There are a number of sites explaining how to rebuild the gooey-wheel capstan for the cartridge drive. They all give dimensions, but no source for these dimensions is mentioned. Does anyone have the original engineering drawings of the CJ drive, indicating the actual measurements of the rubber roller?
Not that I know of (although if someone does, they're probably here). But the dimensions simply aren't that critical. For these tape drives, as long as you're within a millimeter or so, you're good.
I found a page that mentioned 10.7 to 10.9 mm, which equates to 0.425". I just redid two 85 drive spindles with 3/8" ID, 1/2" OD vinyl tubing superglued to the hub and used a diamond nail file to reduce the OD to about 0.425". The drive passed the 85 service ROM tape diagnostics at that diameter.