Stihl Ignition Coil Mounting (Spark Test)
To mount a STIHL ignition coil, align its pole faces with the flywheel magnets, set the air gap to the specification for your saw, and tighten the fasteners to the model’s service data. Then test with a rated spark tester. Many models use a 0.2–0.3 mm gap, but that range is not universal.
You have pulled the starter again and again. The saw still will not fire, and the coil looks close enough to be right. That is where guesswork gets expensive: a gap that is wrong, a damaged wire, or a stop-switch fault can all look like a bad coil.
I check the mounting and spark in a set order. First make the test safe, then fit the coil using the correct model data, and finally test for spark. A spark test can narrow the fault, but it cannot prove that fuel delivery or compression is good. Keep those limits in mind as you work.
Equipment Setup and Spark-Test Safety
This section covers safe preparation and the tools needed to mount and test an ignition coil. The ignition system can produce a high-voltage spark, while fuel vapor can ignite easily. Work outdoors or in a well-ventilated, vapor-free area, and keep the test spark away from fuel, spills, and fumes.
Before opening the saw, let the engine cool, engage the chain brake, switch the ignition off, and remove the spark-plug cap. Keep the saw away from fueling areas. Do not test with a loose plug or exposed lead near fuel vapor.
Tools and model information
A coil air gap is the small space between the coil’s pole faces and the flywheel. An inline spark tester fits between the plug lead and spark plug; an adjustable tester can also show whether spark jumps a set distance. Either is safer and more controlled than holding a plug by hand.
Find the exact saw model and, if available, its engine or serial information. Use the matching STIHL service manual or service data for the coil gap, fastener torque, spark plug, and plug gap. There is no safe universal coil-bolt torque.
Have these items ready:
- The correct service information for your saw
- A suitable feeler gauge or the gap-setting tool specified for the model
- A rated inline or adjustable spark tester
- Basic hand tools that fit the fasteners
- A clean cloth and good lighting
A common range on many STIHL saws is 0.2–0.3 mm (0.008–0.012 in) for coil-to-flywheel gap. Treat that as a reference only; your model’s specification takes priority. The plug electrode gap is also model-specific, though 0.5 mm (0.020 in) is common on many models.
Coil Mounting and Air-Gap Adjustment
Correct mounting puts the coil’s pole faces in line with the flywheel magnets and sets the specified clearance between them. A coil can look centered and still be out of position. Follow the model’s procedure and use its gap and torque values instead of relying on feel or a general rule.
Inspect before you loosen anything
Remove only the covers needed to reach the ignition coil, following the service instructions. Check the spark plug, plug cap, high-tension lead, coil laminations, flywheel magnets, and kill-switch wire for cracks, corrosion, oil, or other contamination. Also look for a pinched or rubbed-through wire.
The kill wire grounds the ignition to stop the engine. Damage or a short in this wire can prevent spark even when the coil and flywheel gap are sound. Note how the wiring and any spacers or clips are routed before moving them.
Set the coil position
- Confirm that the ignition is off and the plug cap is removed. Keep the work area free of fuel vapors.
- Loosen the coil fasteners enough to let the coil move. Do not remove connected wires unless the service procedure requires it.
- Turn the flywheel until its magnets are aligned with the coil pole faces. Position the coil so the faces line up as specified.
- Place the specified gauge between the pole faces and flywheel. Let the magnets draw the coil toward the gauge, then hold the coil in position.
- Tighten the fasteners to the model-specific torque and thread condition in the service data. Remove the gauge and turn the flywheel by hand through a full rotation to check for contact.
Do not use a business card, folded paper, or an assumed torque value unless the model’s service procedure specifically calls for it. A feeler gauge must fit as intended for the specified gap; do not force it between the parts. If the flywheel touches the coil, stop and reset the position before turning it further.
Spark Diagnosis and Model-Specific Maintenance
A spark test checks whether the ignition system can produce a visible spark during cranking. It helps separate ignition faults from fuel or compression problems, but a spark seen in open air does not confirm that the saw will run under load. Use the model’s plug and gap specifications when checking the plug.
Test with a rated spark tester
With the saw off, connect an inline tester according to its instructions. Reconnect the plug lead as directed by the tester maker. Keep hands and tester leads clear of the chain, flywheel, and other moving parts. Pull the starter and look for a consistent spark.
If using the spark plug instead, remove it, connect the plug cap, and press the plug’s metal shell firmly against clean, bare engine metal. Do not hold the plug by hand. A loose or ungrounded plug can arc unpredictably, and an exposed lead can spark near fuel vapor. Keep the test away from fuel and spills.
Interpret what you see
| Test result | What it suggests | Next check |
|---|---|---|
| Consistent spark | The ignition can produce spark during this test | Check fuel and compression if the saw still will not start |
| No spark | Ignition may be interrupted, or a test connection may be poor | Recheck tester connections, plug cap, lead, and coil wiring |
| Spark returns with kill wire disconnected | The stop switch or its wiring may be grounding the ignition | Inspect the switch wire and connections |
| Spark remains absent with kill wire disconnected | The fault may be at the plug, lead, coil, flywheel, or another ignition part | Check each part against the model’s service procedure |
For the kill-wire check, switch off before disconnecting the wire. Disconnect only the coil’s kill wire, then repeat the spark test safely. If spark returns, the switch or wiring is a likely cause, not proof that the coil gap is wrong. Switch off again and reconnect the wire after testing. Do not operate the saw with the kill circuit disconnected.
A plug that is fouled, damaged, or incorrectly gapped can affect the test. Check the specified plug type and electrode gap for your model. Many STIHL saws use a 0.5 mm (0.020 in) plug gap, but confirm before setting it. Replace a plug only when inspection or service guidance supports that step.
Two useful workshop scenarios
These are diagnostic examples, not claims about a specific saw model. In the first, a coil has been moved during repair and spark is absent. The useful next step is to verify pole alignment and the specified air gap, then repeat the tester check. Replacing the coil before checking its mounting could waste money.
In the second, the coil gap is correct, but spark returns only after the kill wire is disconnected. That points toward a shorted stop-switch circuit or damaged wire. The right next step is to trace that circuit, not to keep adjusting the coil. In either case, confirm the result with the correct service procedure.
Reassembly, Storage, and Seasonal Care
Reassembly restores the saw’s stop function and protects the ignition parts from damage. A spark test is only one step in diagnosis. Before starting, reconnect all wires, reinstall covers and safety parts, and confirm that the stop switch works as intended.
After testing, switch off and reconnect the kill wire if it was removed. Refit the plug and cap, then reinstall all covers and components in the correct order. Check that wires are not trapped and that the chain brake and stop switch function normally. Do not run the saw with covers or safety parts removed.
If the saw still will not start despite a consistent spark, stop replacing ignition parts at random. Check fuel condition and delivery, then compression using the model’s service guidance. Where applicable, STIHL specifies a 50:1 fuel mix (2% oil) with STIHL-approved or equivalent quality two-stroke oil. That mix is not a coil-setting specification, and the correct fuel guidance can vary by product.
For storage, follow the saw’s manual and keep the machine clean and dry. Before the next test, inspect the plug lead and wiring for damage. The most useful record to keep is the model number, the specified coil gap and torque, and what the spark tester showed.
Conclusion and FAQ
The dependable sequence is simple: identify the exact model, set the coil with its specified gap and torque, and test spark safely. A common gap range is only a starting reference, not a substitute for service data. If spark returns after disconnecting the kill wire, investigate the switch circuit before condemning the coil.
Frequently asked questions
What is the correct ignition-coil gap on a STIHL chainsaw?
It depends on the model. Many saws use 0.2–0.3 mm (0.008–0.012 in), but confirm the exact specification in the service manual.
Can I set the coil gap with a business card?
Do not assume a card gives the right clearance. Use the gauge and method specified for your saw.
How tight should the coil bolts be?
Use the model’s service torque and thread instructions. There is no safe universal value.
Can I test spark by holding the plug against the engine?
Do not hold it by hand. Secure the plug’s metal shell against clean engine metal, or use a rated spark tester.
What should I do if there is no spark?
Check the tester connection, plug, cap, lead, and wiring. Then follow the model’s ignition checks, including the kill-wire test where appropriate.
What does it mean if spark returns when the kill wire is disconnected?
The stop switch or its wiring may be grounding the ignition. Switch off, reconnect the wire after testing, and inspect that circuit.
Should the flywheel touch the coil?
No. After setting the specified gap, turn the flywheel by hand through a full rotation. If it contacts the coil, reset the position.
What spark-plug gap should I use?
Use the specification for the correct plug and saw model. A 0.5 mm (0.020 in) gap is common, but it is not universal.
Is a visible spark proof that the saw will run?
No. It shows that spark is present during the test. Fuel delivery, plug condition, compression, and other faults can still prevent starting.
Can I run the saw with the kill wire disconnected?
No. Reconnect it and verify normal stop-switch operation before starting the saw.
(This article was written by one of our staff writers, Robert Callahan. Visit our Meet the Team page.)
