Correct valve timing is not about lining up the marks on the pulleys, it's about the correct relative positions of the crankshaft, camshaft and fuel injection pump. Nearly all diesel engines require setting tools to lock the shafts in the correct positions, and to allow correct tensioning of the belt with the pulleys slackened. Failure to do this will mean the timing will move when the belt is tensioned, and will susequently be incorrect. Not enough to cause damage to the engine, but sufficient to have an effect on emissions, and in the case of pump timing far enough incorrect to cause engine knock (advanced) or smoke on starting (retarded).
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