To be certain, you should:
* Check the part numbers: Find the part number on your Mountaineer's starter and compare it to the part number of the Explorer starter. This is the most reliable way to ensure compatibility.
* Consult a parts catalog: Use an online parts catalog (like AutoZone, Advance Auto Parts, etc.) and enter the year, make, model, and engine of both your Mountaineer and the Explorer the starter came from. Compare the listed starters.
* Check the physical dimensions: If possible, visually compare the starters themselves. Even if the part numbers are similar, slight dimensional differences could prevent installation.
In short, while it's *possible* they might be interchangeable, it's not a sure thing. Always verify compatibility before purchasing and installing.