String Trimmer Head Replacement: A Complete How-To Guide (Updated April 2025)

A practical, no-fluff guide to string trimmer head replacement covering every head type, tool-by-tool compatibility, and a clear step-by-step process for getting back to work fast.

String Trimmer Head Replacement: Everything You Need to Know

A worn-out or cracked trimmer head is one of the most common reasons a perfectly good string trimmer starts underperforming. Whether your bump head stopped feeding line, your fixed-line head cracked after a rock strike, or you're upgrading to a blade head for thick brush, replacing it is a straightforward job, if you know the thread direction, the right head type, and the torque specs for your machine. This guide walks you through the whole process, from identifying which head fits your trimmer to spinning the new one on correctly the first time.

Know Your Head Type Before You Buy a Replacement

Not all trimmer heads work the same way, and swapping one type for another can dramatically change what your machine is capable of. The three most common types are bump-feed heads, fixed-line (pre-cut) heads, and blade heads. Bump-feed heads, like the Husqvarna T35 or STIHL AutoCut 25-2, use an internal spool of monofilament line that advances when you tap the head on the ground. They're the go-to choice for standard lawn edging and trimming on half-acre or smaller lots. Fixed-line heads accept pre-cut lengths of heavier line (often 0.095" to 0.130") loaded directly into eyelets, no spool to untangle, which appeals to anyone who's lost 10 minutes fighting a bird's nest of tangled line mid-job.

Blade heads replace monofilament entirely with a plastic or metal blade, making quick work of thick weeds, blackberry canes, and saplings up to about an inch in diameter. Oregon, Husqvarna, and STIHL all make blade heads rated for their own machines, and you'll want to verify your engine displacement (typically 25cc-30cc minimum) before mounting one, a blade head puts significant rotational stress on the drive shaft and clutch drum. For battery-powered trimmers in the 40V-80V class like Greenworks Pro or EGO Power+ units, blade head availability is growing but check the manufacturer's compatibility list first.

Thread Direction and Arbor Size: Getting Compatibility Right

This is where most DIY head swaps go wrong. Trimmer heads thread onto the drive shaft arbor, and the thread direction is intentionally reverse (left-hand thread) on most gas-powered trimmers. That means you turn the head clockwise to loosen it, the opposite of every other fastener you've ever touched. STIHL uses a M10 x 1.25 LH thread on the majority of their curved- and straight-shaft gas models. Husqvarna's most common spec is M10 x 1.25 LH as well, but some older curved-shaft models use a smaller M8 thread. Echo and Ryobi gas units often use M10 x 1.25 LH or a proprietary nut system depending on the model year.

Battery-powered trimmers are less standardized. A Greenworks 80V unit might use an M10 x 1.25 RH (right-hand thread) adapter, while EGO Power+ models use a proprietary twist-lock collar. Always pull the owner's manual or check the stamped spec on the retaining bolt before ordering. Universal-fit replacement heads from Oregon or Forester typically include multiple arbor adapters in the box, it's worth the extra $5 over a single-adapter OEM head if you're not 100% sure of your thread spec.

Tools and Parts You'll Need

String trimmer head replacement doesn't require a full shop setup. Gather these before you start so you're not hunting through drawers mid-job.

Step-by-Step: How to Replace a String Trimmer Head

OEM vs. Aftermarket Replacement Heads: The Real Trade-Offs

When to Replace vs. When to Just Reload Line

A trimmer head that's just out of line doesn't need replacing, it needs reloading. But there are clear signs the head itself is done: cracks or chips in the housing (especially visible stress fractures radiating from the center bolt hole), a bump mechanism that won't advance line even after a firm tap on pavement, a spool that rattles because the internal spring has snapped, or any visible wobble when the head spins at full throttle. Wobble is especially serious, it puts lateral stress on the drive shaft bearing and will eventually cause a much more expensive repair.

On gas trimmers used seasonally for half-acre lots, a quality bump-feed head typically lasts 3-5 years before the housing UV-degrades to the point of cracking. Landscaping crews running trimmers 6-8 hours a day often replace heads every season. For cordless homeowner trimmers in the 40V-56V range, heads tend to last longer because the lower torque puts less stress on the housing, but the bump mechanism can still wear out. If your trimmer is more than 8 years old and showing these signs, also check the drive shaft for flex, a bent shaft will destroy a new head quickly.