I found an old post on quiltingboard.com (https://www.quiltingboard.com/7058873-post17.html) talking about modifying the needle bar in order to accommodate 15x1 needles. I have been successful using Groz-Beckert DBX1 needles on my W&W D9 but it's kind of tedious to set the proper height and to have to align the needle manually (although using a vintage Kenmore needle threader helps).
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Since I recently managed to get a donor machine (W&W D9), I decided to give it a try. I am pleased to report that I was able to modify the needle bar to accommodate 15x1 needles. It was pretty easy and it works great. I tested with a variety of 15x1 needles (Schmetz, Organ, Singer) in different size (11,14 and 16) and also tried with the only 127x1 needle I have with no issue. I didn't try a DBX1 needle as it is even skinnier than the 127x1 and after dressing down the index screw to accommodate the larger diameter of the 15x1 a DBX1 wouldn't be held properly.
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I used a 2mm (5/64") drill bit on a drill press to enlarge the bore, making sure that I only drill enough so that the shorter 15x1 needle sits at the proper height. Since the 15x1 needle is slightly larger that 2mm, I used a cordless drill with the same drill bit to slightly remove some extra material. I finally grinded the little index screw on a sharpening stone, just enough so that the needle clamp assembly would fit on the needle bar.
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After reassembling, I checked the timing and it was perfect!
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Mike Sierra