I have a 2002 Mitsubishi Montero Sport 3.5 engine and I will like to know at what mileage should I change my timing belt only has a hundred and twenty one thousand
Every 60k miles. Remember that's an interference engine. That means if belt brakes or jumps time you will most likely bend some valves. I would be making plans to change it if it was mine.
Also replace the water pump at the same time. Some timing belt kits come with new water pump included.
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