Good question. If the camshaft sprocket is not removed, once you remove the index tools that hold everything at TDC, the tension won't be even and the sprockets will move. If the sprocket is separated from the camshaft so that it can move freely, everything will stay in place. This is why you loosen the injection pump sprocket as well. Cam and injection pump timing are important to proper operation and starting.
The crankshaft sprocket is not touched. Not sure which drive gear you were referring to.
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