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If you have a Ford engine with a flat-tappet hydraulic-lifter camshaft using an iron gear and a Duraspark distributor from a roller cam 5.0L engine, you will have to change the steel distributor gear for a cast version. The opposite also applies. Effectively, you must match the distributor drive gear to the cam. You can identify a steel gear (left) by its smooth surface above the gear compared to the cast-iron gear (right), which has a rougher surface.