Most string trimmer heads use a small compression spring inside the head to keep tension on the spool and bump knob.

• Bump-feed heads: The spring typically sits between the inner hub and the bump knob or under the spool. It lets the spool move when you tap the head, then pushes it back.
• Auto-feed heads: A spring usually lives beneath or behind the spool to keep steady pressure on the line system.

Placement can vary by head style, so it’s a good idea to snap a quick photo during disassembly and check your trimmer’s manual or the product’s parts diagram. Reinstall with the spring centered and seated flat so the head can move smoothly.

1. Power off safely: Remove the battery or unplug the tool. For gas units, disconnect the spark plug boot.
2. Open the head: Press the tabs or unscrew the cap. Note how the spool and spring are arranged.
3. Remove parts: Lift out the spool and old spring. Clean out grass and debris.
4. Match the spring: Compare length and diameter with the new spring to help ensure compatibility.
5. Reinstall: Seat the new spring in the recess, replace the spool, and secure the cap or knob.
6. Test gently: Pull the line and, if it’s a bump head, lightly tap on a safe surface to confirm it compresses and returns.

Steps can vary, so consult your manual. Product pages often list dimensions to help find a compatible replacement.

Choosing the right trimmer head spring usually comes down to matching your head style and dimensions.

• Check your model and head type: Identify whether it’s bump-feed, auto-feed, or fixed line. Springs differ by design.
• Measure your old spring: Note overall length, outer diameter, and wire thickness. Count coils if needed.
• Compare ends and fit: Some springs have closed/ground ends; others are open. Match the seat style in your head.
• Review product details: Many listings include measurements and compatible head types. Compare photos and specs.

If you’re unsure, bring the old spring’s dimensions to the product page and use the item Q&A or support options. A close match can help restore proper line feed, but results may vary by condition and usage.

A non-advancing bump head often points to a few simple issues.

• Tired or misplaced spring: If the spring is weak, missing, or not seated correctly, the spool may not pop back.
• Line diameter or winding: Line that’s too thick or wound in the wrong direction can jam movement.
• Debris buildup: Dirt under the spool or in the cap can block travel.
• Worn cap or bump knob: Rounded or cracked parts reduce the bump action.

Try cleaning the head, rewinding the correct line size in the right direction, and inspecting the spring for damage. Replacing a fatigued spring and any worn hardware can help restore function, though performance depends on overall head condition.

Signs of a failing trimmer head spring tend to show up during use.

• Weak return: The spool doesn’t rebound after a bump or feels mushy when pressed.
• Inconsistent feed: Line advances unpredictably or not at all, even with correct winding.
• Noises or rattling: The head may chatter if the spring no longer holds tension.
• Visible wear: Look for rust, kinks, bends, or a noticeably shorter free length.

If you notice these symptoms, installing a replacement spring during routine head service can help. Many packs include multiple springs so you have a spare. Matching dimensions and head style is important for a proper fit, and outcomes can vary with overall head wear.