Will a 04 Mazda 6 motor fit in my 07 Mazda 6
There are 2 engines made for the mazda 6. There are also changes in the engines over the 2 years that you are describing. If you are able to tear down the engine to a bare block and notice that there are no internal differences, such as cams, then it should work if you put back everything onto your 2004 engine from your 2007 engine. That would mean replacing all of intakes, manifolds, etc, and using the original wiring harness from your 2007. However, I would recommend finding the correct direct swap engine to avoid issues in the long run or messing up during the process.
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