"Chains have a few vital statistics. The first critical dimension is the indentifying number (520, 525, and 530 being the most common motorcycle dimensions). The 5 stands for the pitch in eights of an inch.
Sorry. Pitch is the distance between pins (the little nubs that show through the sides on the hourglass shaped side plates). So, for motorcycle chains, they are 5/8 of an inch apart. Don't recall exactly how they come up with the other digits, but they designate the width of the roller. That is, a x20 is narrower than a x30. Because the rollers are different widths on 530s and 520s, the sprockets that mate into the gaps between the rollers need to be different widths. This is why you can't run a 520 chain on 530 sprockets (gap is too narrow and sprocket teeth won't sit down between the rollers), and neither can you a 530 chain on 520 sprockets (too much gap so their is dangerously excessive freeplay and wear). A 530 is a wider chain which is theoretically stronger, as well as being more durable and spreading the accelleration forces across more area resulting in longer wearing components. A 520 chain is narrower resulting in less overall weight, but specifically less rotating chain weight for less gyroscopic force, less weight which has to be accellerated/decellerated during operation for less driveline power losses, and less unsprung and reciprocating weight at the rear wheel thanks to a narrower sprocket. The effects can be fairly marginal to somewhat noticeable. I figure it's about six of one and a half dozen of the other and see no reason to change unless you're due for a sprocket/chain change anyway. But understand that the higher the power of the machine in question and the harder it is ridden, the more wear will come into play (i.e. a 520 on a literbike is sure to stretch and wear sprockets faster than a 530 on a 600).
The other chain dimension in question is length, which is expressed in the number of links. If you know your length specifically (i.e. 108 links), you can order chains to an exact length. Or more often than not, you'll order the next size up and cut to fit (i.e. ordering a 110 link chain and cutting off 2). Check your manual for your specifications and remember that changing the number of teeth on your sprockets MAY require changing the number of links in your chain. When asking other owners for gearing recommendations, be sure to ask if the stock chain length worked. Usually a tooth one way or the other (i.e. dropping a tooth up front) won't make a difference, but sometimes two and most often three teeth (i.e. going from a 36 to a 39 rear sprocket) will cause you to need an extra link.
An X-Ring is an O-Ring with better marketing. Um...maybe. Not saying it's not better. I'm not an engineer so I don't pretend to know. But, an X-Ring is simply an O-Ring of a different shape that allows more oil surface to come in contact for better lubrication and less friction losses, as well as double the sealing points to better keep lubrication in blah blah blah :sleep: Want to know more, go to D.I.D. or RK or whoever and read up. I like X-Rings just because they seem to be the top of the line no matter the manufacturer and I somewhat buy the marketing hype of not trapping dirt and so on. Are they really better? Dunno, but I like gadgety things anyway and there isn't a huge price difference from quality O-Ring to marketed X-Ring these days.
Last comment before I realize I spent WAY too long typing all this. Change your sprockets and chain at the same time. Seriously. This isn't some mechanic's myth designed to get you to spend more. Now, I'm not talking about changing your 2000-mile-old chain just because you want to drop a tooth off the front sprocket. That would be overkill. But, if one or more components is just about gone, do all three (front sprocket, rear sprocket, and chain) all at once. Not doing so *WILL SEVERLY INCREASE* wear on any new components. I just this week changed all three for a buddy who thought that just changing his rear sprocket would get him by last time...2000 miles ago! The worn chain and front sprocket ate up his brand new rear sprocket like it was a snickers bar in a fat camp. And I've seen new chains go just about as fast when sprockets weren't changed in conjunction.
So, your choices when getting chains are the width and the length. Then you have to get sprockets to match the width of chain you chose, but you can play with the number of teeth if you wish to affect gearing. Good luck, and may I suggest picking up some GOJO or Lava or whatever extreme duty soap you like if you plan on doing the work yourself - chains are NASTY buggers."
