I'm rebuilding the 3.8 engine on my 1960 Mark 9 because a previous owner overheated the engine and smoked the rings. As we rebuild the engine, I want to put an overheating siren on the engine. Rather than fit a temperature switch in the head, I want to protect against overheating due to water loss, so I want to put a temp switch at the block. I'm trying to replace the draincock with a temperature switch, but I'm having trouble finding one that fits the 1/2" 20 UNC thread that the draincock has. I found out the hard way that the ~1/2" outside thread diameter doesn't align with a 1/2" pipe; it's more like a 1/4" NPT thread, although I don't think it's exactly the same.

Has anyone successfully installed a temp switch in the BLOCK of a 3.8 engine?