NOTE - You are mixing two incompatible thread forms with different flank angles, thread depths, and threads per inch and hoping to plug the resulting holes in the dike with sealant. Why beg for leaks?NOTE: YOU CAN'T INSERT AN NPT THREAD INTO A BSPT HOLE, BUT YOU CAN INSERT A BSPT FITTING INTO AN NPT HOLE! IT WILL JUST SEAT .
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Your metric fitting does not seal on the threads and does not need thread sealant. It is a face seal fitting and needs a sealing washer under the head.