Stihl BR 800 Spark Plug (Gap & Replacement)
The STIHL BR 800 uses an NGK CMR6H spark plug with a 0.5 mm (0.020 in) electrode gap. Confirm your blower version and check its current operator’s manual before fitting a plug. Work outdoors with a cool engine, clean around the plug, install it by hand, and tighten it as the manual directs.
For a busy yard, a little luxury is equipment that starts when you need it. A fouled or damaged plug can interrupt work, but replacing it is only useful when the part and gap are right. On the BR 800, check the plug before you blame the carburetor or keep pulling the starter.
I approach plug service as both a repair and a quick inspection. The plug can show signs of wear or deposits, but it cannot diagnose every starting problem. The steps below focus on the BR 800’s ignition parts and safe service, without treating a plug change as a cure-all.
BR 800 Spark-Plug Setup and Safety Preparation
The BR 800’s specified spark plug is an NGK CMR6H, with an electrode gap of 0.5 mm (0.020 in). Check the operator’s manual for your exact model version before starting. The engine is a STIHL 4-MIX design and uses the specified 50:1 fuel mix, not straight gasoline.
Confirm the plug and gap
A spark-plug gap is the space between the center and ground electrodes at the firing end. For the BR 800, the specified gap is 0.5 mm, or 0.020 inch. Use a wire-style gap gauge to check it; do not choose a plug by appearance alone.
Confirm the replacement is marked NGK CMR6H. A plug that looks similar may differ in dimensions or heat range, so it is not a safe substitute based on visual match. If you cannot confirm the correct part for your BR 800 version, pause and check the manual or ask a STIHL dealer.
| Item | BR 800 service detail | What to do |
|---|---|---|
| Spark plug | NGK CMR6H | Verify the marking before installation |
| Electrode gap | 0.5 mm (0.020 in) | Check with a wire-style gauge |
| Engine type | STIHL 4-MIX | Use the fuel specified in the manual |
| Fuel mix | 50:1 STIHL 2-cycle engine oil and gasoline premix | Do not run straight gasoline |
| Tightening | Follow the applicable BR 800 manual | Do not guess a torque value |
Get the blower ready
Work outdoors, away from flames, sparks, and other ignition sources. Stop the engine and let it cool fully before touching the plug. Wear eye protection and gloves, and disconnect the spark-plug boot before removing the plug.
Clean debris from the area around the plug before loosening it. This simple step helps keep dirt from falling into the cylinder when the plug comes out. If the engine is hot, the boot is difficult to remove, or the area is not clear, stop and correct that first.
The 4-MIX engine’s fuel requirement matters during ignition checks. It runs on a 50:1 premix of gasoline and STIHL 2-cycle engine oil. Straight gasoline does not provide the required lubrication. If you suspect the wrong fuel was used, do not keep running the blower; consult the manual or a service dealer.
Key step: Confirm the plug, gap, manual, and fuel requirement before you begin.
Replacement Procedure and Post-Service Start Check
A careful replacement protects the plug threads in the cylinder head and makes the test meaningful. Remove the boot, clear dirt, inspect the old plug, and install the specified replacement by hand. Tighten it only as directed in the manual, then reconnect the boot and test the blower outdoors.
Remove and inspect the old plug
With the engine cool and the boot disconnected, clean the plug well and nearby surfaces. Use the correct plug tool for your machine, then loosen the plug without forcing it. Lift it out carefully so it does not drag dirt into the opening.
Inspect the removed plug in good light. Look for cracked porcelain, damaged threads, heavy deposits, or a worn electrode. Replace the plug if it is damaged or fouled. Do not try to fix those conditions by changing the specified gap.
If the electrode gap needs checking or adjustment, use a wire-style gauge and work gently. The target is 0.5 mm (0.020 in). Avoid bending the center electrode. If the plug is badly worn or has cracked porcelain, replace it rather than attempting to restore it.
Install the replacement without damaging threads
Make sure the replacement is an NGK CMR6H and that the gap is correct. Keep the plug well and threads clean. Set the plug squarely in the opening and turn it by hand for several full turns before using a tool.
The plug should turn freely by hand. If it binds, stop. Back it out, realign it, and try again without force. The cylinder head is aluminum, and forcing a misaligned plug can damage its threads. That repair can be costly and may leave the blower unsafe to operate.
After the plug is seated, tighten it using the instruction in the manual that applies to your BR 800. Do not guess at a torque or copy a value from a different model. Reconnect the plug boot firmly when the work is complete.
Start and check the blower
Move outdoors before starting. Follow the manual’s cold-start or warm-start procedure for your machine. A successful check means the engine starts and runs steadily at idle, then responds to acceleration without an obvious misfire or stall.
If the symptom remains, stop rather than repeatedly changing plugs or altering the gap. A plug replacement cannot correct every fuel, air, ignition, or exhaust problem. Let the engine cool before any further inspection, and use the troubleshooting steps below to decide what to check next.
Key step: Hand-start the plug, follow the manual’s tightening instruction, and test outside.
Spark-Plug Inspection and Ignition Troubleshooting
Spark-plug condition can offer clues, but it does not prove the cause of a fault on its own. A damaged or heavily fouled plug should be replaced with the specified part and gap. If the BR 800 still runs poorly, check fuel, air filtration, ignition, and exhaust rather than changing plug settings.
Read the plug without overdiagnosing
A cracked insulator, damaged threads, or worn electrodes calls for replacement. Heavy deposits also justify replacing a fouled plug instead of widening or narrowing the gap to chase a running issue. Keep the specified 0.5 mm setting.
A deposit pattern is only one piece of evidence. Fuel quality, operating conditions, and other engine issues may affect how a plug looks. Do not use appearance alone to decide that the carburetor needs adjustment, or that the ignition system has failed.
| What you find or observe | Reasonable next step |
|---|---|
| Cracked porcelain, damaged threads, or worn electrode | Replace with NGK CMR6H at 0.5 mm gap |
| Heavy deposits or a fouled plug | Replace the plug; then test the blower |
| Plug appears sound but engine still will not start | Check fuel and air-filter condition; follow the manual |
| Engine starts but runs unevenly or stalls | Check fuel quality, air filtration, ignition, and exhaust |
| Plug will not turn freely by hand during installation | Stop, remove it, and realign; never force it |
| Fault remains after correct plug service | Stop repeated plug changes and seek further diagnosis |
Troubleshoot in a safe order
Start with the basics that do not require dismantling the engine. Confirm that the fuel is fresh and mixed at the specified 50:1 ratio with STIHL 2-cycle engine oil. Check the air filter as the manual directs. If the plug is correct and the problem persists, the fault may be elsewhere in the fuel or ignition system, or involve an exhaust restriction.
Do not work on the plug while the engine is hot, and do not try to mask a running fault by changing the gap. If diagnosis requires testing ignition components or opening the fuel system, use the manual’s service guidance or have a qualified technician inspect the blower.
Key step: A correctly gapped plug rules out one basic issue; it does not prove the rest of the engine is sound.
Seasonal Storage and Fuel-System Care
Seasonal care helps you avoid mistaking a fuel problem for a spark-plug failure. The BR 800 needs the specified 50:1 premix, and storage instructions can vary by model and fuel product. Follow the current operator’s manual for storage, fuel handling, and restart steps rather than relying on a one-size-fits-all routine.
Before putting the blower away, review the manual’s fuel-storage instructions and use only the recommended fuel mix. Label any premix container with its contents and mix ratio. If fuel has been stored for an uncertain period, do not assume it is suitable; follow the fuel maker’s guidance and the STIHL manual.
When bringing the blower back into service, inspect the plug and boot, check the air filter, and confirm the fuel is correct. A clean, correctly gapped plug may still fail to solve a stale-fuel or airflow problem. Change one thing at a time so you can tell whether the repair helped.
Two practical service scenarios
These are illustrative troubleshooting situations, not claims about specific customer repairs. They show how to use plug inspection without assuming that every no-start problem has the same cause.
Scenario 1: The blower will not start after storage. I would first work outdoors with the engine cool, disconnect the boot, clean around the plug, and inspect the installed part. If it is not a CMR6H, is damaged, or is heavily fouled, I would fit the specified plug at 0.5 mm. I would also verify the fuel mix and follow the manual’s start procedure.
Scenario 2: A new plug is installed, but the engine still runs poorly. I would not keep changing the gap. I would confirm that the plug was hand-threaded and tightened as directed, then check fuel quality and the air filter. If the problem remains, ignition, fuel-system, or exhaust checks may be needed.
Key step: Treat the plug as one checkpoint in seasonal service, not as a substitute for correct fuel and maintenance.
Conclusion
The BR 800 plug specification is straightforward: NGK CMR6H, gapped to 0.5 mm (0.020 in). Safe service depends on more than the part number. Confirm the model manual, work on a cool engine, clean before removal, hand-start the plug, and use the manual’s tightening direction.
If the blower still has a starting or running problem after correct plug service, do not force a different plug or alter the gap. Check fuel and air-filter condition, then seek further diagnosis if the fault continues.
Frequently Asked Questions
What spark plug does a STIHL BR 800 use?
The specified plug is an NGK CMR6H. Confirm your BR 800 version and check its operator’s manual before installing a replacement.
What is the BR 800 spark-plug gap?
The specified electrode gap is 0.5 mm, or 0.020 inch. Check it with a wire-style gap gauge.
Can I use a plug that looks like a CMR6H?
Do not choose a plug by appearance alone. Use the specified NGK CMR6H and verify compatibility in the manual for your blower.
Should I adjust the gap to fix a starting problem?
No. Keep the gap at 0.5 mm (0.020 in). A running or starting problem may involve fuel, air filtration, ignition, or exhaust.
How tight should the spark plug be?
Use the tightening instruction in the applicable BR 800 manual. Do not guess a torque or borrow one from another model.
Can I replace the plug while the engine is hot?
No. Stop the engine and let it cool fully. Disconnect the spark-plug boot before removing the plug.
What if the replacement plug will not turn by hand?
Stop and realign it. Do not force it. Forcing a misaligned plug can damage the aluminum cylinder-head threads.
Does the BR 800 run on straight gasoline?
No. The 4-MIX engine uses a 50:1 mix of gasoline and STIHL 2-cycle engine oil. Follow the manual’s fuel instructions.
What should I check if a new plug does not solve the problem?
Confirm the correct fuel mix and check the air filter as directed by the manual. If the fault remains, further fuel-system, ignition, or exhaust diagnosis may be needed.
Should I replace a plug with cracked porcelain or heavy deposits?
Yes. Replace a damaged or fouled plug with the specified NGK CMR6H at the correct gap. Do not try to compensate by changing the gap.
(This article was written by one of our staff writers, Harrison Croft. Visit our Meet the Team page.)
