Replacement intervals by use, reference plugs by model, and the colour chart that diagnoses an engine in two minutes.
Sommaire · 10 parties
01Replacement interval
| Use | Interval |
|---|---|
| Race motocross (4-stroke) | 30-50 h |
| Enduro (4-stroke) | 50-80 h |
| Motocross 2-stroke | 15-30 h |
| Road / adventure | 100-200 h |
02References by model
| Model | OEM part | Alternative |
|---|---|---|
| KTM 250/350/450 SX-F | NGK LKAR8BI-9 | Denso IU27D |
| Yamaha YZ250F/450F | NGK CR8E | NGK CR8EIX (iridium) |
| Honda CRF250R/450R | NGK IMR8C-9HES | — |
| KTM 250/300 SX (2T) | NGK BR8ES | NGK BR8EIX |
| Yamaha YZ250 (2T) | NGK BR8ES | — |
Source: manufacturer manuals, NGK and Denso catalogues
03Symptoms of a worn plug
- Hard starting
- Misfires under acceleration
- Higher fuel consumption
- Eroded or blackened electrode
04Is iridium worth it
Yes. Three to five euros more, twice the service life, a better spark. On a four-stroke motocross engine it is now standard.
05The plug is the fastest diagnosis you have
Pulling a plug takes two minutes. Its colour tells you immediately whether the engine is running properly, which no other check does for the same effort.
06Reading a plug
| Colour | Diagnosis | Action |
|---|---|---|
| Light brown / tan | Perfect | Nothing to do |
| White / grey | Too lean | Richen the jetting (2T) or check injection (4T) |
| Dry black soot | Too rich | Lean the jetting (2T) or check the air filter |
| Wet black (oil) | Oil in the chamber | Worn rings or valve stem seals |
| Melted electrode | Detonation or overheating | Stop, the engine is in danger |
| Red deposits | Fuel additives | Normal with some fuels |
07How to read one properly
1. Ride for five to ten minutes at varied revs, not just idling 2. Cut the engine under load, not at idle 3. Pull the plug immediately 4. Read the base of the insulator, the white ceramic part around the electrode
The second step is what most people get wrong. A plug read after two minutes of idling in the paddock always looks rich, because idling is rich. The reading only means something if the engine was working when it stopped.
08When to change it
- When the electrode is eroded, rounded instead of sharp
- When the engine starts poorly despite correct jetting
- At every top-end rebuild, as a matter of course
- Every 15-30 h on a two-stroke, 30-50 h on a four-stroke
09Fitting, without breaking anything
Aluminium heads are unforgiving. Thread the plug in by hand until it seats, then torque it: on a plug with a gasket, roughly a further half turn; on a taper seat, a sixteenth. Overtightening strips the head, and a stripped head is a workshop job, not a trailside one.
Never fit a plug to a hot engine either, and blow out the plug recess before removing anything: grit that falls into the cylinder does more damage than the worn plug ever did.
10Keep a spare, and log it
A plug costs the price of a coffee and lives in the toolbox. It is the single part most likely to end a day out. Log the change with the engine hours next to it: a plug changed at 45 h on an engine showing 130 h says the machine was watched, and that is worth more at resale than the receipt.
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