I've had a few free days to be doing some reading, researching and poking around. Looks like the KLX300R and the KLX250S are blood brothers. They both share the same stroke, but have different sized bores. The KLX300R has a 78 mm bore, the KLX250S is bored to 72 mm.

Without having the parts in front of me to measure and compare, this next commentary is strictly an assumption.

Is the bottom end - crankshaft, rod and such pretty much the same between the two bikes? If that is the case, and Stroker has their 331 kit, which requires an 83 mm bore, can the cylinder in the KLX250S handle an 11 mm overbore? That sounds like way too much, but I've been surprised in the past.

From what I've read, the 331 kit makes the KLX300R into one screaming beast with reports of 42 horsepower. That would make the KLX250S more roadworthy! Big time!

Not in the plans for the immediate future, but who knows what will happen a few years down the road?

Tony and Todd, I would imagine you guys know the answer to this.

 

I don't see a need for a larger bore at this time, my bike screems just fine. As for the overbore, I though the bikes have an a electro-fusion coated aluminun cylinder. I assume that the cylinder is harder on the inside of the bore, so if you bore it too much you will loose some of that hardness. More learning to do.

 

I think I just figgured it out. Marty you are MBD Aflicted on the Thumper Talk site. Right? Come on don't leave me hanging. LOL

Deric (deej)

 

Guys:
it's a "plated bore" (nikasil) you do not bore these out. If you mess one up you buy a new cyclinder assembly.
The over bore hit is a new cyclinder w/piston & rings.

 

http://www.thumper-racing.net/KLX250300.html offers details on the max bore size for the 250 and 300. They sell kits for various size upgrades.

They will bore and re-plate your cylinder or sell you a new one.

 

So, for the price of the KLX250S and the kit from Thumper Racing, you are getting a street legal KLX300R. The "285" kit makes the KLX250S the same displacement as the KLX300R, 292 cc.

Now I'm wondering if we should be watching for KLX300R motors on EBay? Might they just 'bolt-in' to our existing frames?

 

I just bought my KLX250 and got a free 300cc big bore kit with it

 

ok the kit consists of
1 x barrel p/n 11005-1983
1 x gudgon pin p/n 13002-1109
1 x set piston rings p/n 13008-1071
2 x gudgon pin clips p/n 90033-1054
1 x Head gaskit p/n 11004-1314
1 x cylinder gaskit p/n 11060-1740
1 x Piston p/n 13001-1463

wow they threw in a bike cover too .... ooohhh its like christmas lol
I'm off to pick the bike up now, I think I'm irish or something, I went and picked up the kit first
hope this is of some help Marty