Introduction to reconditioned BMW 420d engines: Understanding the basics. When it comes to engine replacements, a reconditioned BMW 4 Series engine offers a cost-effective and reliable solution for BMW enthusiasts. Reconditioned engines undergo an extensive refurbishment process to restore them to their optimal working condition. This process includes disassembling the engine, cleaning components, replacing worn-out parts, and reassembling it to meet manufacturer standards. The end result is an engine that performs efficiently and reliably while costing significantly less than a brand-new replacement.