When we need a new chain, we visit our favourite workshop and tell them so. Then they ask what size and we do not know. So, how is the motorcycle chain size calculated?

Here is quick guide. 

ANSI or ISO?

There is some confusion when it comes to chain sizing, most commonly because chain sizing uses the Imperial (inches) instead of metric measurements. The system has been used for a long time, hence it continues to be used despite most parts of the world uses the metric system.

Anyhow, US chain manufacturers abide by the ANSI (American National Standards Institute) standards and label their chains as 25, 35, 40, 43 and so forth. The more popular system that we use is the British ISO method which marks the chain sizes as 415, 428, 520, 525, 530, 630 that we are familiar with.

As such, do note that ANSI and ISO chains are not interchangeable.

Chain sizing

Right, let us get down to the subject of this article.

Chains sizes are classified according to their pitch and width (obviously):

• Pitch: The distance between the centre points of two adjacent pins.
• Width: The distance between the inner side plates (the rollers are slightly narrower to allow them to roll on the pin).

What about the numbers?

So good. But what do the numbers mean? For example, 418? The first number, “4” is the pitch, while the last two digits “18” represent the width.

Let us start with the pitch:

1. To understand this easier, begin by draw a 1-inch line.
2. Then, divide the line by 8 equal parts.
3. Place one link of the chain on the paper.
4. The number of parts the two pins fall into is the pitch of the chain, in this case, there are 4, so we get 4--.

As for the width:

1. The same principle applies, however, this time we have also 1/16^th of an inch, in addition to 1/8^th of an inch.
2. However, the two digits, “18” here means it is 1.8 X 1/8^th of an inch. (For comparison, a 420 chain means the width is 2.0 X 1/8^th of an inch).

It can be confusing at first but as how we said, it is much easier if you draw out an inch with 8 or 16 equal parts, then laying the chain on it or counting the parts.

So, from this we can conclude that the 630 chain has longer links but is at the same width of a 530 chain. On the hand, a 418 chain has the same link length but narrower than a 428 chain.