Stihl MS 290 Spark Plug Gap (Spec Guide)

The STIHL MS 290 spark-plug gap is 0.5 mm (0.020 in). Check it with a feeler gauge, with the engine stopped and cool and the plug boot disconnected. The common specified plug is an NGK BPMR7A, but confirm the type for your saw. Replace a damaged plug; do not force its electrodes or tune the carburetor to mask a fault.

Small-engine service has become easier to check with feeler gauges, clear manuals, and replacement parts listed by model. Still, a plug that looks close to right may not be set right. The gap is the space between the center and ground electrodes. Setting it to specification is a quick task, but it must be done with care.

I treat the gap as one check, not a cure-all. A saw that misfires, stalls, or will not idle safely may have another fault. Do not keep cutting while it runs erratically. Use the steps below to check the plug and decide what to do next.

MS 290 spark-plug gap and compatible plug

The spark-plug gap is the distance between the center electrode and the bent ground electrode. For the MS 290, set that distance to 0.5 mm (0.020 in). The commonly specified plug is an NGK BPMR7A, but the saw’s manual and installed ignition setup are the final check.

A feeler gauge measures the gap with a thin metal blade. Use a blade marked 0.5 mm or 0.020 in. Do not estimate the distance by eye. A small error can leave the plug outside the specified setting.

The figures for idle and maximum speed are reference specifications, not instructions to adjust the carburetor. They do not replace the service manual’s tuning limits or a tachometer-based procedure. If the saw will not idle steadily or the chain moves at idle, stop and diagnose the problem instead of changing settings at random.

Before installing a replacement plug, confirm its type in the manual for your saw. Similar-looking plugs are not automatically interchangeable. The key check is both the correct plug and a measured 0.5 mm gap.

Equipment Setup and Safety Preparation

Safe preparation starts with a stopped, cool saw and a clear work area. Disconnecting the plug boot prevents accidental ignition while you handle the plug. Before any later cutting, check the safety parts and the work area; a correctly gapped plug does not make an unsafe saw safe to use.

1. Stop and cool the saw. Set it on a stable surface, switch off the engine, and let hot parts cool before service.
2. Disconnect the plug boot. Pull the boot from the plug, not by yanking the wire. This helps prevent accidental starting.
3. Remove and inspect the plug. Use the correct plug wrench. Look for a cracked insulator, damaged electrodes, heavy deposits, or other visible damage.
4. Measure the gap. Slide the 0.5 mm (0.020 in) feeler gauge between the electrodes. The gauge should fit at the specified setting without being forced.
5. Adjust with care. If the gap is off and the plug is undamaged, move only the ground electrode a small amount with a suitable tool. Recheck the gap after each adjustment.
6. Replace a damaged plug. Do not try to straighten a cracked insulator or force an electrode that resists movement.

Do not pry against the center electrode or insulator. If the gauge shows the wrong gap after careful adjustment, check the plug again or replace it. A cracked plug, damaged insulator, or incorrectly set gap can cause misfire or loss of ignition.

Reinstall the plug according to the saw’s manual. Use the manual’s specified installation torque; do not guess at a torque value. Refit the plug boot securely. A plug that is damaged, uncertain, or hard to set correctly should be replaced, not forced into service.

Field Cutting Operation

A safe pre-cut check confirms that the engine starts and idles as it should before the chain reaches wood. The plug gap is only one part of that check. Use the correct fuel mix, follow the starting steps in the manual, and stop if the chain moves at idle or the engine runs erratically.

For the MS 290, use a 50:1 fuel mix with suitable air-cooled two-stroke oil. That is 20 mL of oil for each 1 L of gasoline. Measure both liquids accurately and mix them as directed by the oil and saw makers. Do not use straight gasoline.

Before cutting:

• Check that the plug and boot are fitted.
• Follow the model’s starting procedure and keep the saw stable while starting.
• Confirm that the engine holds a steady idle and the chain stays still at idle.
• Check the chain brake, throttle trigger lockout, and chain catcher.
• Wear a helmet with eye and face protection, hearing protection, chainsaw protective legwear, gloves, and boots.
• Keep bystanders clear, and check for overhead hazards and a safe work area.

Use only a bar and chain combination listed for the saw. The bar’s pitch and gauge, drive-link count, and the installed drive sprocket must be compatible. These fitment details do not change the spark-plug gap, but an incorrect cutting setup is not safe to use.

If the engine misfires, stalls, or will not idle safely, stop the saw. Engage the chain brake, shut down, and let the engine cool before inspection. Do not keep cutting and do not alter carburetor settings to compensate for a damaged or incorrectly gapped plug. Resume only after the cause is addressed and the saw idles safely.

Spark-Plug, Chain, and Engine Maintenance

Routine maintenance means checking the plug and other service points at the times listed in the manual. A correctly measured gap can help rule out one ignition issue, but it cannot prove that the whole engine is healthy. Keep plug checks separate from carburetor adjustments and chain service.

Use this comparison to choose the next step:

Illustrative service example: A plug may look sound but measure wider than 0.5 mm. A feeler-gauge check gives a clear result; careful adjustment may bring an undamaged plug back to specification. If the insulator cracks during adjustment, the safe choice is replacement, not continued bending.

Another diagnostic example: If the gap is correct but the engine still runs erratically, the gap alone does not explain the symptom. I would stop using the saw, check that the plug type is correct, and follow the manual or use qualified service to investigate. I would not compensate by changing the carburetor.

Keep the chain sharp and correctly tensioned according to the manual, but treat those as separate maintenance jobs. A dull chain can strain the operator and cut poorly; it does not change the required plug gap. Measure the plug first, then deal with any other fault on its own terms.

Storage and Seasonal Care

Storage care helps keep the plug and fuel system ready for the next use. Follow the manual’s storage directions, since procedures can depend on the saw and fuel used. Before putting the saw away, stop the engine, engage the chain brake, and let the machine cool.

Clean dirt from around the plug before removing it, so debris does not fall into the cylinder area. Check the plug for damage and deposits, and measure its gap during scheduled maintenance or when troubleshooting ignition problems. If you remove the plug, reinstall it using the manual’s guidance.

For a saw that stalls or misfires after storage, do not assume the plug gap is the cause. Check the plug type and gap, then follow the manual’s troubleshooting steps. Do not keep running a saw that cannot idle safely. A clean, correctly installed plug is useful; it is not a substitute for diagnosing the full problem.

Conclusion: The quick MS 290 plug check

The MS 290 spark-plug gap is 0.5 mm (0.020 in). Confirm the plug type in the saw’s manual; NGK BPMR7A is the common specified plug. With the saw cool and the plug boot disconnected, inspect the plug and use a feeler gauge to measure the gap.

Adjust only an undamaged plug, and do not force the electrodes. Install the plug to the manual’s instructions, use the specified 50:1 fuel mix, and confirm safe idle before cutting. If the saw continues to misfire or will not idle safely, stop and diagnose it rather than changing the carburetor.

Frequently Asked Questions

What is the spark-plug gap on a STIHL MS 290?

The specified gap is 0.5 mm (0.020 in). Measure it with a feeler gauge rather than estimating by eye.

Which spark plug does an MS 290 use?

The common specified plug is an NGK BPMR7A. Confirm the plug type against the manual and the saw’s installed ignition configuration.

Can I set the gap by eye?

No. Use a 0.5 mm or 0.020 in feeler gauge. A visual estimate cannot confirm the specified gap.

Which electrode should I adjust?

If the plug is undamaged, adjust the ground electrode carefully. Do not pry against the center electrode or insulator.

What if the plug’s insulator is cracked?

Replace the plug with the correct specified type. A cracked insulator can cause misfire or ignition failure.

Does the gap change if I use a different bar?

No. Bar and chain fitment do not change the spark-plug gap. The specified gap remains 0.5 mm (0.020 in).

What fuel mix does the MS 290 use?

Use a 50:1 mix with suitable air-cooled two-stroke oil. This equals 20 mL of oil per 1 L of gasoline.

Can I keep cutting if the saw misfires?

No. Stop the saw and let it cool before checking it. Do not continue cutting with persistent misfire or an engine that will not idle safely.

Should I change the carburetor if the saw runs badly?

Do not use carburetor changes to mask a damaged or incorrectly gapped plug. Follow the manual’s troubleshooting steps or seek qualified service.

What torque should I use to reinstall the plug?

Use the installation torque specified in the MS 290 manual. Do not guess or substitute an unverified torque value.

(This article was written by one of our staff writers, David Hensley. Visit our Meet the Team page.)