All motorcycle chain sizes are indicated by the series to which they belong, either 400, 500 or 600. Each series has slightly different specifications that relate directly to the sprocket on which the chain fits. Embedded within the series number is another subdivision that denotes another important specification of the chain. Series 420, for example, is the smallest motorcycle chain available, and so on up to 630, the largest. Encoded within these numbers are the exact specifications a mechanic needs to know to find the correct chain replacement.
Turn the chain on its side and measure the length between links. On the outside of the chain is a plate roughly in the shape of a figure eight that holds two links together, and each link is held by a pin inside. Measure from pin to pin; this gives the pitch of the chain. The pitch relates to how wide the teeth are on the sprocket.
Convert the pitch to a comparative fraction. For example, if the pitch measures 1/2-inch, this is equivalent to 4/8-inch. The 4 corresponds to the 400 series. A pitch of 5/8-inch corresponds to the 500 series, and a 3/4-inch pitch converts to 6/8-inch, and therefore corresponds to the 600 series. Each comparative fraction corresponds to the first digit in the three-digit series.
Turn the chain so the roller of each link faces up and measure the width of a roller. Lighter chains have a roller width of 1/4-inch; the next size up is 5/16-inch. The largest roller width is 3/8-inch. This covers every chain size with regards to roller width. Roller width relates to how deep the teeth are on the sprocket.
Convert the roller width in inches as the last two digits in the three-digit series. For example, 1/4-inch converts to 20, 5/16-inch converts to 25 and 3/8-inch converts to 30. We now know that a 420 chain has a pitch of 1/2-inch and a roller width of 1/4-inch, and a 630 chain has a pitch of 3/4-inch and a roller width of 3/8-inch. Each conversion in the series system is standardised to fit the universal three-digit convention known by motorcycle mechanics worldwide.