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I am in the same business, that is creating an extruder for my E3D v6, and I guess he means this coupling:
The dimensions got me a bit confused at first as well, but it does say thread size 1/8" BSPT (British Standard Pipe Thread), not plain 1/8" measurement. According to http://en.wikipedia.org/wiki/British_Standard_Pipe 1/8" BSPT means a thread major diameter of 9.728 mm, and a minor ditto of 8.565 mm. Mystery solved.