bulk line, skip the winding process. Check that its line ends are held in the spool notches and that the spool shape matches the old one.

Step 5: Reinstall the Spool and Test the Trimmer

Guide each free line end through its matching outlet eyelet in the trimmer-head housing.
Seat the spool flat inside the housing without trapping line beneath it.
Align the cap and press it down until both release tabs lock.
Pull each line end gently to free it from the temporary holding notch.
Reinstall the battery.
In a clear outdoor area, start the trimmer and let the guard-mounted cutter trim any excess line.

Stop immediately if the head shakes, the cap loosens, or one line retracts into the head. Remove the battery before reopening the spool housing.

If Your Kobalt Trimmer Has a Different Head Style

Check the physical design of your head rather than assuming every replacement works the same way.

Direct-Load Heads With a Winding Knob

Some heads let you feed line through openings in the head and wind it with an external knob.

Remove the battery.
Clear grass from the line-loading openings and winding knob.
Align any arrows or index marks required by the head design.
Feed nylon line through the head so the two free ends are even.
Turn the winding knob in the direction shown by the molded arrow.
Keep light tension on the line ends as the knob stores line inside the head.
Stop winding when both ends remain outside the outlet openings and the line is stored cleanly inside the head.
Reinstall the battery and check that the line feeds normally.

If the knob becomes difficult to turn, do not force it. Remove the line and check for a crossed strand or packed debris in the loading path.

Heads That Need a Replacement Spool

A prewound spool is often the simplest option when the old spool is badly worn or the line is fused tightly enough that it will not unwind cleanly.

Release the spool cap.
Lift out the old spool and compare it with the replacement.
Thread the replacement spool’s line ends through the outlet eyelets.
Seat the spool level in the head.
Snap the cap fully into place.
Install the battery and test the line feed in a clear area.

Keep the old spool only if it is undamaged and can be rewound smoothly. A warped spool can cause repeated feeding problems even with new line.

How to Tell Whether the New Line Is Loaded Correctly

Check the trimmer before returning to regular yard work.

Both line ends should extend from the head’s outlet eyelets.
The spool cap should sit flush and stay latched when you pull gently on the line.
The trimmer head should not wobble or rattle when the trimmer runs.
The line should advance through the bump-feed system rather than pulling back into the head.
The guard cutter should trim excess line without the line wrapping around the shaft or guard.

To prevent future jams, clean grass from the head after use and avoid letting the spool run completely empty before reloading. Short, buried line ends are harder to retrieve and can make a simple refill take much longer.

Troubleshooting Kobalt Trimmer Line Problems

Symptom	Likely Cause	Fix
Line will not advance when bumping the head	Line is crossed, fused, or trapped under itself	Remove the battery, open the spool, and rewind the line in neat channels
Only one side of the trimmer cuts	One line end broke inside the head or was not threaded through its eyelet	Open the cap, reload or reposition the line, and thread both ends through the outlets
Line feeds out all at once	Spool was wound in the wrong direction or cap is not seated	Rewind using the spool arrow and lock the cap firmly
Spool cap keeps coming loose	Broken retaining tabs or wrong replacement cap	Replace the cap with one matched to the trimmer model
Head shakes or vibrates	Uneven line loading, damaged spool, or debris under the spool	Clean the housing, rewind evenly, and replace cracked parts
Line keeps breaking near the head	Eyelets are worn or line is pinched during assembly	Inspect the eyelets and make sure the line moves freely before use

Frequently Asked Questions

How do I lengthen the string on a Kobalt weed trimmer?

On a bump-feed trimmer, line advances when you lightly tap the trimmer head’s bump head against the ground while the trimmer is operating in a clear area. If no line comes out, turn the trimmer off, remove the battery, and check whether the line is jammed, too short inside the spool, or caught in a holding notch.

How do I remove a Kobalt trimmer head without a tool?

You normally do not need to remove the complete cutting head to replace line. With the battery removed, press the spool cap’s release tabs and lift out the spool; do not pry or twist the entire head if it does not release normally, because head-retention systems vary by model.

How do I replace line on a bump-feed trimmer?

Remove the battery, release the spool cap, take out the spool, and wind new nylon line in the direction of the arrow molded into the spool. Thread both line ends through the outlet eyelets, lock the cap in place, reinstall the battery, and test the trimmer where the guard cutter can trim excess line.

Why does my new trimmer line keep coming out?

Line usually spills out when it is wound backward, crossed over itself, or not secured in the spool’s holding notches during reassembly. Remove the battery, rewind the spool neatly in the marked direction, and make sure the cap locks fully before testing it again.