Your stock chain will probably last about 20,000 miles if properly maintained. When you replace the chain you will need to replace the front sprocket. The rear sprocket will last through a couple of chains.
Going from a 525 chain to a 520 chain will save you about 2 pounds. The new 520 chains will last as long as the 525 chain. The new ZX-6R actually comes with a 520 chain. About 1 1/2 pounds of the 2 pound saving comes from using an aluminum instead of steel rear sprocket. So you can get about 3/4 of the benefit of a 520 conversion by keeping the 525 chain and just buying an aluminum rear sprocket.
If you are changing sprockets for better acceleration you can go up two teeth in the rear or doen one tooth in front or both and keep the stock chain.
I went +2 in the rear and -1 in the front and changed the stock chain to the DID 520 gold chain. This change will increase definitely performance. Guys I race with have gone +5 in the rear. I do like to drag, but I also wanted to keep it rideable and the -1, +2, 520 conversion has maed it quicker on the track and more fun to ride on the street. Good luck in whatever you choose to do.
Yeah, the 520, 525, 530, etc. basically tell you how wide the chain is. A 520 chain is a little narrower than the stock (for your bike) 525. If you want to go to a 520 chain you will have to change both sprockets. The difference in weight between a 520 chain and a 525 chain is about 8 ounces.
X-ring is just a more sophicated type of O-ring. I think both DID and RK make X-ring chains now. I changed the chain out on my '95 ZX-6R recently and went with a 520 DID X-ring chain with a Renthal 15 (stock) on front and a Sprocket Specialist hard anodized 39 (one down) on the rear. The chain at 20,000 miles was still within spec but the front sprocket was hooked some. The rear sprocket was like new.
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