I replaced my coolant flange about a month ago, but it still had a slow leak. When I was bolting the flange to the head I noticed that on the left side of the flange (viewing from overhead) I could see the o-ring sticking out between the head and flange. It looked like it was not sealing properly on that side. It leaked slightly, and has grown progressively worse, to the point were I cannot drive. This weekend I took the flange off to see if I could reseat the O-ring, but it looked pretty beat up, and flat on one side.
What am I doing wrong to cause the O-ring not to seal properly? It fits in the groove just fine, but when I snug up the bolts the oring starts to bulge on one side.