I changed my oil yesterday and with all of the problems reported on stuck CRD Engine Oil Pan drain plugs, I took the time to use my machinist tools to determine the correct size. It looked close to 16mm but when I checked for both 1.25mm and 1.5mm pitch, the gauge would not fit, then I tried the 18TPI and it fit like a dream. Next I used a micrometer to check the diameter and it was 0.625 = 5/8".

CRD Engine Oil Pan Drain Plug Size is: 5/8" X 18 TPI

Steve

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2006 Pearl Green CRD
Magnaflow 2 1/2" Cat Back
KJ Extra Leg Room Brackets, Carter Lift Pump, V6 Airbox, ORM
Fuel cooler, Oil Separator, Progard 7
Gauges EGT Boost Trans Temp Oil Pres, Michelin LXT AT2 245 70 R16
7,000# Draw Tight hitch, PML EX Deep Trans Pan
Centrifuge, SunCoast, Transgo, RAM TCM, InMotion Stage 2
Wife's 99 TDI VW Beetle