Stihl 08 Chainsaw Spark Plug (Cross-Reference)
For a classic STIHL 08 or 08S, Bosch WSR 6 F is a commonly listed spark plug, and NGK BPMR7A is a commonly cited cross-reference. Before fitting one, confirm your saw’s exact model and compare plug dimensions and terminal type. The common gap is 0.5 mm (0.020 in), but use the specification in your saw’s manual.
A plug listing can get you close, but “cross-reference” does not always mean every detail is identical. If the old plug is fouled or the saw will not start, check the plug and its fit before assuming the replacement will solve the problem. The right part must match the saw, not just the name on a seller’s chart.
I treat the plug number as a starting point, then verify dimensions, terminal, and gap. This short check helps prevent a mismatch and avoids forcing a plug into the cylinder head.
Common spark plugs and cross-references
A cross-reference is a parts listing that identifies another brand’s plug as a possible replacement. For the classic STIHL 08 and 08S, Bosch WSR 6 F is commonly listed, while NGK BPMR7A is commonly cited as an alternative. Neither listing, by itself, proves that every version of the saw accepts both plugs.
Check the model-specific STIHL parts list or service manual when available. Model year, ignition parts, or previous repairs can affect what is fitted. Compare the replacement to the original plug and confirm the fit before installation.
| Plug information | Common listing | What to check |
|---|---|---|
| Bosch reference | WSR 6 F | Match thread, reach, seat, and terminal |
| Common NGK cross-reference | BPMR7A | Confirm dimensions and terminal suit your saw |
| Thread | M14 × 1.25 | Diameter and thread pitch must match |
| Reach | About 9.5 mm | Compare with the correct model specification |
| Gap | Commonly 0.5 mm (0.020 in) | Set to the saw manual’s specification |
A plug’s heat range and construction may differ between brands. A cross-reference is not a guarantee of identical performance or design. If a listing conflicts with the STIHL manual or parts list, follow the model-specific information and ask a qualified dealer to confirm the part.
Next step: Use the plug number as a lead, then verify the fit against your saw’s documentation and the old plug.
How to verify fit before installation
Plug reach is the distance from the seating surface to the end of the threaded portion. The seat is the surface that seals against the cylinder head. These details matter because a plug can have the right thread but still be the wrong part. Check dimensions and terminal style before you put a wrench on it.
With the engine cool and ignition off, remove the plug cap and inspect the old plug. Look for its brand and number, then compare the replacement’s thread, reach, seat, and top terminal. Do not rely on appearance alone, and do not force a plug that does not turn smoothly by hand.
Check the plug and cap
The terminal is the top connection that meets the plug cap. Some plugs use a removable terminal nut, while others have a different top shape. Make sure the replacement connects securely to your saw’s cap; a loose or incompatible connection can interrupt ignition.
Inspect the cap and lead for cracks or damage. Check the plug’s ceramic for cracks and the metal threads for damage. If the plug will not start by hand, stop. Remove it and check alignment rather than using a wrench to overcome resistance.
A resistor plug, marked with an “R” in many plug codes, contains a built-in electrical resistor. Do not assume resistor type is interchangeable just because two plugs appear in a cross-reference. Confirm the required type for the saw’s ignition system.
Set and inspect the gap
The spark gap is the small space between the center and ground electrodes. A common gap for this saw family is 0.5 mm, or 0.020 inch, but the exact model manual takes priority. Use a wire-style gap gauge and adjust gently; do not lever hard against the center electrode.
| Check | What to do | Stop if… |
|---|---|---|
| Thread and reach | Compare plug and manual | The size or reach differs |
| Seat and terminal | Match the plug and cap | The seat or connection is incompatible |
| Electrode condition | Inspect for wear or damage | Ceramic is cracked or electrodes are damaged |
| Gap | Measure with a gauge | You cannot set the manual’s value safely |
A plug that fits the threads is not automatically a correct plug. Confirm all key features before moving to installation.
Safe removal and installation
Removing and fitting a plug is a small service task, but cross-threading or overtightening can damage the cylinder head. Let the engine cool, switch the ignition off, and remove the cap before service. Use a suitable plug wrench, keep dirt out of the plug opening, and follow the model manual for tightening torque.
- Cool and prepare the saw. Place it on a stable surface outdoors or in a well-ventilated work area. Keep the ignition off and the chain clear of people and objects.
- Remove the cap and plug. Pull the cap by its body, not by yanking the lead. Turn the plug out carefully. Brush away dirt around the opening so debris cannot fall into the cylinder.
- Inspect the old part. Note the plug number and condition. Check the cap, lead, and cylinder-head threads for visible damage.
- Verify the replacement. Confirm thread, reach, seat, terminal, and resistor type against the model information. Set the gap to the manual’s specification.
- Start the plug by hand. Turn it gently until it seats. If it binds or feels crooked, stop and back it out. Do not use a wrench to force it.
- Tighten to the manual’s value. Plug torque depends on the specification and seating arrangement. If you cannot find the correct value, obtain it instead of guessing or overtightening.
- Reconnect and test safely. Refit the cap fully. Follow the saw’s cold- or warm-start procedure outdoors, with the chain away from people and objects.
Do not use a cross-reference to infer carburetor settings, engine speed, or other service values. Those require their own model-specific specifications.
What plug condition can tell you
A spark plug is one clue in a starting or running problem, not a full diagnosis. Carbon, wetness, or wear can point to conditions that need checking, but a plug’s appearance alone does not identify the cause. Inspect it along with the air filter, fuel condition, ignition lead, and other parts called out in the manual.
| Plug condition | What to check next |
|---|---|
| Dry, heavy carbon deposits | Check the air filter and follow the manual’s fuel and running guidance |
| Wet with fuel | Confirm the starting procedure and inspect for an ignition issue |
| Oily deposits | Check the fuel mix and investigate recurring fouling |
| Cracked ceramic or damaged electrodes | Replace with a confirmed, compatible plug |
| Plug repeatedly fouls | Look for an underlying issue instead of repeatedly changing plugs |
The fuel mix deserves special care on an older two-stroke saw. A historical specification commonly associated with these older models is 1:25, or 4% two-stroke oil, but do not treat that as a universal instruction. Follow the exact saw manual and the oil maker’s instructions. Do not assume a modern 1:50 recommendation applies, and never run straight gasoline in the two-stroke engine.
Next step: If a new plug fouls again, investigate the cause rather than treating the plug as the only problem.
Two practical fit-check scenarios
These examples describe common service situations, not confirmed repairs on a specific saw. They show how I would use a cross-reference listing while avoiding guesses. In both cases, the model manual or parts list remains the final check for the individual machine.
The old plug matches the listing
Suppose an 08S arrives with a Bosch WSR 6 F installed, and the replacement listing names that plug. I would still compare the replacement’s dimensions and terminal, inspect the plug cap, and check the gap. If those details match the saw’s information, I would hand-start the plug and tighten it to the specified torque.
If the old plug is badly fouled, I would also check the air filter and fuel mix. Replacing a fouled plug without checking why it fouled may only provide a short-lived fix.
A seller lists an NGK BPMR7A
If a parts seller lists NGK BPMR7A as a cross-reference, I would not treat the listing as final proof of fit. I would compare the plug dimensions, seat, terminal, and resistor type with the STIHL parts information and the plug removed from the saw. If one detail differs or cannot be confirmed, I would pause and verify before installation.
That approach is slower than trusting a label by itself, but it helps avoid a wrong part or damaged threads. A cross-reference is useful when it agrees with the saw’s specific requirements.
Before returning the saw to service
A correctly fitted plug cannot make an unsafe or poorly maintained saw safe. Before cutting, check the controls and cutting equipment according to the STIHL manual. Wear chainsaw chaps, a helmet with face and ear protection, gloves, and protective boots. Keep bystanders clear and plan an escape route before felling.
Confirm the chain brake, throttle lockout, chain catcher, tension, bar, and controls are in good condition. Use a sharp chain that matches the installed bar and chain. Shut the engine down before repositioning or clearing debris. Keep the guide-bar nose away from wood where possible; contact with its upper quadrant can cause rotational kickback. Hold the saw with both hands and do not cut above shoulder height.
For storage, let the engine cool before cleaning. Brush away sawdust and oil, inspect the plug cap and lead, and follow the manual’s fuel-system instructions for extended storage. Use fresh, correctly mixed fuel when returning the saw to service.
Frequently asked questions
What spark plug is commonly listed for a STIHL 08?
Bosch WSR 6 F is commonly listed for the classic STIHL 08 and 08S. Confirm the exact saw model and ignition setup with its parts list or manual before buying, because a common listing does not guarantee fit for every machine.
Is NGK BPMR7A a cross-reference?
NGK BPMR7A is commonly cited as a cross-reference for these saws. Treat it as a candidate, not a guarantee. Check thread, reach, seat, terminal, and resistor type against the model-specific STIHL information before installation.
What thread size does the plug use?
The commonly listed plug thread is M14 × 1.25. Confirm this against the exact model’s service information. A matching thread alone is not enough; plug reach and seat must also be correct.
What is the plug gap?
A common setting is 0.5 mm, or 0.020 inch. Use a gap gauge and follow the specification for your exact saw. Do not use the common value as a substitute for the manual when it gives a different figure.
How far should the plug reach into the cylinder?
The common reach is approximately 9.5 mm. Verify it against the correct parts list or manual. Do not install a plug with a different reach simply because its thread fits.
Can I use any plug labeled equivalent?
No. “Equivalent” on a seller’s listing is not proof that the plug matches your saw’s dimensions, terminal, resistor type, or heat range. Compare the details and confirm against STIHL information before fitting it.
What fuel mix should I use?
A historical mix commonly specified for these older saws is 1:25, equal to 4% two-stroke oil. Check your exact manual and oil maker’s instructions. Do not assume a modern 1:50 mix applies, and never use straight gasoline.
Should I replace a plug that is wet or black?
Not automatically. A wet or carbon-fouled plug can have several causes. Check the starting procedure, air filter, fuel mix, and other manual-listed items. If fouling returns after cleaning or replacement, investigate the cause rather than changing plugs repeatedly.
How tight should I install the plug?
Use the torque specified in the saw’s manual. If you cannot find the correct value, obtain it rather than guessing or overtightening. Start the plug by hand first; if it binds, stop to prevent cross-threading.
Can a plug cross-reference tell me how to set the carburetor?
No. A plug cross-reference does not specify carburetor settings, operating RPM, or chain and bar fitment. Use the correct service information for each adjustment, and do not change settings based only on the replacement plug.
(This article was written by one of our staff writers, David Hensley. Visit our Meet the Team page.)
