Black+Decker 20V Max 12″ String Trimmer

Safety

The Black+Decker LST300 is a light cordless trimmer, but the rotating line can still throw stones, glass, mulch, wire, and hard stems. Inspect the work area before installing the battery and keep bystanders, pets, and vehicles outside the throw zone. Use the guard supplied with the trimmer; operating without it increases line length, motor load, and debris risk.

• Wear eye protection, long pants, closed shoes, and gloves.
• Remove the 20V Max battery before clearing the head or replacing line.
• Keep hands away from the spool until it stops fully.
• Do not use metal blades, wire, chain, or oversized line in the AFS head.
• Avoid wet grass when possible; it loads the head and reduces footing.

If vibration changes suddenly, stop and check for tangled line, cracked spool parts, or debris wrapped around the shaft.

Unboxing & first run

Unpack the trimmer, guard, auxiliary handle, charger, battery if included, and documentation. Confirm the guard is secured before running the tool. Charge the 20V Max battery fully, then slide it into the handle until the latch clicks. Do not test the head indoors; the automatic feed may advance line immediately.

For the first run, extend the line only through the eyelet, stand clear of loose debris, and run the trimmer at low load along a small patch of grass. The cutter on the guard trims excess line to the correct length. Listen for a clean, even motor tone. A harsh slap usually means the line is too long, crossed inside the spool, or the guard is missing.

• Voltage: 20V Max
• Cutting swath: 12″
• Line size: 0.065″
• Head: AFS automatic feed
• Weight without battery: 5.2 lb

Controls layout

The LST300 layout is simple: battery at the rear handle, trigger and lock-off near the grip, auxiliary handle on the shaft, telescoping or adjustable shaft sections, guard at the cutting head, and an AFS spool under the cap. Learn the battery latch and trigger lock before trimming near fences or beds.

The cutting head is designed for nylon line only. The guard sets the correct cutting radius and carries the line-trimming blade. The auxiliary handle should be set so the head floats level without forcing the wrists down. If using the trimmer as an edger, rotate or reposition the head according to the tool’s adjustment mechanism and keep the wheel or guide, if fitted, against the pavement edge.

• Rear handle: trigger, lock-off, and battery socket.
• Front handle: balance and steering.
• Head: spool cap, line eyelet, and automatic-feed mechanism.
• Guard: debris shield and line cutter.

Routine maintenance

Routine care keeps the automatic feed reliable and reduces battery drain. Remove the battery before any cleaning. Brush grass paste from the guard, shaft joint, air slots, and spool cap after each use. Packed clippings around the spool increase friction and may prevent the AFS system from feeding line correctly.

Check the line before storage. Brittle, flattened, or kinked 0.065″ line breaks often and feeds poorly. Store spare line away from heat and direct sun. Inspect the guard for cracks and confirm the line cutter is present and not bent into the cutting path. Wipe the battery rails clean and let the pack cool before charging after heavy trimming.

• Use only round or approved trimmer line in 0.065″ diameter.
• Keep motor vents open.
• Replace cracked spool caps promptly.
• Do not spray the tool with a hose.
• Store with the battery removed.

Blade or chain replacement

This trimmer does not use a blade or chain. The service item is the spool, line, or AFS head assembly. Remove the 20V Max battery, turn the tool over, press the spool-cap tabs, and lift the cap straight off. Pull out the empty spool, clear grass from the pocket, and check that the spring and feed parts remain seated.

Install a prewound spool or rewind with 0.065″ line in the direction marked on the spool. Do not overfill; line packed above the spool rim drags against the cap. Feed the line end through the eyelet, seat the spool, and snap the cap fully into place. Pull the line lightly to confirm it moves, then run the tool outside so the guard cutter trims it.

• Never fit brush blades or metal wire.
• Replace the cap if tabs are loose.
• Use AFS-compatible spool parts.

Sharpening or cleaning

There is no cutting blade to sharpen during normal LST300 service. The fixed cutter on the guard only trims nylon line; if it is damaged or missing, replace the guard or cutter part rather than filing a new edge freehand. A dull or bent cutter leaves the line too long, which slows the motor and shortens runtime.

Cleaning is more important than sharpening. Remove the battery, open the spool cap, and clear grass fibers, melted line fragments, and dirt from the spool pocket. Wipe the shaft and head with a damp cloth, then dry the tool before storage. If line has fused inside the spool from heat, discard that section and rewind with fresh 0.065″ line.

• Clean after trimming wet grass or weeds.
• Keep the line eyelet smooth and free of burrs.
• Do not oil the spool; oil attracts grit.
• Replace worn plastic parts instead of reshaping them.

Troubleshooting

Most LST300 problems trace to battery state, line condition, or a dirty AFS head. Start with the battery removed, then inspect the head before assuming the motor or switch has failed. Reinstall the battery only after the head turns freely by hand.

• No start: battery is discharged, not latched, overheated, or the trigger lock is not fully engaged.
• Short runtime: grass is too heavy, vents are clogged, line is oversized, or the battery is aged.
• No line feed: spool is empty, line is crossed, spool pocket is dirty, or cap tabs are not seated.
• Line breaks often: line is old, eyelet is burred, or trimming is too close to masonry.
• Excess vibration: spool is cracked, line lengths are uneven, or debris is wrapped under the cap.

If the head still will not feed with a clean spool and fresh line, replace the AFS head assembly.

Compatible parts

Use parts sized for the Black+Decker AFS system and 0.065″ line. The trimmer’s 12″ cutting swath depends on the guard and line cutter setting the proper line length; thicker line or non-AFS spools can overload the motor and make feed unreliable. Battery compatibility should be checked by platform and latch style, not only by voltage printed on the pack.

Compatible APOROZONA catalog SKUs are AP—AFS—HEAD, AP—065—LS, and AP—BT—V20. The head and line SKUs cover normal wear items, while the battery SKU supports the 20V Max platform when the pack style matches the tool.

• Line: 0.065″ nylon trimmer line.
• Head: AFS automatic-feed style.
• Battery: matching 20V Max slide-pack interface.
• Avoid universal metal-blade conversions.