If that doesn't work, the threaded bores may have elongated, or 'stretched', and then you might try to helicoil it. These things are one of the greatest inventions to come down the pipe. I had to do this on the fly with my first whizzer going across the country. The head bolts were over torqued when I recieved it, and in wyoming I purchased the helicoil kit and did this. It helped immensly.
Useta' use 'em in the machine shop all the time when the milling fixtures would wear after running twenty million of the same part...