This ^

But if BB has some doubts about the environmental conditions you're in I think i would err on the side of caution and go with the 340 or 331. There really isn't going to be a huge difference in the amount of power but like you and Bill said there might be a noticed difference in cooling capability in said climate conditions.

 

I went with the 300 because it's hot where I live in the summer, and I already struggle with the range of the stock tank. I would have loved to go 331 but the 300 was more practical for my situation.

 

If I do the big bore, which I think I'm going to do in the near future, I am going to keep the bike for a long time. Which one of the big bores do you think will provide the best longevity? Do I stick to the Kawasaki 300 kit if longevity is a concern?

 

If you are asking me:

I researched the Kustom Kraft with the high-tech nickasil lining. Then, I looked at the KK warranty of that lining...it's only 1 year. KK's Sleeves are warrantied for 5 years. Also, Refurbishing the nickasil is much more expensive than re-sleeving. Put all that info into a brain that reads between the lines, and I'd trust a Bill Blue 330 more than a Kawasaki 300. Heck, I'd trust a Bill Blue 351 more than a Kawasaki 300...something about Mr. Blue makes me feel very trusting, compared to the unknowns at the Kawasaki engineering department (ever hear of the Doohickey, or the cam timing chain automatic tensioner, or the KACR?).

 

Or the MILLIONS of bikes that Kawasaki has sold over the years with no problems?
Not sure I understand your train of thought.

 

Probably referring to the issue of a manufacturer not having a perfect record in regards to some design and component issues when comparing them to a good aftermarket component comparison. That may not be a fair comparison to some degree, however, as a manufacturer has to address untold numbers of variables and products in their lineup. That said, I think the KLX300 cylinder is a fairly well proven item, but I'd have no hesitation in using a Kaw 300 cylinder, a KK kit, or a BB kit. Currently I'm using a Kaw 300 cylinder because the price and availability worked out best at the time. I don't think I've heard of any consistent problem with any of these kits, and certainly none of them have turned out to be duds. You probably can't go wrong any way you go.

 

I'm just saying I'll take a proven independent craftsman's diligent work over a giant corporation's mass marketed work (which is driven solely by profit).

When you get right down to it, getting the cylinder, piston, and rings to play nicely together on this small motorcycle engine isn't exactly cutting edge rocket surgery.

If I were to flip a coin, any choice would work fine. If I can instead choose, I'll take Mr. Blue's work every time....he has more skin in the game.

 

There are many answers to this question I guess, I found some things during my research.
Bill Blue has the biggest, baddest at 351cc, and by all accounts it seems to be the dogs nuts. Some good reviews, but just as many, if not more, unanswered questions. Longevity? Heat? Vibration? I'm sorely tempted myself, and as I live in Japan, with the greenback virtually worthless it wouldn't be so expensive for me.
However, I can't find any big bore kit in Japan bigger than 315cc, which raised some questions, main one being .... why not?
Immediately available for me are:
292cc (klx 300) bolt on, no need to elaborate
315cc Beet Japan, new cylinder, piston, gaskets, expensive.

I did stumble across a great thread on here, written by someone who did the 300cc upgrade, then bigger. The difference? 250 to 300, good. 300 up apparently not so much, until the head was reworked to supply sufficient air.
That makes sense, maybe thats why the Japanese market doesn't bother with bigger than 315?

So I'm considering firstly 300cc, then this week I found this:
http://thumbnail.image.rakuten.co.jp...29/2111236.jpg

292cc sleeve, Wiseco piston, rings, gasket set. Cost 21000yen
OK as of today, thats $260, and need to get the cylinder bored out, but it does offer reasonable cost for big bore.
So I'd have the same question as the OP, just how far should you go before the extra cost becomes meaningless?

 

I think the big gain is in going to at least a 300 (292) kit. I speculate that the head and other components make the intended match for the 300 piston, because it appears that the '06 and later KLX250's are just downsized 300's with everything being basically 300 components. Still, more displacement seldom goes unrewarded. Even if peak horsepower were to be limited by cams and valve size to some degree, torque should increase notably and lower in the power band. My fully farkled 250 ran a ton better than a bone stocker, but the 300 jug made a surprising difference for a measley 43cc increase. That's why I think there's a better head and cam match to the 300 as compared to the 250 piston...and I checked compression on my 250 before I tore it down for the 300 jug, and it was at max compression by the manual. The OEM 250 head pipe is probably restrictive, but mine already had a Muzzy header on it. I think the most radical thing I'd do to a big bore kitted KLX is install those 1st level Web cams that were recommended by Rossler. I think the stock head probably flows plenty well for any sane use of this bike, but the milder Web cams probably perk up the extra displacement to a noticeable degree.

 

I had the 331 kit and ran a east coast slow enduro in 90f temps with no signs of over heating. I also had fcr pumper with a fourstroke works header and pipe. In general the bike ran nice and cool, not like my stupid ktm 450.
I would get the 351- and have a ball.