The bolts are all the same length. I don't think the holes are stripped, because If I remove the cover, I can thread the bolts into the holes in the head. I've checked the routing of the wires thinking they could be interfereing with the position of the and don't think this is the problem, but will look at this again after work today.
No, #6 plug was not oliy; all were in great condition after one year of service.
Mark