Originally Posted by etherhuffer

I had a friend put a Technomic in his lathe and bring it down to 21.1. For safety reasons I cannot tell others to do this. That said, I think putting a caliper on an old 21.1 stem and then the Technomic will tell you how much wall you have to work with. If no thinner than the OEM 21.1 stem you should be safe.If you look at old Schwinn stems, the top is larger, then it steps down to 21.1 Of note, I polished that thing like crazy after we cut it down. I worry that a cut surface can lead to crack formation. The bike its on does not get used at extremes, I don't stomp and reef on the bars. That said, its my noggin I have to deal with, not yours.
It would really be nice if Nitto would do a 21.1 stem. I asked Grant at Rivendell and he said there is just not enough demand. Same at Velo Orange. Good luck on the project, its really a pain in the rear.