Cost to replace a stripped bump knob

A stripped bump knob on a string trimmer is usually a small, relatively inexpensive repair. The total cost depends on whether you replace only the knob, install the part yourself, or have a shop replace the complete trimmer head. Before buying anything, confirm that the replacement part matches your trimmer head—not just the trimmer brand.

What affects the replacement cost?

The main cost factors are:

• Knob-only replacement: Usually the least expensive option when the head body, spring, and spool are still in good condition.
• Complete head replacement: Often more practical if the knob threads are damaged along with the head, or if the head is cracked or badly worn.
• Labor: Do-it-yourself replacement avoids labor charges. A repair shop may charge a minimum service fee even for a quick part installation.
• Compatibility: The correct knob may depend on the head design, spool size, attachment method, and thread or arbor specification.
• Shipping and availability: A low-cost replacement part may not be worthwhile if it is difficult to source or takes longer to obtain than a compatible replacement head.

Because trimmer-head designs vary, there is no reliable single price for every stripped bump knob. Compare the cost of the exact replacement knob with a compatible complete head before ordering.

When replacing only the bump knob makes sense

A knob-only repair is reasonable when:

• The knob is visibly stripped, rounded off, or no longer engages the spool.
• The head body is not cracked.
• The spool still rotates normally.
• The spring and retaining hardware are present and undamaged.
• A replacement knob is listed for your exact head or confirmed compatible by the manufacturer.

Remove the spool and inspect the knob’s mating surfaces. If the knob is the only damaged piece, replacing it can restore bump-feed operation without replacing the whole assembly.

When to replace the entire trimmer head

A complete head is usually the better choice when:

• The knob and the threaded portion of the head are both damaged.
• The head has cracks, severe wear, or missing retaining parts.
• The spool does not seat correctly.
• The spring is weak, broken, or missing.
• The correct replacement knob is unavailable.
• The complete compatible head costs little more than the replacement part and shipping.

Replacing the entire head also reduces the chance of a mismatched knob causing line-feed problems later.

Check compatibility before buying

Do not assume that a universal-looking bump knob will fit your trimmer. Check the following:

1. Trimmer make and model: Use the model number from the product label or owner’s manual.
2. Current head model: Some trimmers accept more than one head design.
3. Attachment specification: Confirm the required thread, arbor, or mounting method from the manufacturer’s parts information.
4. Spool and knob design: Compare the shape, retaining method, and contact surfaces with the replacement listing.
5. Line-feed system: Make sure the replacement supports the same bump-feed arrangement.

If the listing does not clearly identify compatibility, measure and compare the original parts only if the manufacturer provides the dimensions needed. Do not force a knob onto a head with a different thread or mounting system.

Basic replacement steps

Disconnect the trimmer from its power source before working on the head. Remove the battery, unplug the unit, or shut off and cool a gas trimmer according to the owner’s manual.

Then:

1. Remove the spool and old bump knob using the procedure in the manual.
2. Inspect the head, spring, spool, and retaining parts for cracks or distortion.
3. Clean grass and debris from the head without damaging the mounting surfaces.
4. Install the replacement knob and spool in the correct order.
5. Confirm that the spool turns and that the knob moves as designed.
6. Load the specified line and test-feed it briefly in a safe area.

Never operate the trimmer with loose parts, a cracked head, or a knob that does not securely retain the spool. If removing the head requires a procedure you cannot identify, follow the manufacturer’s instructions or ask a qualified repair professional.

Is it worth repairing?

A knob-only repair is generally worth considering when the head is otherwise sound and the correct part is easy to obtain. Replace the complete head when the head is damaged, compatibility is uncertain, or the combined part and labor cost approaches the cost of a suitable new head.

If the trimmer is older and also has poor power, battery problems, repeated line-feed failures, or significant housing damage, replacing only the knob may not solve the larger reliability issue. In that situation, compare the repair cost with the cost of replacing the trimmer itself.

Related guides

• Replacement bump head