General Information

A chain's number refers to the width and length of the links in 1/8 inch increments where the first digit represents the length, or pitch from pin to pin, and the second two the roller width. For example, a '520' link would be 5/8" long and 1/4" (2.0/8) wide.

So, a 520 chain is bigger and stronger than a 428. Unfortunately, it's also heavier. With a lighter chain you'll notice faster acceleration & quicker throttle response with less drive line lash.

O-ring and X-ring chains use a rubber gasket to trap grease between the pins and bushings to extend chain life significantly beyond that of other chains. O-ring and X-ring just refer to the gasket design used. Typically, X-ring chains are reported to have less rolling resistance & longer life spans than the equivalent O-ring chain. However, both have more rolling resistance than a non O-ring chain and this is racing after all so, lifespan is not an issue.

I've had some of my fellow racer claim that they found an extra 1 to 2 horsepower just from this mod alone, but I have not had an opportunity to confirm or deny this on a dyno.

Specifics

Since the 428 pitch is shorter than the 520, we had to have more teeth to retain similar diameters (and gear ratios).

Those running a 14/47 (3.35:1 ratio) will run a 17/57 in 428
Those running a 14/45 (3.21:1 ratio) will run an 18/58 (3.22)

This "mod" really became popular in the beginning of the 2001 season when Mike Norman started carrying 'em at G-Force (much to the chagrin of some riders). Paul Somerville up'ed the ante by running a 420 kit in the 2002 season in which he won the championship - I decided to stick with the more popular 428 so, that it would be easier to mooch parts at the track if I needed too.

References