1. When the chain stretch (check it when you do your valve job) can't be corrected with a factory offset woodruff key. Do a search on camshaft woodruff key or chain stretch

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT