Big Bore kits

[dirtkid651]

When I send my bike in for a rebuild I am thinking about having them install a big bore kit. I've been looking at the Athena 290 kits but I'm not sure what all I would need. Obviously new valves and springs, since I am rebuilding, what else? Any pro's and con's would be appreciated, thanks to all! ?

[WFOracer]

I am in the same boat as you. Although, I don't think I would replace my valves unless they are out of spec. I had a chance to ride an '03 YZ250f with an Athena 290 kit.....it was a blast. The amount of power it adds makes the bike feel lighter.

[250Thumpher]

A rebuild doesn't always nesecarrily mean new valves and springs. Are they shimmed? Are they too worn out for shimming?

On my rebuild, I also did the timing chain, and oil pump, as well as a new crank, piston, rings, wrist pin, and gaskets. My valves were in spec so I left them.

-Philly

[tarry]

A rebuild doesn't always nesecarrily mean new valves and springs. Are they shimmed? Are they too worn out for shimming?

On my rebuild, I also did the timing chain, and oil pump, as well as a new crank, piston, rings, wrist pin, and gaskets. My valves were in spec so I left them.

-Philly

This is my son's bike,which we bought used.We have had them adjusted once since buying,but are currently on the second cross country race, I think it is four more until the summer break.I thought that it would be better to get it all done at one time,than risk having to go back in at a bad time. I have never torn into a engine before but I am not completely incompetent either.Should I really think about doing it myself or have it done and not risk screwing it up? ?

[WFOracer]

This is my son's bike,which we bought used.We have had them adjusted once since buying,but are currently on the second cross country race, I think it is four more until the summer break.I thought that it would be better to get it all done at one time,than risk having to go back in at a bad time. I have never torn into a engine before but I am not completely incompetent either.Should I really think about doing it myself or have it done and not risk screwing it up? ?

Tarry, as long as you have your Yamaha manual it is a piece of cake. Just make sure you have all the tools and stuff. There are also some good TT threads for this.

[tarry]

Tarry, as long as you have your Yamaha manual it is a piece of cake. Just make sure you have all the tools and stuff. There are also some good TT threads for this.

I have not searched all the threads yet,but will before making my final decision.I am not to concerned about the top end as much as the crank,I have been in the top end of a two stroke,although that was 20 years ago.By the way,I always have my manual handy,the gray matter gets a little soft sometimes! ? Thanks for your input!

[Doug.]

It depends on the year of the bike, but I dont see why you will need new valves unless you have shimmed two or three times before. The stock Yamaha titanium valves will last a very long time if you take care of your bike (hell, they'll even last a long time if you treat it like crap!). You might want to invest in stiffer valve and clutch springs to handle the extra horsepower...but that's only if you're boring it out far.

290cc is a good size, but at that bore the bigger piston really slows the revs down and takes away a good amount of top-end. Makes a crapload of torque, though. Plus, I have heard some not-so-good things about Athena's service. If you have something you don't like about the cylinder/piston kit...don't expect to get any help from them. I'd reccomend a 272cc kit...from LA Sleeve. Everyone who has tested it seems very happy with the quality and the fact that it doesn't slow down revs as much as a bigger bore job does. You still get lots of torque, and you get to keep some top-end.

[Doug.]

On my rebuild, I also did the timing chain, and oil pump, as well as a new crank

You rebuilt the crank??? Why? How much did it cost?

[Ambluemax]

I have a 262 wiseco kit and love it!!! I like it b/c there is a butt load of extra power. However you can't feel it just riding around, but if you ride one back to back with a 250 I notice that the bike seems to have much more reach to get over obsticles. For example those trick doubls out of corners, jumps I could barely clear on my 250, were no sweat on the 262. They feel like a perfectly tuned works 250 with a much better bottom end.

A couple other good tings about the 262: one, its cheaper b/c you can cut the cyl you already have, and two I've heard they are more reliable b/c they aren't as hard on cranks.

[ADMIRAL]

Are there any big bore kits where you dont lose any top end but still have a good gain in power?

[Ambluemax]

262, you get more bottom with basically the same top end.

The guy I bought mine from described it as a 250 pinger bottom end with a 250F top end power band. I've never ridden a 250 pinger, so I have to take his word for it.

[av]

262, you get more bottom with basically the same top end.

The guy I bought mine from described it as a 250 pinger bottom end with a 250F top end power band. I've never ridden a 250 pinger, so I have to take his word for it.

Pingers in general don't have much bottom (unless it's a big bore like a 500cc monster), they tend to hit in the mid to wot - then you hang on for the ride... ?

[Ambluemax]

Like I said, I wouldn't know. I do know he said that and he also told me awhile back that if you use the bottom and mid range in the powerband, the 262 was awsome and if you are the type who bangs it off the rev limiter all the time you shouldn't bother.

Bazooka Joe, help me out here....

[BigIvyI]

I have the 262, but feel like I still need to really rev it hard to go fast in the woods. I want to try the 290 on my next bike.

I'm not yet convinced that the 290 is harder on cranks. It has a heavier piston, but you won't be reving it as much on average, so it's hard to say. It will definately make your valve-train last longer though.

I rode a KTM 400 a while back and that thing had the sweetest woods power I've ever felt. So much smoother and easier to ride than the other 450's I've been on, yet still more than enough power to handle anything. Only problem is the bike, while surprisingly light feeling, is not as light as a 250f. I'd like to try to get my next yz250f's motor as close to that one as possible.

[tarry]

It depends on the year of the bike, but I dont see why you will need new valves unless you have shimmed two or three times before. The stock Yamaha titanium valves will last a very long time if you take care of your bike (hell, they'll even last a long time if you treat it like crap!). You might want to invest in stiffer valve and clutch springs to handle the extra horsepower...but that's only if you're boring it out far.

We bought the bike used, not sure if the valves had been shimmed or not,the bike was in very good condition though.We have adjusted them once and are racing cross-country,40 to 50 miles a race, not counting practice, so the hours will add up fast. It is a '04, and I would hate for my son's bike to disintegrate under him during a race.

[sndmn2]

My opinion and experiances..

I have the Athena 290 in my 02 wr and a 262 in my sons 02 yzf. My experiance has been that the 262 finds its best performance mid to top. A little sleepy on the bottom.The 290 has more grunt but signs off early. If your track rideing and are at the top of the revs go with something like the 262. For trail rideing the 290 is a better way to go. I am very seldom on the rev limiter while rideing single track through the woods. My sons 262 will run very hot in tight single track work. So much that I"m realy considering going back to stock bore. I"m talking when rideing under 8-10 miles an hour. Others have said that thiers never overheat but his does run hot unless it gets plenty of air through the radiators.

[Doug.]

We bought the bike used, not sure if the valves had been shimmed or not,the bike was in very good condition though.We have adjusted them once and are racing cross-country,40 to 50 miles a race, not counting practice, so the hours will add up fast. It is a '04, and I would hate for my son's bike to disintegrate under him during a race.

I have an '04 too...I race motocross and some desert grand prix events. I havn't needed to adjust my valves yet. I ride twice a week and race about once a month. Of course, if you bang your bike off of the rev limiter often...you will wear out your valvetrain much quicker than the person who doesn't. It all depends on how you ride, but to be honest, I wouldn't worry about new valves in your case if you've only adjusted them once and they're good. Just check them periodically to avoid disaster.

[oldnbold]

does anyone know the particulars about the LA sleeve big bore? My concern is whether the walls are thick enough to keep it's shape. Eric Gorr was saying a while back that if the walls were too thin, the cylinder couldn't hold it's shape and got out of round resulting a a huge loss of power in a very short amount of time.

[grh450]

I have the 290 kit on my WR and it has been entirely trouble free. Added a YZ pipe and could not be happier. Just bolt on ,and rip

glenn

[Flynall]

GRH,

I have a Thumper Racing 280 set up in my YZ. It also has WR gearing in it. I like it very much. I am thinking about splurging and doing the full 315 bore and stroke from RPMs next......

What sprockets are you running front and back?

Thanks,

RH