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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):

picture

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.

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    You'll want to measure the distance between stack bolts (the "C-C") to use the 1.701 multiplier. If you find the the two holes at the bottom of your tape are separated by 2 5/16 inches, your BCD is 110mm.
    – Jeff
    May 28, 2019 at 1:45
  • That looks a very large BCD, and matches the results you are calculating.
    – mattnz
    May 28, 2019 at 1:53
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    @Jeff There are two chainrings with their own BCD. OP is measuring for the “large chainring” whereas you are commenting on the small one
    – Swifty
    May 28, 2019 at 6:51
  • Kyle it would be interesting to know how many teeth are on the large chainring, sounds like a whopper!
    – Swifty
    May 28, 2019 at 9:26
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    @Swifty Yes, the large chain ring, thanks for clarifying! Unfortunately, I lost the chainring on the ride, so don't know the number of teeth. It looks just like the one pictured in Andrew's answer though, so I believe its likely also 52. A big ring, just wish it was a little sturdier!
    – Kyle
    May 28, 2019 at 22:49

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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.

drive side crank on Schwinn World Sport

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    That looks exactly like my crankset! And the smaller ring has 40 teeth, so I'm guessing my outer had 52. Luckily, come to find out, my city does have a co-op! Thank you so much for the suggestion, and the note on the square taper spindle. I'll take this info to the co-op, and see if I can get lucky.
    – Kyle
    May 28, 2019 at 23:08
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    Just an update... Could not find a chain wheel that fit, but did have great luck at my local co-op finding a full new-to-me crankset that did! Thanks again for the suggestion!
    – Kyle
    Jun 1, 2019 at 1:32
  • Good to hear. Co-ops are sometimes underappreciated as sources of vintage parts (and bikes).
    – Andrew
    Jun 1, 2019 at 15:25

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