250 Gearing / Sprocket Answers & Information
250 replacement Rear Sprockets are available as part number JTR802-XX
The XX is the numer of Teeth. They are available as: 32, 34, 36, 37, 38, 39, 40, 42, 43, 44, 45, 46, 47, 48, 49, 50
They are for a 428 Chain; The current Jetmoto factury chain will fit (but you need extra links to make longer)
There are two sets of 4 holes, properly spaced. The small set are 8.5 mm and will fit the current allen head factory screws (NOT RECOMMENDED)
Tale the new sprocket to a machine shop and have them drill out the smaller holes with 3/8" bit. (VERY HARD material - you MUST have a drill press and MACHINIST grade drill bits)
Take the sprocket hub with you and have them drill it 21/64 ths and tap for 3/8ths National Fine (24 threads per inch)
Purchase (4) 3/8 ths x 1-1/2" Fine Thread GRADE 8 Bolts, (8) 3/8 ths Fine Thread GRADE 8 Nuts, and (4) 3/8 ths Stainless Steel Washers (Thin, to use as shims under bolt head - if neccessary, depending on gear selected, thickness, and length of shoulder at bolt head)
Grind off about 1/16" off bolt head to ensure clearance (depends upon exactly how far your bearings are set into the carrier from the factory)
IF gear is thinner than shoulder on bolts, you must use a thin washer UNDER bolt head to ensure tight fit against sprocket hub - if gear is as thick as bolt shoulder then dont use the washer.
Thread through new gear into sprockt hub and tighten 35 to 40 foot pounds.
Install first set of nuts on outer side of sprocket hub and thighten to 25 to 30 foot pounds. Install second set of nuts (hold inner nut with an open end wrench) and tighten to 25 to 30 foot pounds.
You will NEVER have the sprocket bolts EVER back out.
The JT Gears are available thru a search on Amazon, or MAW (Motorcycle Accessory World). I think that shipping might be cheaoer thru Amazon.
Cost is about $18 plus about $12 S&H.
I would recommend a 46 Tooth for a rider under 150 pounds, a 48 Tooth for a rider 150 to 200 pounds, and a 50 Tooth for over 200 pounds.
Since you need a longer chain anyhow, I would recomment a DID 428V O-Ring chain with 130 links (you cut to suit) This chain is rated at something like 4300#'s and is an O Ring design that keeps the dirt out and is suppost to last up to 18 times longer than stock. (the master link comes with the chain) It is about $60 plus $15 to $20 S&H at the same places as above.
The 46 tooth sprocket requires no modification to the skid guard. The 50 tooth might require a slight amount of bending and / or a few washers between the skid plate and the trailing arm on the left side.
The JT Sprocket web site is w.jtsprockets.c
The JTR 802 fits a Kawasaki G3, G4, G5, USA 1970 to 1975
I will do another post with front sprocket information.
I will see if I can get some pictures of both the 46 and 50 tooth sprocket installed as above (depending upon frustration level of me with camera and figuring out how to post them)
I hope that this will be helpful to some - It makes a HUGE difference in the TOTAL way the ATV operates.

 

Well I for one appreciate your hard work. Thanks!

BTW, I'm running 22" tires on the back of my son's 250 Sport. Going by his weight, he weighs 135#, you say the 46 tooth would be the one to get. Would the larger diameter rear tires affect which sprocket I should get?

 

Just like the kids with the big swamper tires on their pickup trucks - why did I burn out my clutch / trans etc ???
You would need a 50 tooth rear gear just to stay even with the 46 tooth, if you went from 20" to 22" diameter tires.
Doing the math: 20" X 3.14 = 62.8" Circumference / 22" X 3.14 = 69.1" Circumference
69.1 / 62.8 = 1.10 (a 10% increase in the circumferance)
46 tooth rear gear X 1.10 (to compensate for the rear tire increased circumference) = 50.6 Teeth needed.
Answer: go for the 50 Tooth rear sprocket = JTR 80250
Hope this helps.

 

great info, thanks. how did you track these down, by luck or hours of searching?
heres a link for everyone else..
http://www.jtsprockets.com/52....l?&L=0&sel_uid=4551&p=

 

You don't need to do the DIAMETER*PI for the calculation. Just use diameter because if you multiply*Pi on both sides of an equation, it's the same percentage as not doing it at all.

So if you go from 20" rears to 25" rears, it's a 25% increase (either way) and if you like a 14/40 setup, then a 14/50 will compensate for rollout, but not any weight added because of bigger tires.

If you are looking for more climbing power, keeping the 20" with a 50 tooth will probably let us rev out 4th gear. What have you found with a 250ish lb rider?

Thanks for the top quality info man! I heard about this, and I'm glad it got posted!! [img]i/expressions/face-icon-small-smile.gif[/img]

I wish we could get a line on some 3/8", grade 8 flat head bolts so we could countersink the sprockets.

-JNY

 

Yes thanks for the information, I'm sticking put with my gearing for now but this will come in handy for a lot of people I'm sure!!

 

Welcome to the forum NJDale! [img]i/expressions/face-icon-small-smile.gif[/img]

Thanks for the sprocket info.

 

Glad to be of help! No, the info did not come easy. Three or four weekends ago, I spent 15 to 20 hours looking around on line. First I found all the info for the gears at a gear manufacturer called "AFAM", but then could not find any retail distribution on them - they turned out to be owned by a Dutch company. So, then I started all over again and came up with JT Sprockets.
The front sprocket is a JTR 259 XX. I did a whole complete posting last night about it & hit the post button and the whole thing went to electronic dust heaven - server error! I won't have time tonight to redo it, but tomorrow night (Thursday) I will repost it.

 

I went and bought one of the 50 tooth JT sprockets for my 250 Sport. It looks to be a high quality sprocket. I drilled out the smaller holes to 3/8" and so far have it and the hub back on the axle.

One thing I'm not too happy with is the hole for the hub is bigger diameter than the hub itself. I don't think it will be an issue, as I don't think the 250 has enough grunt to shear off 4 grade 8 3/8" bolts. I suppose you could have a machine shop make you a spacer ring if it becomes necessary. I'll keep an eye on it for awhile though. If your running the 14 tooth front sprocket you will be about 7-8 links short on the chain. The sprocket is pretty close to the swingarm toward the front end, again shouldn't be a problem though. This also might solve some of the chain slap on the swingarm because of the larger diameter sprocket. You will have to stack some flat washers between the swingarm and the sprocket/brake rotor guard to gain clearance.

I'm going to get the new chain this afternoon and put it on tonite or tomorrow, and try this setup out.[img]i/expressions/face-icon-small-tongue.gif[/img] I'll keep you all posted on the results.

 

00XJ - Glad to hear of your progress. Yes, I forgot to mention about the 52 MM center "Borehole" verses the 44 MM hub center flange. I too don't think that it will be a problem - IF, you use the 3/8" grade 8 bolts. I see more of a potential problem with the disk brake rotor hub that the chinees are trying to use as a sprocket hub. All the Japanese quad manufacturers (and aftermarket "premium" axle manufacturers) use a sprocket hub that is "pressed" onto the axle. The reason being that it eliminates the clearance "play" nescessary for a slide on sprocket. Over time, I have fears that the spline "play" will lead to sprocket failure. In addition, the use of a dick brake rotor hub as a sprocket hub leads to too small a mounting circle for the bolts to spread the torque to. In general - many potential future problems due to poor design decisions.
I think that the most probable longer term solutin will turn out to be getting used Honda 250R rear axles with gear sprockets, disk brake rotor hubs and outer wheel hubs (complete axle assembly - but without swing arm or bearing hub) off ebay. Should be about $35 to $100. However you must get a complete assembly with wheel hubs (different years have different spline counts). This looks like it would be the correct and low cost answer - but will require more research and possible test purchase and test fitting to be sure. Some time this winter (if no one before me), I will try this out and let everyone know what the right replacement answer is. Until then, I think that the JT Sprocket will be good enough - unless you are an extreme rider, jumper - in which case you will probably break something because at this price point this quad was not meant to do this.
ASSEMBLY TIP: If you are putting a new master link on a new "O ring" type chain - get two square type stove bolt nuts (probably for about 1/8" bolts) and a 1-1/2" to 2" small "C" clamp. Assemble new master link with the 4 "O rings". Slip the second side liink over the 2 pins. You should be ready to put the little clip on now - but it won't go, because to new "O rings" are too fat. Put the 2 little square nuts over the post ends and use the small clamp to compress the second side link into place. Leave it clamped that way and take a break. Come back later and have the little slider clip and needle nose pliers, ready to go. Release the clamp, take the 2 nuts off, and slip the slider on. (NO problem).