[Lifetime Warranty] 537338701 Trimmer Head Bump Knob - Compatible with Husqvarna T25 Heads (124L, 125L, 125LDx, 128L, 128LD, 128LDx, 128R, 128RJ, 223L, 223R, 26LC, 32LC) - Pack of 2

537338701 Trimmer Head Bump Knob - Pack of 2

Key Features

• High-Density Wear Polymer: Engineered from specialized, abrasion-resistant plastic to withstand the high-velocity friction of tapping against concrete, asphalt, and rocky soil.
• Internal Spring-Loaded Interface: Designed with precision-molded internal slots to securely house the trimmer head spring, ensuring a consistent and responsive "snap-back" after every tap.
• Balanced Impact Surface: Features a reinforced, flat-bottom design that ensures even wear and prevents the trimmer head from wobbling at high RPMs.
• High-Visibility Blue: The signature factory-match color allows for easy inspection of wear levels and ensures the knob is easy to locate during field maintenance.
• Tool-Free Installation: A direct-fit replacement that snaps into the existing T25 housing by hand, allowing for rapid repairs without the need for additional tools.

Replaces / Cross-Reference Part Numbers

• 537338701
• 537 33 87-01
• 537338702
• 966674401 (Associated Head)

Compatibility & Fitment

This bump knob is the standard replacement part for the Husqvarna T25 (Tap Advance) series trimmer heads. Key compatible models include:

• Husqvarna 120 Series: 124L, 125L, 125LDx, 128L, 128LD, 128LDx, 128R, 128RJ.
• Husqvarna 200 Series: 223L, 223R, 26LC.
• Husqvarna 300 Series: 32LC, 322L, 323L, 325L.
• Note: This part is specifically for the T25 head. If you have the larger T35 or T45 head, you will require a different part number.

When to Replace

• Ground-Through Surface: The bottom of the blue knob has worn down so thin that the internal spring is visible or has already fallen out.
• Broken Retaining Tabs: The plastic clips that hold the knob inside the trimmer head housing have snapped, causing the knob to fly off during operation.
• Seized Mechanism: Dirt and dried sap have fused the knob to the internal spool, preventing it from being "tapped" to release new line.
• Rounded Edges: Significant wear has rounded the edges of the knob to the point where it no longer triggers the internal indexing mechanism reliably.
• Cracked Housing: Visible stress fractures in the blue plastic which can cause the knob to shatter at high RPMs.

Installation Tips

• Clean the Housing: Before installing the new knob, use a stiff brush to remove all dried grass, mud, and debris from inside the trimmer head and around the spring.
• Check the Spring: Inspect the internal metal spring for rust or loss of tension. If the spring is weak, the new knob will not retract properly, causing the head to dump all the line at once.
• Tab Alignment: Align the retaining tabs of the new knob with the slots in the T25 housing. Press down firmly until you hear a distinct "click" on both sides.
• Manual Feed Test: Once installed, press the knob in by hand and pull the line to ensure the internal spool is releasing the line correctly before starting the engine.
• Avoid Dragging: To prolong the life of your new knob, avoid letting the trimmer head "ride" on the ground while trimming; only make contact with the surface when you need to advance the line.