Depends on your motor and preferred spark plug type. I've got an Optimax 225 that runs NGK iridium spark plugs. Other than after the initial break-in period, I don't think you have to change them unless they're damaged. For other plug types and motors, I just pulled them, inspected them, re-gapped them, and only replaced them if they were really fouled and I couldn't clean them.
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I just put those in my 04' Explorer at 70K, made a heck of difference in my gas mileage!The problem is the gapping. Specs are done in metrics and you have to dig around online to find the conversion for our SAE 100ths of an inch gapping tools.
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