Again I agree with your logic leathermag upon reflection. The pounding should back off the loose adjuster. Probably does in fact. Since the really old gas adjusters were moving probably up till the 130 gas engine mercedes did add a much more aggressive thread interference. Either through wear or the combination of wear and perhaps some adjuster movement the clearence did not seem very simular from valve to valve in comparrison after 10k miles. Perhaps they were also subject to seat errosion to some extent . In some cases just trying to clean up for burnt valves meant some valve seats were cut pretty deep by the time I got some of those cars from the previous owners. Usually only on one or two exhaust valve positions though. That indicated to me either the valve had failed many times in that location or more likely for whatever reason the seat had required pretty aggressive cutting to restore to sealing condition by someone at some time. Wonder as well if the bad valve guides may have been a factor as they were almost always pretty bad and I had a new set installed into each head I repaired. Not just marginal as the recessed area that the valve stem seal mounted on was pretty well wore away from the loss of guide material..