How to Change a Dirt Bike Spark Plug

Step By Step

1. Clean around the plug

   Engine cold. Pull what your bike needs to reach the plug, then blow or brush the dirt out of the plug well before the plug comes out. Anything sitting there drops straight into the cylinder.

   • Compressed air or brush

   Careful. Yamaha’s manual says to clean around the plug before removal so dirt can’t fall from the plug hole into the cylinder.

2. Pull the cap and the plug

   Twist and pull the cap straight off, never by the wire. Fit the plug socket square on the hex and turn it counterclockwise until the plug spins free, then lift it out with the socket.

   • Plug socket
   • Ratchet

3. Read the tip

   Look at the insulator around the center electrode. Light tan or gray is healthy. Dry black soot means rich, too cold a plug or lots of idling. Wet black means flooding or oil. White or blistered means lean or too hot.

   How often

   Bike	Spec	Source
   KTM / Husqvarna / GASGAS 250 / 300 SX, XC, TC, MC, EX two-stroke (race)	Replace every 15 h	KTM 250/300 SX-XC 2024 OM service schedule, PDF p. 36; GASGAS MC 250 / EX 300 2026 OM, PDF p. 36 (2022 carb 250 SX: every 20 h, PDF p. 33)
   KTM / Husqvarna / GASGAS 250 / 450 SX-F, XC-F (race)	Replace every 45 h	KTM 250 SX-F 2024 OM, PDF p. 42; 450 SX-F 2024 OM, PDF p. 41
   Yamaha YZ250F / YZ450F (race)	Check every 3rd race (7.5 h)	Yamaha YZ250F / YZ450F 2026 OSM maintenance intervals, PDF p. 93 (replace as required)
   Honda CRF250R / CRF450R (race)	Inspect each race (2.5 h)	Honda CRF250R 2025 OM maintenance schedule, PDF p. 42
   KTM XC-W / EXC, Husqvarna TE 2-stroke (enduro)	Replace every 45 h	KTM 2026 250 XC-W / 300 EXC / 300 XC-W OM, PDF p. 55; Husqvarna TE 250/300 2026 OM, PDF p. 55
   KTM 450 / 500 EXC-F, XCF-W (enduro)	Replace at 90 h	Changed in the 90 h engine service, no separate plug line. KTM 450 XCF-W / EXC-F / 500 EXC-F 2025 OM, PDF p. 54
   Honda CRF450X, CRF250F (trail)	Inspect every 600 mi	Every 600 mi (1,000 km) or 6 months. Honda CRF450X 2025 OM, PDF p. 45; CRF250F 2025 OM, PDF p. 52
   Honda CRF450RL (on-road schedule)	Inspect at 1,800 mi	At 1,800 mi (12 months) and 3,600 mi (24 months). Honda CRF450RL 2025 Off-Road Handbook, PDF p. 37

   Check your manual’s schedule. Racers often check the plug far more often than the schedule asks.

   Shop tip. To read jetting on a carb two-stroke, KTM says fit a new plug, make a short full-throttle run, shut off and then read it. A plug that has idled in the pits tells you nothing.

4. Use the right plug

   Match the NGK number in your manual exactly. In NGK numbers, the digit is the heat range: a higher number is a colder plug with a shorter nose. Only change heat range once the jetting, air filter and premix are known good.

   Spark plug (NGK)

   Bike	Spec	Source
   KTM / Husqvarna / GASGAS 250-300 two-stroke	NGK BR 8 ES	KTM 250/300 SX-XC 2024 owner’s manual, PDF p. 132 (same plug in the 2026 manual; 2022 carb 250 SX used BR 8 ECM)
   KTM / Husqvarna / GASGAS 250 / 450 four-stroke	NGK LMAR9AI-10	KTM 250 SX-F 2024 OM, PDF p. 149; 450 SX-F 2024 OM, PDF p. 154
   Honda CRF250R / CRF450R	NGK SILMAR9A9S	Honda CRF250R / CRF450R 2025 owner’s manual, PDF p. 80
   Yamaha YZ250F	NGK LMAR9E-J	Yamaha YZ250F 2026 owner’s service manual, PDF p. 55
   Yamaha YZ450F	NGK LMAR8G	Yamaha YZ450F 2026 owner’s service manual, PDF p. 53

   Use the exact plug your owner’s manual lists. Part numbers change between model years, and two-strokes and four-strokes use different thread sizes.

5. Check the gap

   Even a new plug can be off. Slide a gauge between the center electrode and the ground strap. It should drag slightly. If it’s out, bend only the ground strap, gently, and never pry against the center electrode.

   • Feeler gauge

   Electrode gap

   Bike	Spec	Source
   KTM / Husqvarna / GASGAS 250-300 two-stroke	0.60 mm (0.024 in)	KTM 250/300 SX-XC 2024 OM, PDF p. 132
   KTM / Husqvarna / GASGAS 250 / 450 four-stroke	1.0 mm (0.039 in)	KTM 250 SX-F 2024 OM, PDF p. 149
   Honda CRF250R / CRF450R	0.8 to 0.9 mm (0.03 to 0.04 in)	Honda CRF250R / CRF450R 2025 OM, PDF p. 80
   Yamaha YZ250F	0.6 to 0.7 mm (0.024 to 0.028 in)	Yamaha YZ250F 2026 OSM, PDF p. 55 and 103
   Yamaha YZ450F	0.7 to 0.8 mm (0.028 to 0.031 in)	Yamaha YZ450F 2026 OSM, PDF p. 53 and 103

   Check your manual for the gap. A gap that is too wide is on KTM’s list of no-start causes.

6. Start it by hand

   Check the new plug’s washer is in place. Thread the plug in by hand until the washer touches the head. If it won’t turn easily, back it out and start again, because a cross-threaded plug ruins the head threads.

   Shop tip. Push a short piece of fuel hose over the plug top and use it to spin the plug in. If the thread binds, the hose slips instead of forcing it crooked.

7. Torque it

   Finish with a torque wrench to the manual figure. NGK warns that over-tightening can crack the shell, and under-tightening stops heat leaving the tip, which can overheat it and cause pre-ignition.

   • Torque wrench

   Spark plug torque

   Bike	Spec	Source
   KTM / Husqvarna / GASGAS 250-300 two-stroke (M14x1.25)	25 Nm (18.4 lbf ft)	KTM 250/300 SX-XC 2024 OM, PDF p. 134
   KTM / Husqvarna / GASGAS 250 four-stroke (M10x1)	12 Nm (8.9 lbf ft)	KTM 250 SX-F 2024 OM, PDF p. 151
   KTM / Husqvarna / GASGAS 450 four-stroke (M10x1)	10 to 12 Nm (7.4 to 8.9 lbf ft)	KTM 450 SX-F 2024 OM, PDF p. 156
   Yamaha YZ250F / YZ450F (M10)	13 N·m (9.6 lb·ft)	Yamaha YZ250F 2026 OSM, PDF p. 64 and 103; YZ450F 2026 OSM, PDF p. 62

   Honda’s owner’s manual prints no plug torque. NGK’s chart for a 10 mm gasket plug in an aluminum head is 7.2 to 8.7 lb-ft, and 18.0 to 21.6 lb-ft for a 14 mm. Use your service manual figure if it has one.

8. Refit the cap and test

   Push the cap on until it clicks over the terminal, then refit what you removed. Start the bike and check it idles clean. KTM changes the plug connector along with the plug at each scheduled change.