What does "burning the midnight oil" mean?
Over the next nine months they tuned just about every moving part, replacing the suspension, radiator, and alternator, fitting it with front struts, a hatch spoiler and a seamless roll cage, and picking a proper array of rally tyres. It was quite an undertaking, but well worth the effort.
After burning the midnight oil for nearly a full year, making sure not a single loose screw could be found, the big day came.

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It means they worked late into the night. In this case it means they worked many late nights throughout the year.

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"To burn the midnight oil" means to stay up late at night in order to work/study. The "oil" refers to the oils used in oil lamps.