I recently broke the large chainring on my vintage bike, and need to replace it, but am having trouble determining what size I need. I lost the chainring on the ride home, so am measuring the bolt circle diameter on the crankset to determine the size.
In this picture, you can see that the distance between adjacent bolt holes is roughly 109.5mm (4.3125 inches):
According to the Sheldon Brown site...
"Multiplying this dimension by 1.701 will give you the actual BCD for a 5-bolt chainring"
...making my BCD 186.2595mm.
However, I'm not finding anything online with a size anywhere near that big. Can someone confirm my method of measuring is correct, and if so, any tips on sourcing a new chainring of this size? Or is my chainring size just obsolete at this point? I'm hoping to just replace the chainring, and not the whole crankset, if possible.
The crank arm is branded as Silstar, but I'm not finding any chainrings of that size under that brand either. The bike was a hobby project for the previous owner, who built it from scratch, and I no longer am able to contact him for any info on the parts.
Thank you for any info.