drain the water lever in the block down below the heads and take the bolts out one at a time and put pipe thread sealant on them and re-install and let set per directions on the sealant.
I use a thread sealant made by Loctite called Loctite Thread Sealant:-D, basically liquid teflon. Also I would figure out what the thread size is and buy a bottoming tap to carefully clean out the threaded hole before applying.