1" pipe is not 1" OD, but closer to the ID. Given that 1" pipe has an OD of about 1.3", the coupling that it screws into is about 1.5" OD. Aside from how old the thread is, if anyone is newly reading this, I think a pipe coupling is about the worst solution for a heating element. Some of the better alternate solutions are triclover, welding spuds, and even just face soldering a 1" NPT locknut in a pinch. All of these will allow for gasket contact to the face of the fitting rather than relying on an NPS to NPT seal which requires a LOT of teflon tape to seal.