Troubleshooting leaks

Yes, overtightening can cause leaks and the dreaded bolt break. Cheap replacement parts can also attribute to a leak. I remember changing a water pump on my old 280ZX and it leaked like crazy. We did everything you mentioned and it still leaked. Found out that the part we replaced it with was the problem. We had bought the part at Chiefs Auto Parts when they were still in Dallas. We went to another parts store, cant' remember the name and the replacement part worked fine. Try a thermo from another parts store and see if that helps. Also get a new gasket too. And possibly a torque wrench so you don't overtighten. I remember one of my friends over tightened a bolt while replacing a exhaust manifold. The head of that bolt went snap....I was like..."Later dude..have fun with that."

I know a lot of guys use rtv silicone like that but I won't anymore. It can come loose and plug up your cooling system (I've had it happen). Get some 'Right Stuff', it won't come loose and you won't have any leaks.