Interesting stuff, one more question. Would Tamiya 501X diff pulley be a better option since they are readily available. It is 37T, so combine that with a 18T maybe the stock belt would fit?
That should fit. 2 tooth smaller from 39 to 37.
I had to get a 2 tooth larger belt to use the 39T pulleys.
let me know the part # for those 501x pulleys..
the only problems I see is that Tamiya uses 3mm diff balls and HB is 2.4mm.
but it should fit by removing all the bearing shims.