Which chain does my motorcycle take?

A pitch and a link count, both set by your machine and published in its manual. The pitch — 520, 525, 530 and so on — is the distance between pins and the width of the rollers. The link count follows from the geometry and the gearing. Get either wrong and nothing fits.

Reading the pitch

It is stamped on the side plates of the chain already fitted, and it is in the manual. The three digits are a convention rather than a measurement you would take with a ruler: the first describes the pitch, the last two the width. A 525 chain does not go on a machine specified for 520, and the difference is not adjustable.

Reading the link count

Also in the manual, and countable on the chain you have. A chain is supplied long and shortened to the count needed — which is why it can be shortened and never lengthened, and why ordering short is the mistake that cannot be recovered.

The sprockets, which come with it

Chains and sprockets wear together, and fitting a new chain onto hooked sprockets ruins it quickly. That is why kits exist. The sprockets add a bolt pattern and a tooth count, both published, and changing the tooth count changes the gearing — a deliberate decision rather than a variation between suppliers.

What is genuinely a choice

The sealing ring. A plain chain has none and needs constant lubrication. An O-ring chain traps grease behind round-section seals. An X-ring uses a profiled seal that makers publish as lower drag and longer life. All three exist at the same pitch, and the difference is maintenance frequency rather than fit.

The detail that decides whether the job can be finished

Which joining link is in the box. A rivet link needs a riveting tool; a clip link does not. Makers publish which is supplied, and a kit that arrives with a rivet link and no tool is a job that stops halfway.