How Is Komatsu 4D102E-1 Engine Different From Cummins 4BT3.9 Engine?

In short, the Komatsu S4D102E-1 engine and Cummins 4BT3.9 engine use the same basic engine components (cylinder head, valves, valve inserts, block, crankshaft, con-rods). 

The Komatsu possibly uses a different camshaft and maybe pistons, and it definitely uses a different fuel system. There will be differences in accessories between the engines and maybe even front and rear housings. Things like switches, belt tensioners, alternators and starters, for virtually all engines, can be acquired from several aftermarket manufacturers, for far better prices than OEM.

When Cummins and Komatsu went into Joint Venture initially, they formed 2 companies - Komatsu-Cummins in Japan (51% Komatsu shareholding and 49% Cummins shareholding), and Cummins-Komatsu in America (51% Cummins and 49% Komatsu).

Komatsu-Cummins built Komatsu-Cummins engines in Japan and Cummins-Komatsu built Komatsu-Cummins engines in the U.S. You will probably find differences in brands of accessories depending on where the engine was built. Komatsu and Cummins all use the same basic engine block components, but Cummins have developed their fuel injection and electronics along different lines to Komatsu.

Initially, the original (mechanical injection) Komatsu-Cummins engines did not use Cummins PT fuel injection, they used inline Bosch-type fuel injection (Diesel Kiki, or Nippon-Denso, under licence to Bosch). This meant, that although the Cummins and Komatsu engines of that era used the same basic block components, the cylinder heads and pistons were different to accommodate the different style of fuel injection. Since the development of electronic injection and lower emission engines, the two engines have got closer in design and interchangeability. However, always expect to get a surprise when you find differences, when you expected the same component.

The Japanese love changing the design of a part or component twice a year, simply because they think a new design will work better, or is cheaper to manufacture.

Be aware that the 4BT3.9 is a product of the CDC (Consolidated Diesel Company) that started in 1980 as a Joint Venture between Case and Cummins.

In 1999, Case Corporation was purchased by Fiat Group and merged with New Holland NV to form CNH Global, and Case became Case IH and Case Construction. CNH Global kept Case IH, Case Construction, and New Holland, operating as separate manufacturing entities.

However, CNH Global arranged to sell its share in CDC to Cummins in 2008, so CDC became fully-owned by Cummins after that date.