These Duramax shafts feature a new type of radial oil transfer passage which provides full time oiling to the roller lifters via the adjusting screw and pushrod. Without this new oiling feature, the stock oiling passage in the shaft is shut off as soon as the rocker arm starts to open the valve. This can be seen when the lifters are taken apart ;The bushings will be discolored from excessive heat. In effect, any oil that reaches the bushing at this time is being overheated, contaminating the oil supply with damaged oil. These rocker arm shafts use a threaded plug to seal each end, eliminating any chance of an oil pressure loss from worn or poor fitted press plugs. Sold Individually, requires 2 to make a set