537185801 Bump Knob Replacement - Pack of 4

Key Features

• High-Impact Blue Polymer: Manufactured from a high-density, abrasion-resistant plastic specifically designed to withstand thousands of high-velocity "bumps" against asphalt, gravel, and packed soil.
• Reinforced Structural Ribbing: The internal geometry features reinforced walls to prevent the knob from cracking or shattering under the extreme centrifugal force of commercial-grade trimmers.
• Precision Spring Seating: Engineered with a centered internal recess to perfectly secure the T35's heavy-duty spring, ensuring a consistent and responsive line-advancement "snap" every time.
• Commercial-Grade Multi-Pack: This 4-piece kit provides a cost-effective solution for landscaping fleets and large residential properties, ensuring you always have a spare on hand to prevent downtime.
• Snap-In Tool-Free Installation: Designed as a direct-fit replacement that clicks into the existing T35 housing by hand, allowing for rapid field repairs without the need for specialized tools.

Replaces / Cross-Reference Part Numbers

• 537185801
• 537 18 58-01
• 537185901 (Associated Spool)
• 531300194 (Associated Head)

Compatibility & Fitment

This bump knob is the standard replacement for the professional "T35" series trimmer heads. Key compatible models include:

• Husqvarna 100/200 Series: 123L, 123LD, 225L, 232L, 232R.
• Husqvarna 300 Series: 322L, 323L, 324Lx, 325L, 325Lx, 326L, 326Lx, 327LS.
• Husqvarna 500 Series: 522L, 525L, 525LS, 525LST, 535LS.
• Historical Production Dates: Compatible with T35 heads manufactured during the 2003-05, 2005-01, and 2007-04 production cycles.
• Note: Specifically for the T35 head. If you have the smaller T25 or larger T45, this knob will not fit correctly.

When to Replace

• Worn-Through Bottom: The center of the blue knob has ground down from pavement contact to the point where the internal metal spring is visible or falling out.
• Broken Retaining Tabs: The plastic clips that lock the knob into the housing have snapped, causing the knob to fly off while the trimmer is spinning at high RPM.
• Seized Mechanism: Significant buildup of dried sap, dirt, and debris has fused the knob to the internal spool, preventing it from releasing new line.
• Rounded Indexing Teeth: The internal plastic "teeth" that click against the spool have rounded off, causing the head to dump all the line at once during use.
• Visible Stress Fractures: White lines or cracks appearing in the blue plastic, indicating the material has become brittle and is prone to catastrophic failure.

Installation Tips

• Clean the Housing: Before installing the new knob, use a stiff brush or screwdriver to remove all dried grass and mud from the locking slots and spring area to ensure a 100% secure lock.
• Inspect the Spring: Ensure the internal metal spring is not rusted or collapsed. If the spring is weak, the new knob will stay "compressed," causing the line to feed uncontrollably.
• Snap-Lock Check: Press the new knob firmly into the housing until you hear a distinct "click" on both sides. Give the knob a firm tug by hand to verify it is fully seated before starting the engine.
• Manual Feed Test: With the engine off, press the knob in and pull the trimmer line manually to ensure the indexing mechanism is releasing the line smoothly.
• Avoid "Riding" the Head: To maximize the life of your new knobs, try to avoid letting the trimmer head drag on the ground while trimming; only make contact with the surface when you need to advance the line.