Stihl FS 450 Spark Plug (Gap & Replacement)
The STIHL FS 450 commonly uses a Bosch WSR 6 F or NGK BPMR 7 A spark plug, with a specified electrode gap of 0.5 mm (0.020 in). Check your model-year manual before fitting a plug. Work with a cool engine, clean around the plug, start its threads by hand, and investigate repeated fouling rather than replacing plugs again and again.
A dependable spark helps your brushcutter start and run as intended. Replacing a worn plug is a small repair, but fitting the wrong type or forcing its threads can turn it into a costly one. Keeping the ignition system clean and using fresh, correctly mixed fuel also helps avoid wasted parts and fuel.
The FS 450 is a 2-stroke brushcutter, not a chainsaw. Its engine displacement is 44.3 cm³, but plug recommendations can vary by market or machine version. Treat the model-specific STIHL manual as the final word, especially if its guidance differs from a general parts listing.
Confirm the right plug and gap
The spark plug carries electrical current across a small space between its electrodes. That space is the electrode gap. For the FS 450, the listed gap is 0.5 mm (0.020 in); confirm the plug type and gap in the manual for your exact machine before installation.
Commonly listed plug types for the FS 450 are Bosch WSR 6 F and NGK BPMR 7 A. These are recommendations to check, not a reason to choose a plug based only on a matching thread. Plug design and heat range matter, too. If the manual or a reliable STIHL parts reference for your model year specifies a different plug, follow that information.
| Check | FS 450 reference | What to do |
|---|---|---|
| Engine type | 2-stroke brushcutter | Use the FS 450 manual, not a chainsaw guide |
| Displacement | 44.3 cm³ | Confirm your machine’s model details |
| Plug types commonly listed | Bosch WSR 6 F or NGK BPMR 7 A | Verify the applicable manual and market |
| Electrode gap | 0.5 mm (0.020 in) | Measure with a wire feeler gauge |
| Fuel mix | 50:1 with STIHL 2-cycle engine oil, or as specified for the machine | Follow the manual and oil instructions |
A plug that fits the hole is not necessarily the correct plug. Check its full code, package, and manual recommendation. If you cannot confirm the specification, pause and check with a STIHL dealer rather than guessing. Key takeaway: verify the plug type and the 0.5 mm gap before installation.
Prepare safely before removing the plug
Safe preparation keeps fuel vapors away from sparks and helps prevent dirt from entering the cylinder. Stop the engine, let it cool, and keep the machine steady on level ground. Clean the area around the plug before loosening it, and do not work near spilled fuel or an open fuel container.
Set the stop switch to the stopped position. Move the brushcutter outdoors or to a well-ventilated work area away from ignition sources. Keep the cutting attachment clear of people and objects. Have the correct plug socket, a clean brush, and a wire feeler gauge ready.
Brush grass and dirt away from the plug and its recess. This matters because debris can fall into the cylinder when the plug comes out. Pull the spark-plug boot off by gripping its cap. Do not yank the ignition lead; pulling the wire can damage the connection.
Use a socket that fits the plug securely. If the plug resists turning, do not force it. Check that the socket is seated straight and that the engine is cool. Next step: remove the plug carefully, then inspect both the plug and the opening for damage or debris.
Remove, check, and replace the spark plug
A sound replacement starts with careful inspection and correct threading. Look for cracked porcelain, damaged electrodes, heavy deposits, or fuel wetness. Set the new plug to the specified gap, turn it into the cylinder by hand, and tighten it according to the applicable STIHL manual.
- Remove the old plug. With the boot off, turn the plug counterclockwise using the correct socket. Lift it out without letting dirt fall into the cylinder.
- Inspect it. Replace a plug with cracked porcelain or damaged electrodes. Heavy buildup or repeated fuel wetness also calls for attention. A plug’s appearance can offer clues, but it cannot identify every engine problem on its own.
- Check the new plug’s gap. Use a wire feeler gauge to measure the space between the center and ground electrodes. The target is 0.5 mm (0.020 in), unless the machine’s manual specifies otherwise.
- Adjust only if needed. If the gap is wrong, make small changes to the ground electrode. Do not press or lever against the center electrode or porcelain; either can be damaged.
- Start the threads by hand. Hold the plug straight and turn it gently into the cylinder. It should thread in smoothly. If it binds, back it out and realign it. Never use a wrench to force a plug into the threads.
- Tighten and refit. Tighten the plug as directed by the applicable STIHL manual. Do not guess at a torque value. Refit the boot fully so it grips the plug connection.
A loose plug can let combustion gases escape, overheat, or damage the cylinder-head threads. A cross-threaded plug can also damage those threads and may not seal correctly. If the plug will not turn in smoothly by hand, stop and get help rather than forcing it. Key takeaway: hand-start the threads and use the manual’s tightening instruction.
Diagnose recurring fouling or hard starting
Fouling means deposits or fuel have made it harder for a plug to spark. A new plug may restore operation for a time, but repeated fouling suggests another issue needs checking. After replacement, assess fuel quality, the air filter, and the ignition lead before changing more parts.
| What you find or notice | What to check next |
|---|---|
| Plug is wet with fuel | Check starting procedure and fuel condition; repeated wetness needs further diagnosis |
| Heavy deposits return | Review fuel mix and maintenance; inspect the air filter and seek carburetor service if needed |
| Plug porcelain is cracked or electrode is damaged | Replace with the manual-specified type and gap |
| New plug fits, but the engine still will not start | Check fuel quality, air filter, ignition lead, and carburetion |
| Plug loosens or will not thread smoothly | Stop; inspect the threads and get qualified service |
I use a simple rule when a brushcutter comes in with a “bad plug” complaint: first confirm the plug is the right type and correctly gapped; then look for a reason the old one failed. This avoids treating a symptom as the whole repair.
Two practical repair scenarios
These are diagnostic examples, not claims about a particular FS 450 owner or a recorded repair. They show how the same replacement steps can lead to different next actions.
- The old plug is damaged, and the new one starts the engine normally. Confirm the correct plug and gap, install it carefully, and monitor operation. Continue routine checks at the intervals in the manual.
- The replacement plug becomes wet or fouled again. Do not keep fitting plugs and expect a lasting fix. Check fuel freshness and the specified mix, inspect the air filter and ignition lead, and have the carburetor or ignition system assessed if the fault continues.
If the engine still runs poorly after a correct plug replacement, stop repeated plug changes and move to diagnosis. Key takeaway: a plug is a maintenance part, but recurring fouling is a clue to check the system around it.
Maintain the plug and prepare for seasonal storage
Routine checks help catch wear before the brushcutter is needed for a full day’s work. Follow the inspection and service intervals in the correct FS 450 manual. Keep the plug boot and lead clean and undamaged, and use fresh fuel mixed at the ratio specified for your machine and oil.
The commonly listed fuel mix is 50:1 with STIHL 2-cycle engine oil, or the oil and ratio specified for the machine. Measure the fuel and oil carefully; do not estimate the amount by eye. Follow STIHL’s fuel-storage guidance and the instructions on the oil. Use fresh, correctly mixed fuel when preparing the machine for operation.
Before storing the brushcutter, follow the storage steps in its manual. Do not assume fuel can be left in the machine for any set period; storage advice depends on the fuel and product guidance. Keep the plug area clean, and check the boot and lead for looseness, cracks, or other visible damage.
If you remove a plug for seasonal inspection, keep dirt out of the cylinder and reinstall it using the correct type, gap, and manual instruction. Key takeaway: correct fuel and scheduled inspection can help prevent seasonal starting trouble, while repeated deposits deserve diagnosis.
Frequently asked questions
What is the spark-plug gap on a STIHL FS 450?
The listed gap is 0.5 mm, or 0.020 in. Confirm it in the manual for your exact machine before installation.
Which spark plug fits the FS 450?
Bosch WSR 6 F and NGK BPMR 7 A are commonly listed types. Verify the recommendation for your model year and market before buying.
Can I use another plug if its threads fit?
No. Thread fit alone does not confirm the right type or heat range. Check the manual or a reliable STIHL parts reference.
How do I measure the plug gap?
Use a wire feeler gauge between the center and ground electrodes. Compare the result with the specified 0.5 mm gap.
Which part of the electrode can I adjust?
Adjust the ground electrode only, and make small changes. Avoid pressing on the center electrode or porcelain.
Should I use a torque wrench to install the plug?
Use the tightening direction in the applicable STIHL manual. Do not assume a torque value if the manual does not provide one.
Why is my FS 450 spark plug wet?
Fuel wetness can point to a starting or fuel issue, but it does not prove one cause. If it keeps happening, check fuel condition and seek further diagnosis.
Why does a new spark plug foul quickly?
Repeated fouling may relate to fuel, the air filter, ignition lead, or carburetion. Replacing the plug again without checking those items may not solve the problem.
Can I check for spark beside fuel vapors?
No. Keep spark checks away from spilled fuel, fuel vapors, and open fuel containers. Follow the manual’s safety procedure or ask a qualified technician.
What should I do if the plug will not thread in by hand?
Stop and realign it. Do not force it with a wrench. If it still binds, the threads may need inspection by a qualified service provider.
(This article was written by one of our staff writers, Harrison Croft. Visit our Meet the Team page.)
