The guy that mentioned different expansion rates nailed it. A lot of guys helicoil all the threaded holes right from the get go. You could try selling that original kit and get those Stage 8's. They have a e-clip groove cut into the wrench flats and locking tangs that slip over the bolt heads. The locking tangs have a 12 point hole so they can be positioned properly on the bolt head. Then you lock it in place with the e-clip. If the bolt loosens the tang will hit the manifold and stop the bolt from turning. So it can't loosen more than 1/12 turn. Pretty darned fail safe.
http://www.stage8.com/products_lhb.htmlSafety wire works too.
