How can I tell what size chain is already on my bike??
A 520 is 1/4" across the plates, a 525 is 5/16" across the plates.
Chains are still made in imperial sizes, 5/8" by 1/4" = (5)x1/8" by (2.0)x1/8" hence 520. 5/8" by 5/16" = (5)x1/8" by (2.5)x1/8" = 525 and so on for other sizes.