Most engines I've worked on have at least 1/8" clearance when the valves are closed to the top of the piston at TDC. Even if your valves was tight I doubt they could hit the piston unless your timing chain was off several teeth or streched beyond reality!

__________________
87 300SDL (115,000mi. 7/1/2008)
03 Jetta Wagon TDI (62,000mi. 7/1/2008)
75 240D(sold, sad to sell it, needed the garage for the SDL!)

Smokey