Federal-Mogul developed an extension of electro-chemical machining that for the first time makes this approach feasible for high volume automotive production. The first application is machining the wrist pin hole of Monosteel® pistons for diesel engines. This allows the piston to tolerate pressures above 200 bar without a bushing - and at lower production cost, which helps open the design envelope for diesel engines. The machined surface holds tolerances within a 4-micron band; the tiniest high spot will form a pressure point that diminishes engine life, and that can lead to catastrophic failure. To do this, the Federal-Mogul team started from the basic science to modify the prior art of electro-chemical machining so substantially that they chose to call it electro-erosion machining. The process is applicable to other ultra-high tolerance machining of suitable geometries.