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old timer
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THe fact is...all Big Twin Harley Davidson Overhead Valved engines since the knucklehead in 1939 (I think) have been built with one camshaft that had four lobes. It was gear driven by the pinion shaft from the right flywheel and controlled the operation af all four valves.

The Twin Cam is only different in that there are now two cams...one is geardriven by the pinion shaft and the second cam is chain driven from the first cam.

Several aftermarket companies are marketing gear driven twin cams for the motor because theoretically gear driven cams are more precise, from a timing point of view, due to chain stretch and slop perspectives.

That should explain the difference between the single cam vs. twin cam concept. THere are a few other differences between EVO and T-C configuation but the cam design is basically the 'real' difference. Heads, various valve train components and case and timing cover design also difer but the real difference is i the number of cams and the geometric implications on such things as pushrod angles, length and such.

I hope that helps....

manny
 
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