Russ, the spindle dimensions are readily available online per e.g. here
**LINK**
and suggest you read this whole thread before expending too much effort on a metric banjo:
**LINK**
(there's a dimensioned drawing of the metric banjo on page 2 of that thread if you do feel the need to make one)
Edited By Bikepete on 29/05/2023 11:12:34