Make sure you use new copper crush washers every time. Push the valve cover up towards the fuel distributer when you tighten down and do not over torque, i prefer using feel use a 1/4 inch socket and tighten to pinky finger strength you will feel the crush washers give that is the perfect amount.
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Martin Ingram
Colorado Springs
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)