Most of the fittings on the water side of the block are pipe thread. Pipe dope, aka, pipe thread sealant from plumbing suppliers usually work best for American NPT pipe. The block plugs, bypass hose, big block heater hose fittings are all standard tapered pipe threads. If the female side is worn, damaged or tapped too deep, you'll have a leak. Teflon tape, wound in the correct direction starting about 3/4 thread back with pipe dope over it can help seal minor problem threads.
