A 525 O-ring chain is a specific size of motorcycle chain, commonly used in motorcycles and other powersport vehicles. The "525" refers to the size of the chain, which is based on the pitch and width of the links. Here's a breakdown of what "525" means:

• 5: Refers to the pitch of the chain, which is 5/8 of an inch (approximately 15.875mm).

• 25: Refers to the width of the chain, which is 1/4 of an inch (approximately 6.35mm).

The "O-ring" part of the name indicates that the chain uses rubber O-rings between the links. These O-rings serve to reduce friction and wear by keeping the lubricant in and contaminants out, which helps extend the lifespan of the chain. O-ring chains are popular in motorcycles because they require less maintenance compared to non-O-ring chains and provide better performance, especially in harsher conditions.