Engamax — Car engine maintenance schedules: service | engamax.com
Plug life is set by electrode material — anywhere from 30,000 to 150,000 km — while the timing belt answers to kilometres and calendar years at once.
Spark plug replacement distance depends on the electrode. Nickel plugs are due at 30,000–40,000 km, platinum at 60,000–100,000 km and iridium at 100,000–150,000 km. The spread is large enough that the first step is finding out which material the engine was built with.
The material is the schedule. Two otherwise identical engines can have plug intervals three times apart, and fitting a longer-life material changes the interval rather than the engine. The electrode wears kilometre by kilometre, which is why plug failure is gradual rather than sudden.
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The timing belt works on a double limit: 60,000–100,000 km or 5–7 years, whichever comes first. A low-mileage car does not escape the calendar half of the rule, because the belt ages whether or not it turns.
The reason the belt interval is treated as absolute is the interference engine. In that design, pistons and open valves share the same space at different moments; when the belt snaps, they meet, and the valves bend. There is no warning noise and no cheap version of the repair.
Plugs and belt sit on the long end of the schedule; the short end is the oil change interval, which is worth checking against the normal-versus-severe table, and the home oil change procedure for the one job most owners do themselves.
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