Joseph,

MB says every 15,000 miles. But I think as that the engines get older, the engines need valve adjustments at less frequent intervals.

Example: After 25,000 miles since the last adjustment on my '79 300SD, I was about to adjust them again. But before I loosened any of the adjustment nuts, I check the clearances. Not one of the valves needed adjustment. They were all within the factory specification + or - .001". I'm waiting for another 25,000 miles to occur so I can measure the clearances again, but that will take another year.

So the true answer is how often the valves need adjustment is: whenever the clearance is out of specification. How often should the clarances be checked, I would recommend 25,000 miles. But you should determing how many if, any are out of spec and how far they are out of spec to determine what the best frequency is for your engine. To do this you have to have your mechanic give you a measurement of each valve's clearance or at least how many valves actually needed adjustment.

P E H