So, on to the bottom brackets. Start with a piece of 1.75” OD tubing, and cut off two pieces a bit proud of 68mm. Squared the ends, cut bearing seats, and gave the edges a bit of sanding. Repeat for second. These use bmx bottom brackets.

Added to the BB are two plates from sendcutsend.com . These needed just a quick touch with a 1.5” drum sander in a cordless drill and voila!

These allow 0.6” of fore and aft adjustment for chain tensioning. They clamp to a TBD place on the frame per below.