Hello,
I recently received a group of TA chainrings from a friend whose father used them in the 70's. I thought that the crank that his dad used was a TA triple touriste. But don't know much about the TA line, so just guessed. I now have 12 chainrings ranging from 52 down to 26, all with the same 5 hole BCD.
Using the method of measuring from the center to center and multiplying by factor of 1.701 for 5 hole bolt pattern, For 38mm between two adjacent bolt holes, I got a BCD of 66.33 If I used 39mm, 64.6. Does this sound right? I cannot find this BCD anywhere! What am I doing wrong?
Please see attached screen shot for how I determined my calculations.


I do plan to offer these rings to BF members in the future and want to have correct BCD.

Many thanks in advance.

38-40mm rough estimate from center to center.