Vintage bike - help with chainring replacement 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.
I believe I have that same crank on a roughly 1983 Schwinn World Sport (see image). I got it used, so I don't know if it was the original, but in any case, mine has 40 teeth on the small ring and 52 on the big ring, so that's likely the size you're looking for. 

That said, it might be easier to find a replacement for the entire crank+chain rings rather than match that chainring. Mine is a standard square taper spindle, and many vintage cranks that would fit are available on eBay etc. 

If you live in a city with a co-op that salvages old bikes, they often have loads of cranks from that era that would fit. 

Furthermore, since the square taper spindle was a standard for so long, a more recently made crank is also an option. 

UPDATE: Images online seem to indicate this crank was standard on early 80's World Sports. Given how many of those bikes are still around, which means the cranks should be relatively easy to find, or you could easily find a whole bike and take the crank off if it's really important to you to match your original.