The head on the end of your shaft decides how much time you spend cutting versus fiddling with line, and it decides whether the machine can take a metal blade at all. Four broad families cover almost everything on the market: fixed line, bump feed, auto feed (semi- and fully-automatic), and blade adapters. None of them is universally “best” — each solves a different problem.
Fixed line (manual) heads
A fixed-line head holds pre-cut lengths of line in a spool or a set of holes, with no feed mechanism at all. When the line wears down, you stop the engine, open the head, and thread in fresh pieces by hand.
How it works: the line sits in a drum or clips into slots around the head, held by friction or a simple locking cap. There’s no spring, no ratchet, no bump knob.
Pros:
- Simplest mechanism on the market — almost nothing to break
- Among the cheapest heads you can buy
- No line jams, no tangled spools, no spring fatigue
- Easy to carry spare pre-cut lengths in a pocket for a quick swap
Cons:
- You must stop and manually rethread every time line breaks or wears short
- Slower in heavy, coarse growth where line snaps often
- Some designs only take short lengths, so you’re stopping more often than with a full spool
Best for: occasional users, light garden edging, anyone who wants a head that essentially never needs repair, and pros who deliberately avoid feed mechanisms because they’re one less thing to fail mid-job.
Bump feed heads
Bump feed is the mechanism most homeowners already know: tap the head on the ground while it’s spinning and a spring-loaded ratchet releases a short length of line, which a fixed blade on the guard trims to size.
How it works: the spool sits under spring tension against a ratchet plate. A bump on the ground pushes the spool up momentarily, letting it rotate slightly and unspool a set amount of line before the spring pulls it back into place.
Pros:
- Feed on demand, exactly when you need more line
- Mechanically simple compared to auto-feed, so fewer failure points
- Widely available, and replacement spools are cheap and easy to find
- Works with both single-line and dual-line spools
Cons:
- Requires you to bump the head against the ground, which is awkward on smooth lawns or overhead trimming and can scuff paving
- Ratchet teeth and the spring wear out after a season or two of heavy use
- Line can jam or weld together inside the spool if it overheats from friction (see line welding together)
Safety note: never tap the head against your boot, a fence post at hand height, or anything you wouldn’t want the blade guard’s cutting edge near. Bump only against the ground, with the head pointed away from your legs.
Best for: general-purpose trimming and edging — the default choice for most petrol and cordless brush cutters sold today.
Semi-automatic feed heads
Semi-auto heads release line automatically once the existing line gets too short to maintain full cutting speed — no bumping required — but you still need to manually pull out extra line occasionally or after fitting a fresh spool.
How it works: centrifugal force plus a spring-loaded catch releases line once RPM drops (a sign the line has worn short and lost mass). Some designs need a light tap to trigger the release; others do it purely on speed sensing.
Pros:
- Less manual intervention than bump feed
- No need to stop and bend down to bump the head
- Good compromise between reliability and convenience
Cons:
- More moving parts than bump feed, so more that can wear or jam
- Slightly more expensive to replace when the mechanism fails
- Can over-feed in some conditions, wasting line and needing more trims of excess length by the cutting blade
Best for: users doing long trimming sessions who don’t want to stop and bump but don’t want the added cost of a fully automatic head either.
Fully automatic feed heads
Fully automatic heads feed a fixed increment of new line continuously or at set intervals, with zero input from the operator — no bumping, no manual pulling.
How it works: a spring-and-ratchet mechanism inside the head senses when the line has worn short and advances a little more, which the guard blade then trims to length. Black+Decker’s AFS system on its electric trimmers is the best-known example.
Pros:
- Genuinely hands-off — ideal for continuous heavy-duty trimming
- No downtime bumping or rethreading
- Consistent line length throughout the job
Cons:
- The most expensive head type to replace
- Usually tied to one brand’s own spools, which cost more than bulk line
- More complex mechanism means more that can go wrong, and repairs usually mean replacing the whole head rather than a simple spring or spool
Best for: contractors and landscapers running brush cutters all day, where time saved not stopping outweighs the extra line consumption and higher unit cost.
Comparison at a glance
| Head type | Feed method | Relative cost | Failure points | Best use |
|---|---|---|---|---|
| Fixed line | Manual rethread | Lowest | Almost none | Light, occasional use |
| Bump feed | Tap to release | Low (spools cheaper still) | Spring, ratchet teeth | General trimming/edging |
| Semi-automatic | Releases as line wears, sometimes with a light tap | Medium | Spring, catch mechanism | Long sessions, less stopping |
| Fully automatic | Advances as line wears | Highest | Whole feed assembly | Hands-off trimming |
| Blade adapter | N/A — rigid blade | Low to medium | Nut, blade balance, gearbox stress | Woody growth, brambles, saplings |
Quick-load spools
Quick-load isn’t a separate feed category — it’s a design feature that can appear on bump, semi-auto or auto heads. Instead of unwinding old line and manually winding new line around a spool by hand, you pop out the empty spool and clip in a pre-wound replacement spool.
Pros: fastest possible line change, useful for pros who carry spare pre-wound spools in a pouch.
Cons: pre-wound spools cost more than buying bulk line and winding your own; you’re paying for convenience.
Blade adapters
A blade adapter replaces the line head entirely, bolting a rigid metal blade — commonly a 3-tooth or 4-tooth brush blade, or a specialist saw-tooth blade — directly onto the drive shaft.
How it works: the adapter plate mounts to the gearbox output shaft, the blade sits on top, and a retaining nut (often left-hand threaded so cutting force tightens rather than loosens it) locks it down.
Pros:
- Cuts through woody stems, brambles and saplings that would strip line instantly
- Faster clearance in heavy, overgrown ground
- No feed mechanism to jam or wear
Cons:
- Only suitable for machines with sufficient torque and a gearbox rated for blade use — check your manual
- Higher risk of kickback and thrown debris than line
- Requires the shaft, clutch and gearbox to be in good condition; a worn clutch drum or loose shaft bearing under blade load is a real hazard
Safety note: a metal blade throws debris much harder and further than line. Clear the 15 m working radius of bystanders, wear a face shield plus safety glasses underneath, and never fit a blade to a machine not rated for one — the extra rotational mass and torque can shatter a gearbox designed for line only.
For the mechanics of actually swapping a head or blade safely, see how to change brush cutter line, how to load a bump feed head and how to fit a brush cutter blade. The heads we compare are in our brush cutter head guide, and blades in the blade guide.
Choosing the right head for your work
- Lawn edges and light garden trimming: fixed line or bump feed — cheap, simple, rarely needs attention.
- Mixed trimming and edging, moderate use: bump feed or semi-automatic — the everyday sweet spot.
- Long professional days, hedgerows, verges: fully automatic — worth the extra line cost for the time saved.
- Brambles, saplings, dense woody growth: blade adapter, provided the machine is rated for it.
- Uncertain terrain with mixed grass and woody stems: many operators carry both a line head and a blade adapter, swapping depending on what the next section of ground demands.
When to call a dealer
Take the head to a dealer or small-engine shop if:
- The bump mechanism won’t release line even after a full strip-down and spring check
- A blade adapter’s retaining nut is seized or the threads look damaged
- You notice play or wobble in the gearbox output shaft itself — that’s not a head problem, it’s a shaft or bearing problem, and running a blade on it is unsafe
- You’re not confident the machine’s gearbox rating supports the blade you want to fit
Quick checklist
- Match thread size and rotation direction before buying any replacement head
- Keep the spring and ratchet clean and lightly greased on bump and semi-auto heads
- Never overfill a spool — overpacked line overheats and welds together
- Confirm your machine is rated for blade use before fitting an adapter
- Carry spare line and, if you use blades, a spare blade and the correct spanner