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How to Use a String Trimmer Properly Without Breaking the Line!

Gas and electric corded trimmers

Gas and electric corded trimmers

What Is a String Trimmer?

This garden power tool has many names depending on where you live in the world, but one thing is sure: It certainly beats a hedge shears for trimming and edging! Like most power tools nowadays, string trimmers are available in gas, electric corded and rechargeable battery cordless versions.

The machine was invented in the early 1970s by George Ballas of Houston, Texas. He attached fishing line to the head of a lawn edger and was able to cut grass and weeds with his concoction. The tool works on the principle that a line spun at high speed becomes stretched and taut due to centripetal acceleration. The tension in the line makes it stiff, and the inertia of the line gives it the energy to cut. So basically, it is like a spinning whip.

This guide shows you how to wind the spool and cut, without breaking the line!

Alternative Names for a String Trimmer

A string trimmer is also known as:

  • Weed Eater (brand name)
  • Weed Whacker
  • Strimmer (In the UK and Ireland)
  • Weed Trimmer
  • Lawn Trimmer
  • Weed Whip
  • Whipper Snipper (Australia and Canada)
  • Line Trimmer

How to Use a String Trimmer Safely

Follow these safety tips in order to operate your string trimmer properly.

Wear PPE (Personal Protective Equipment)

  • The most important thing is to protect your eyes because even a small piece of snapped line, or debris such as a pebble travelling at 200 to 300 mph has the potential to cause a severe eye injury. A visor provides total face protection.
  • Don't wear shorts or sandals for obvious reasons. Heavy trousers will give some protection if you get hit by debris (even though you will still feel it!). Even broken off fragments of trimmer line will cut skin.
  • Wear safety shoes which have steel caps and soles, as these will protect your feet if you walk on glass, metal, nails or other nasty stuff while working in an overgrown area. When cutting, angle the trimmer so that cut material is thrown away from you.
  • Finally, wear ear muffs to protect your ears as long term exposure to loud sounds can cause hearing damage.

Check the Ground Before Trimming

Bits of glass, metal, pebbles etc. can be caught by the line and become missiles

Take Care of Cables

Take care not to hit the cable while using an electric trimmer. Electric trimmers are less powerful than gas versions and the line probably wouldn't cut the cord, however it could damage the insulation.

Watch Out for Bystanders

People standing near you are more likely to be hit by debris than yourself. This is because debris may be travelling on an upwards trajectory and eventually reach eye level by the time it reaches the other person. This is especially the case if you are trimming or using a lawn mower on elevated ground. So pause your work until they have moved away.

Debris thrown by a trimmer can also ricochet off hard surfaces. Young children should never be in the vicinity when you are using powered garden equipment.

Avoid Windows and Cars

Pebbles thrown by a trimmer can easily chip paintwork and glass, and may even crack or break windows in your house or car. Check where debris is flying and direct it safely away from anything which could be damaged

How to Mix Fuel for a Gas Trimmer

Most trimmers have two stroke engines which don't have a sump filled with oil for lubrication (four stroke trimmers however are available). So lubricating oil must be mixed with gas before filling the tank.

In general, the oil to gas ratio for a 2-stroke engine should be 1:50. This equates to 20 ml per liter or 2.6 US fluid ounces per US gallon. Some manufacturers recommend a 1:25 ratio for their engines so check your manual. 1:40 or 1:30 is probably a good compromise.

  1. You can buy small bottles of two stroke oil which are sufficient to make up about a gallon or 5 liters (litres) of mix in a jerrycan.
  2. Alternatively you can buy larger liter or quart bottles of oil. Oil can then be measured out into a graduated measuring cup or any other suitable container which you know the volume of.
  3. Add the oil to the gas can and then swirl it around to mix thoroughly.
  4. It's probably a good idea to wipe out any remaining oil from the measuring cup with a paper towel; otherwise, all sorts of dust and dirt, spiders and insects will have stuck to the oil by the time you use it the next time.

What About Four-Stroke Motor Oil?

Don't use 4 stroke motor oil in your engine. It contains additives which can contaminate the cylinder over time and also it can tar up the piston. Two stroke oil is designed to burn as cleanly as possible with the fuel.

Gas and Two-Stroke Oil Needed for a 50:1 Ratio

For a 25:1 ratio, double the amount of oil

Amount of Fuel Mixture RequiredGasoline/PetrolTwo Stroke Oil

1 Litre

1 Litre

20 ml

5 Litres

5 Litres

100 ml

1 US Quart

1 US Quart

0.6 US Fluid Ounces

1 US Gallon

1 US Gallon

2.6 US Fluid Ounces

How to Start a String Trimmer

A general procedure for starting two-stroke engines. There may be some variation as regards setting choke positions, so consult the instructions that came with your machine.

  1. Mix fuel as outlined above and fill the tank at least half full.
  2. Turn on the ignition switch.
  3. Turn on the choke.
  4. Prime the engine by pushing the primer button about 8 times.
  5. Pull the starter cord repeatedly until the engine kicks and attempts to start or runs.
  6. If it runs, allow it to continue running for about 10 seconds and then turn off the choke.
  7. If it cut out with the choke on, try turning off the choke and re-pulling the cord. If it doesn't start after a few attempts, turn the choke back on and re-starting.

What If I Can't Start the Engine?

If you still can't start the engine, check out my fault-finding guide to string trimmers: String Trimmer (Strimmer) Won't Work: 2-Stroke Engine and Carburetor Troubleshooting

Troubleshoot issues with your trimmer.

Troubleshoot issues with your trimmer.

What Type of Line Do I Use With My Trimmer?

Trimmers make use of a monofilament nylon line for cutting. You can buy this line in spools or rolls of varying lengths and cut it to size. Lower powered electric trimmers are normally used with fairly light gage line, about 1.5 mm / 0.06 inch (about 1/16 inch). This is fine for trimming light grass growth around curbs, edges of lawns and flower beds.

Higher powered gas trimmers can also be used with light gage line, but for cutting thick vegetation in overgrown areas such as long grass, nettles, docks and other weeds, heavier gage line is pretty much essential to avoid breaking or tangling the line. This is available in sizes up to 4 mm / 0.16 inch (about 3/16 inch ). You can also buy line which has a square profile rather than round, and supposedly the "corners" cut better.

What I Use

I use a 2.4 mm line (about 0.1 inch) with my gas trimmer, which is a low cc model, and it gives good results with light and small amounts of heavy growth. Thicker gage line requires a correspondingly higher powered trimmer so that the head can spin sufficiently fast without the engine struggling and slowing down in thick vegetation. As always, its better to consult your manual to see what the manufacturer recommends.

Trimmer Line Replacement: Winding the Spool

  • Make sure the end of the line is fully attached to the spool by feeding it through the thread hole or guide designed for this purpose. The line should then be wound tightly and evenly around the spool in the direction of the arrow (pull the line tight to tension it as you are doing so).
  • If the line is wound slackly, excess line will feed out of the spool during an auto feed, or when you bump the cutting head to release line, and just end up getting snapped off by the knife on the guard of the trimmer. Neat winding helps to ensure that line feeds out ok and doesn't bind. You don't have to be overly fussy about this. Just make sure all the line isn't bunched up at one end of the spool or is very loosely wound around it.

Photo Tutorial

Thread the end of the line into the hole and wind it in the direction of the arrow.

String trimmer spool

String trimmer spool

Evenly wound spool. You can wind on lots more line than this.

String trimmer line on spool

String trimmer line on spool

Replace the spool into the cutting head. Feed the end of the line through the hole in the cutting head of the trimmer and replace the cap.

Feed the line through the hole in the cutter head

Feed the line through the hole in the cutter head

Replace the cap on the cutting head (if fitted).

Replace the cap on the cutting head

Replace the cap on the cutting head

How to Replace a String Trimmer Line

The Cutting Knife on a Trimmer

With some trimmers, the cutting head must be bumped down onto the ground to release new line. Other models auto-feed as line is used up. In any case, a cutting knife at the perimeter of the guard on the trimmer keeps the line trimmed and prevents it hitting or extending beyond the guard (which will slow or stall the machine and create a hazard). This knife should be sharp, otherwise it won't cut the line. If you notice this problem, you can sharpen it with a file, sharpening stone, or angle grinder.

Cutting knife on trimmer

Cutting knife on trimmer


Long-Lasting Square Trimmer Line

This 0.095 inch diameter (2.4mm), 3 pound, 685 foot spool of professional trimming line manufactured by Oregon and available from Amazon should last you years and years. It has a high strength inner core and an outer coating to prevent welding. In other words it is less likely to stick as it unwinds from the spool. The square edges also improve cutting efficiency compared to standard round profile type.

Check your trimmer to see whether it is compatible with this diameter line.

How to Cut With a Trimmer

  • When using an electric trimmer, cut with the bare tip of the line because this is the section which moves at the highest speed
  • Don't let the line slap into grass. Ease the line into vegetation, bit by bit from the edge. You shouldn't allow all the line to come in contact with grass as this will tend to slow it down causing it to lose momentum and it won't do a good job of cutting
  • When cutting around thick vegetation, it's vital to slowly move the line into the material being cut, otherwise it'll become tangled or break. A gas trimmer has more power, is usually fitted with heavier gage line, and you can allow more of the line into the vegetation without it slowing down. You just need to listen to the sound of the machine and when the line slows down, back off out of the grass
  • The line often breaks when trimming around pavements, kerbs, pots etc, This is because it tends to slap against the hard surface instead of soft growth and this can shear or abrade it rapidly. This is why it is important to just cut with the tip of the line. It still wears rapidly, but just a little bit at a time rather than in large pieces
  • Thinner gage line will snap more readily. You can use a thicker line but it is less effective when cutting at low speeds. The advantage of a gas trimmer is that you can vary the speed depending on whether you are cutting fine grass or thick vegetation
  • When cutting, angle the trimmer so that the cutting head isn't rubbing off the ground. Adjust the distance of the head from the ground so that the full extent of the line isn't rubbing off the ground surface either. Let the tip of the line do the work and ease it into the area being cut
  • When cutting around shrubs and trees, take care you don't strip the bark which can kill the plant
  • Creepers and climbing plants can sometimes get tangled around the cutting head of a trimmer, although the line usually cuts through these, however if you have young cabbage palms (cordylines) in your garden, keep well away from the leaves! These are long tough and stringy and always seem to get wrapped around the head, stalling the engine
Cutting with the tip of the line to avoid breaking it

Cutting with the tip of the line to avoid breaking it

Angle the trimmer when cutting

Angle the trimmer when cutting


Lighter Gage Line for Gas or Electric Trimmers

Home-owner grade trimmer line by Rino-Tuff from Amazon. This line is lighter gage 0.065 inch (1.65 mm) and more suitable for electric trimmers. You can use it with a gas trimmer, but it will snap and get used up quicker when cutting heavier vegetation. Thinner line is more suitable for cutting light growth and trimming around flower beds and curbs.

Check your trimmer documentation to ensure this diameter line is suitable.

How to Avoid Getting Covered by Grass

The line generally turns clockwise (viewed from above), so when it's to right of the cutting head it's heading towards you and when on the left, it heading away from you. If you imagine a circle through which the line turns, try to keep this circle positioned so that you're cutting in the 10 o'clock to 12 o'clock positions and grass is being thrown away from you or perpendicular to you.