What replacement chain/sprockets should I consider?
ANY O/X/W-ring chain should give you 25-30k miles of use WITH PROPER CARE. Those same chains should give you 10-13k miles with only the most minimal care possible (that being adjusting it from time to time when it gets so loose as to slap the swingarm).
Bill H says to take a look at Tsubaki Omega chains.
BrianM likes RK chains because they're cheaper and last every bit as long as the more expensive D.I.D. chains (that a lot of people seem to prefer). There are two kinds of O-ring RK chains. The SO uses just an O-ring, the XSO uses an X shaped O-ring. The theory is that the X ring has 2 sealing surfaces for retaining the factory lube. This is vs. just one sealing surface on O rings. This supposedly allows for longer life by retaining more lube for a longer period of time. That's seriously the Only difference. The XSO pricing seems to have come down, and the O-ring chains are being slowly dropped from the lineup.
The RK and Tsubaki are relatively close in price. Sometimes. There can be a difference of $20-30 from one site to another. Search high; search low. eBay has many motorcycle parts dealers, too.
DID O-ring chains are in the same price ballpark with the other two. Their X-rings, on the other hand, are another story.
Some companies sell kits that have a chain and both sprockets all in one package. Not, of course, for the EX250. Kits just aren't made for our little bike. And you can't use an EX500 kit, either. The chain is too short. You'll have to source them separately. And it'll be a good day for you, too, if you can find one vendor who has both the chain and sprockets that you want.
The best way to find a chain on the net is to be very specific at first, then get more general. You can look at your favorite store first if you want, but then Google, say, RK XSO and see what happens. This might save you some time. You'll need it when you start looking for sprockets.
Chain size: You need a 520-106 chain. 106 is the # of links you need. The chain size is 520. What the zero means is lost with the secrets of the dead British motorcycle industry. (ie... it doesn't matter, or you could Google it if you cared). The other two numbers represent size, in eighths of an inch. So, "5" is the length of a link from pin to pin (5/8"), and 2 is the width of the link (2/8, or if you were there in third grade when they taught you how to reduce fractions, 1/4").
If you have an EX250H (ZZR250), you need a 520-108.
Generally speaking, it's an Exceptionally good idea to replace both sprockets along with a new chain. Not doing so will promote advanced chain wear. This is even more true when you have a completely shot chain that's being replaced. Some people replace the rear sprocket every 2 chains, but you'll be better off just spending the dough and changing it.
On a worn sprocket the teeth get thin and hooked, kinda looking like the dorsal fin on a shark, or the teeth on a saw blade. When that happens, the sprocket is SHOT and will kill any new chain in short order (a couple thousand miles).
It's strongly recommended that you stay away from aluminum (a.k.a. alloy) sprockets (yes, even the hardened/anodized sprockets) as they wear quickly and take out the chain when they go. There's a Tiny amount of weight savings that Maybe a racer could use, but it's not useful on the street.
There are many choices when it comes to sprockets. Those most used by board members are JT and AFAM. Both are of good quality and will last the life of your chain. AFAM gets a better nod on quality, but JT's aren't nearly as expensive. It's also hard to find AFAM's for the 250, and the rears are usually only listed in Aluminum, which is about twice as expensive as steel, doesn't last as long, and is unnecessary for street use. JT's, on the other hand, are much easier to find.
Here's a list of the manufacturers that make sprockets that fit the Ninja 250, from least to most expensive:
Your local dealer
By the time you factor in shipping, they might not seem so expensive. They'll have to order them, too, but it shouldn't take too long, and there's no freight charge.
Hints & Tips
Finding sprockets online
This is fun. If you've bought mostly gear on the web before, you're in for a rude shock when it comes time to buy parts. Most of the places that sell hard parts have web sites that have never heard the term "user-friendly". What really helps is having the part number, and some patience.
Actually, finding a front sprocket from any of the brands listed above (except AFAM) is pretty simple. Getting a steel rear one is getting harder, as many people have jumped on the aluminum bandwagon. JT and Sunstar are readily available, and Sprocket Specialists sells their own, so it's not too bad. Just be forewarned that many of the stores that come up on a search have websites that are hard to use. The bigger, more well-known sites are usually easier to navigate. Do either a part # search or a search-by-bike.
Sprocket Part Numbers
A Great Big Hint: For REAR sprockets, EX500 and EX250 have the same part numbers. So, if you can't find what you're looking for listed under EX250, try EX500. It will bolt right up. Not applicable to front sprockets or chains.
Note: For any of these part #'s, XX means the number of teeth you want.
Front: JTF 516.xx
Front: 562-xx Chart says 13 & 14T only, but if you click on one of those it also shows 15T. Best to call for 15T.
Call 800 290-9040 to order directly or find a part #. Their website hasn't been updated in over three years, and as of June, 2006, it hadn't been online in months.
Ok, so what's the price difference between the sprocket companies?
Here is a survey, done of four of the "Big Boy" parts houses. (Sprocket Specialists is direct-sale, so their prices are from their web site.) The price listed is the average price, before shipping. These places all had the same basic shipping rates. This comparison is to give you a general idea of what you're looking at before you start shopping. Prices will change, of course.
JT = $14
JT = $31
JT = $38