Image Credit: Inside Japanese Factories / YouTube.Modern performance engines are often associated with computer-controlled machinery, automated assembly lines, and sophisticated electronic tools. At OS Giken in Japan, however, some of the most interesting engine work still comes down to skilled technicians carefully assembling components by hand.The Okayama-based company has been producing high-performance automotive parts since 1973, building a reputation for products including clutches, limited-slip differentials, and specialized engine components. A recent video from the Inside Japanese Factories YouTube channel offers a detailed look at another side of the operation.The nearly hour-long video follows the construction of an L-Series inline-six destined for OS Giken's S30 Nissan Z project car. Rather than rushing through a polished montage, the footage documents the methodical process of turning an original engine block into a carefully assembled performance engine.AdvertisementAdvertisementAt the center of the build is one of OS Giken's most significant creations: the TC24-B1Z cylinder head. Its history stretches back decades and connects the company's modern engineering capabilities with an ambitious design originally developed by founder Masaharu Okazaki.A Modern Version Of A Rare OS Giken DesignImage Credit: Inside Japanese Factories / YouTube.The TC24-B1Z is a modern evolution of OS Giken's original TC24-B1, a dual-overhead-cam cylinder head created for Nissan's L-Series engine during the 1970s. According to the company, only nine examples of the original version were produced.OS Giken revived the concept decades later, introducing the updated B1Z version in 2017. Modern manufacturing techniques allowed the company to refine the design while preserving the basic idea behind the extraordinarily rare original.The workshop itself combines that modern technology with a distinctly small-scale approach to production. CNC machinery and computer-aided design are part of the operation, yet the engine assembly shown in the video resembles the careful work found inside a dedicated racing shop rather than a highly automated factory.The Engine Comes Together One Component At A TimeImage Credit: Inside Japanese Factories / YouTube.The process begins with an original L-Series engine block that is thoroughly cleaned before assembly starts. From there, technicians install new internal components, including the crankshaft, connecting rods, and pistons, while working through the engine piece by piece.AdvertisementAdvertisementConsiderable attention is also given to the cylinder head, including porting and polishing work before final assembly. Every stage is completed slowly and methodically, with technicians relying heavily on their own experience and attention to detail throughout the build.That hands-on approach continues when the completed engine is prepared for its first startup. The engine is initially turned over to confirm that oil is circulating properly before it is allowed to run under its own power.Once it fires, two technicians listen closely for anything that might indicate a problem during the crucial first minutes of operation. Only after testing is completed is the engine ready to be installed in the S30 Z project car.Four Months Of Work For One Inline-SixAccording to the original report, the entire engine project took approximately four months to complete, potentially because it was carried out alongside OS Giken's primary business of producing individual performance components. That timeline also reflects how different this process is from the rapid production environment of a modern automotive assembly plant.AdvertisementAdvertisementFor enthusiasts fascinated by mechanical craftsmanship, the resulting video offers an unusually detailed view inside a respected Japanese performance workshop. Seeing an L-Series inline-six assembled largely by hand is compelling enough, although hearing the finished OS Giken engine finally come to life may be the most satisfying part of the entire process.If you want more stories like this, follow Guessing Headlights on Yahoo so you don't miss what's coming next.