How do You Change the Head on a Stihl Weedeater?

Changing the head on a Stihl weedeater is a straightforward maintenance task that usually takes less than 10 minutes. You’ll need a few basic tools and the correct replacement head for your model. This guide walks you through the entire process, from removing the old head to installing a new one, and covers common troubleshooting tips.

What Tools Do You Need to Change a Stihl Trimmer Head?

Before you start, gather these simple items. Most Stihl trimmers use a locking pin or a screwdriver to hold the shaft in place while you unscrew the head. Many new heads come with the needed locking pin.

Locking pin (often included with a replacement head, or you can use a 4mm hex key or a small flathead screwdriver)
Safety glasses to protect your eyes from debris
Work gloves for grip and protection
Replacement trimmer head (choose one compatible with your Stihl model)
Trimmer line (if the new head doesn’t come pre‑spooled)
Rags or paper towels to clean the shaft threads

If you don’t have a locking pin, you can buy a Stihl universal locking pin set or use a hex key of the correct size. For stuck heads, a pair of pliers with padded jaws can help, but these are rarely needed.

How Do You Remove the Old Trimmer Head on a Stihl Weedeater?

Removing the old head is the only tricky part. The shaft is held by a plastic or metal retaining screw that threads clockwise or counterclockwise depending on your model. Follow these steps carefully to avoid stripping the threads.

Step‑by‑Step Removal

Turn off the engine or disconnect the battery – Safety first. Make sure the trimmer cannot start accidentally.
Remove any remaining trimmer line – Cut or unwind the old line from the head so you can see the retaining parts.
Align the shaft holes – Rotate the trimmer head until the small holes in the gearbox (or shaft) align with the holes in the head’s base. You’ll see a small hole or a notch.
Insert the locking pin – Push the locking pin through the aligned holes. This locks the shaft so it cannot spin.
Unscrew the head – Grip the head firmly and turn it clockwise (for most Stihl models) to remove it. Some older models turn counterclockwise. If you meet resistance, check that the pin is fully seated.
Remove the pin and set aside the old head – Once the head is free, pull out the locking pin and discard the old head or keep it as a spare.

If the head won’t budge, spray a little penetrating oil around the threads and wait 5 minutes. Never use excessive force – you can damage the shaft. If the head is completely seized, consider taking the trimmer to a Stihl dealer.

How Do You Install the New Head on a Stihl Weedeater?

Installing the new head is the reverse of removal, but there are a few important details to get right.

Preparation

Check the threads – Make sure the new head’s threads match your trimmer shaft. Stihl uses two common thread sizes: standard (10mm left‑hand) and SpeedFeed (12mm right‑hand). If you’re switching head types, confirm compatibility. Most Stihl models accept the SpeedFeed 400‑2 or AutoCut series.
Clean the shaft threads – Wipe away any old grease, dirt, or debris with a rag.

Installation Steps

Place the new head onto the shaft – Slide it on without forcing.
Hand‑tighten the head – Turn it counterclockwise (for most models) until it feels snug. Do not overtighten yet.
Align the shaft holes again – Rotate the head so the locking pin can be re‑inserted through the holes. This locks the shaft in place while you final tighten.
Tighten the head firmly – With the pin inserted, give the head an extra ¼ to ½ turn using your hand only. Do not use a tool – overtightening makes future removal harder.
Remove the pin – Pull out the locking pin and verify the head spins freely.
Load the trimmer line – Follow the instructions for your specific head type. Most Stihl heads use a bump‑feed mechanism or a SpeedFeed system that loads line automatically.

After installation, start the trimmer and test at low throttle. Tap the head on the ground to feed line. If the line doesn’t advance, check that you loaded it in the correct direction.

What Are the Different Types of Stihl Trimmer Heads?

Stihl offers several head designs, each with pros and cons. Knowing the types helps you choose the right replacement for your trimming style.

Head Type	How It Works	Best For	Common Stihl Models
Bump Feed	Tap the head on the ground to advance line	General purpose, heavy weeds	AutoCut 25‑2, 30‑2
SpeedFeed	Line loads automatically without disassembly	Quick line changes, light trimming	SpeedFeed 400‑2, 500‑2
Fixed Line	Requires manual reloading when line wears	Low cost, simple design	Standard on some entry‑level trimmers
PolyCut	Uses plastic blades instead of line	Edging, clean cuts on grass	PolyCut 20‑2

Bump feed heads are the most common choice for homeowners. The SpeedFeed system is popular because you can reload line in seconds by simply pulling a new strand through the eyelet – no disassembly needed. If you have an older Stihl, you can often upgrade to a SpeedFeed head if the threads match.

How Do You Troubleshoot a Stuck or Damaged Head?

If your old head refuses to come off or the new one doesn’t fit, check these common issues.

The Head Won’t Unscrew

Locking pin not fully inserted – Re‑align the holes and try again. The pin must go all the way through to stop shaft rotation.
Threads are rusted or gummed up – Apply a small amount of penetrating oil (like WD‑40) to the threads from above and below. Wait 5–10 minutes, then try again.
You’re turning the wrong direction – Stihl uses both left‑hand and right‑hand threads. Check your model’s manual. For most Stihl trimmers, removal is clockwise.
Head is cross‑threaded – If the head was previously forced on, the threads may be damaged. This often requires a new shaft or a professional repair.

The New Head Doesn’t Fit Tightly

Wrong thread size – SpeedFeed heads require a special retaining screw (usually included). Standard AutoCut heads screw directly onto the shaft. Verify you have the correct version.
Missing washer or spacer – Some models have a metal washer that sits between the head and the gearbox. If you lost it, the head will wobble. Replace it with a Stihl trimmer washer set.

The Line Won’t Feed After Installation

Line loaded backwards – In bump feed heads, the line must wind in the direction of the arrow on the spool. For SpeedFeed, the line should slide easily through the eyelet without kinking.
Line too thick – Stihl heads are designed for specific line diameters, usually 0.080″ to 0.105″. Check the head’s label. Too thick a line will jam; too thin a line won’t feed properly.

How to Maintain Your Stihl Trimmer Head for Long Life

Regular care prevents most problems and extends the life of your head and trimmer.

Clean the head after each use – Remove built‑up grass, dirt, and debris from the line slots and the bump knob. A stiff brush works well.
Lubricate the bump knob – Once a season, apply a drop of light machine oil to the bump mechanism (the part you tap on the ground). Wipe away excess.
Replace worn line – Don’t wait until the line breaks off inside the head. Replace when it’s shorter than 4 inches.
Avoid hitting hard objects – Striking concrete, rocks, or fences can crack the head housing. If you see cracks, replace the head immediately.
Store the trimmer upright – Hang the trimmer or lay it on its side so the head doesn’t rest on the ground. This prevents the line from taking a permanent crimp.

Tips for a Smooth Head Replacement on Your Stihl Weedeater

Changing the head is a simple procedure once you understand your specific Stihl model. The key steps – align the shaft holes, insert the locking pin, and turn the correct direction – apply to nearly every gas‑powered and battery‑powered Stihl trimmer.

Always keep a locking pin and a spare head in your toolbox so you can swap heads quickly when the line runs out mid‑job. If you regularly trim thick brush, consider upgrading to a bump feed head with heavy‑duty line for better durability.

For the most common Stihl trimmers (FS 45, FS 55, FS 70, FS 90, and compatible battery models like the FSA 45), the SpeedFeed 400‑2 is a direct replacement that saves time on line loading. You can find it online at Amazon or at your local Stihl dealer.

By following the removal, installation, and maintenance steps above, you’ll keep your weedeater running efficiently and avoid unnecessary repairs. Changing the head on a Stihl weedeater is something any owner can handle with the right tools and a little patience.