How To Measure Chain Size. Motorcycle and atv chains have a width number of either 15, 20, 25 or 30. Number of drive links, pitch, and gauge.
You should check the cog+chainring for a marking of 1/8 or 3/32, and if it has one or the other, pick up a single speed or 8 speed chain respectively. Use a dial caliper to measure the thickness of a drive link. Change the back derailleur on the smallest cog.
If you do not have access to peer part number, then you may need to measure.
Measure from the first to the third, then divide that. Below are roller chain size charts for ansi roller chain in single, double and triple strand. Roller width the roller width is the measurement w in the drawing above. The number of drive links.