The service gasket kits often have a two peice oil pan gasket. When changing out a timing chain, you have to replace the front section of the pan gasket. What I've always done: peen the bolt holes on the pan with a ball-peen hammer back into their original shape. The pan has to be FLAT where it joins. Now, use a small amount of GM gasket sealer/slicone and attach the gasket to the pan. Let it set up long enough for the gasket to stay in place, but not cure/set up completely. Add a small bead of sealer at the corners where the timing cover meets the pan and where the crank flange area is. Put a small thin coat of sealer on the block side of the gasket, and install, tightening from the center of the pan in GRADUAL light increments. The number one cause of leaky pans is overtightening bolts and distorting the pan/tearing the gasket. Slow, meticulous, and neat will get'er done. Not TOO much sealer!!!