Big Bore Kits.

Well big bores make more power so that puts more stress on the bottom end. Not enough to cause any issues, but you may need stiffer clutch springs as you may wear out the clutch faster do to the increased torque. The biggest issue with bore bores if they went too big is the cylinder wall gets thin and gets out of round faster.

Not if you have an Athena Kit, I had one on my old 07 CRF250 mated to a full blown ported race spec head from Tom Morgan Racing, it even had a cam you could adjust or advance/retard timing with on the sprocket but I ended up putting a stage 2 Hotcam in there to compensate for the lack of top big bores can cause and let me tell you WHAT!!!!! That bike was stupid fast!! It had gobs of torque and power everywhere.It was the best of both worlds, very close to 450 power and torque (due to the reduced weight of the bike) this thing snapped forward and it obviously retained the handling of the 250f.

I used to own a 99 YZ400f and to this day I swear it was identical to that engine in power and torque. A lot of the power was due to the TMR head too because at the time I also had an 04 CRF250 as a 2nd bike I did the exact same big bore kit on it and used the 07 Head that I had leftover and this bike was great but nowhere near the built 07, that bike was insane but we parted ways after she pulled the infamous "Honda-Bog" on the face of a 60'+ peak to peak double at perris resulting in the worst crash of my life ........after that I went back to the 2 stroke and continued on with life happy.

Id say you better replace the bottom end with new if you're gonna bolt on a big bore. Otherwise the difference in compression might blow apart that oem rod that is used to taking stock punishment....

But if it's already on there, see how many hours it has. If it's a lot do bottom end. im not an engineer I don't know. But it seems logical to replace fresh oem bottom or some other bottom end that's rated for big bores.

I had a 07 kx250f with a hotrods crank with ~40 hours and snapped the rod with a 290 kit.

I was running an athena 290 kit, which had the worst engineering I have ever seen. They claimed 12.7:1 comp...after CCing everything it was actually 10.8:1... I had to mill the piss out of the cylinder and get a different piston to get it to 13.2:1... I made a thread on that build, but yeah...wont ever buy athena again. Their customer service is shit.

Stress? Who cares when you can hit the triple with a 125 from the inside......braaaaap

I've got a couple seasons on a 270 kit that was installed on my 2013 rmz250. It's a cylinder works kit with stage 1 hot cams. Perfect bike for an average vet rider like myself. The bottom end is OEM and that's all I go back with. A good rule of thumb that seems to work well is new top end rebuild ever 25 hrs and new bottom end rebuild every 75 hrs. Kinda pricey solution but seems to keep things together and rolling smooth.

I have used RPM's ( Rick Peterson's) Ice Cube 270 kits with great success. They are now sold through Max Power.
These are the best kits on the market.

Paw Paw

X2!