There is a twist to your setup that might be beneficial. I have used a UG 1st position cog with a HG cassette with mixed results.
I don’t have any 10 speed cassette bikes, as opposed to real 10 speeds, as I have not found any value in them for me. However, if you can get a UG 12t, I think they made one in 34.6mm, sand it down to give the proper spacing, thread it on to your 6401 UG/HG freehub body to hold the cassette tight, “and” then add a lockring for a bit of extra security.
The issue I had with a UG 1st is the teeth are so tall that shifting on and off was not as smooth. I had contemplated modding the teeth, but I found a 14t HG 1st position with a built in spacer. Friction shifting might make things shift better.
Since I have a couple of Cannondales this thread by miamijim was invaluable for me to understand how to fudge the OLD to fit into 126mm dropouts. I even copied and pasted it into a Word document. The reality of what I discovered was the answer isn’t necessarily getting to 126mm, but getting close. My magic number is 128mm or less. On a Shimano freehub it is pretty easy to find a couple to a few mm’s. 2mm’s is nothing especially since frame mfg’s allowed for that much on their late 80’s frames spaced to 128mm.
John
Last edited by 70sSanO; 08-21-21 at 09:11 AM.