I kinda know my way around an engine, but I make no claims to be an expert.
If I were to bolt on a 290cc big bore kit on my yz250f what other parts should I upgrade to maintain durability and reliability? Big bore kits are the easiest way to get more power without spending a fortune but I don't want to detonate a motor cause I neglected to upgrade certain parts that would cope with the added stress that more displacement would put on the motor. I just want to bring the ol' 07 250 back to life with some added firepower. Lord knows she needs it!
Keep in mind everything you do to the engine performance wise, needs to either be dyno'd or you need a O2 sensor welded into the header pipe for the bike to reprogram itself (oh and youll need the programmer of course). If you dont do that your pretty much wasting your money as you wont reach the full potential of the aftermarket addons. I never realized this untill I went and did my motor.
Just like a 2t, but those where tons easier and less expensive, youd just jet up.
Larger piston will have more mass which is going to equate to larger forces on the crank, rod, bearings, etc... Not sure that means you need to get an aftermarket crank or not though but it wouldn't hurt to look at suggestions. For reliability, that's the first thing that comes to mind.
Mason you damn idiot... It's an 07. What is going to need reprogramming again?
Yes, do the crank too. Get something balanced from Falicon or somewhere else reputable. Adding the extra weight to the top of that poor old crank will for sure grenade your motor. You could probably get away with just freshening up what you've got in the top end though. High compression pistons kill your top end, but don't think adding a high compression piston to a 250 will make it a 450 down low. It really just kills the top end. So stay away from those for a small bike.
Yeah, I will most likely just end up doing a standard top end rebuild. The big bore kit sounds like fun, but the more I think about it the more it seems like a pain in the ass to do it correctly.
I saw a really good price on the big bore kit though, almost got me.
Def get a crank, trust me, ive killed a few of them in my 07 in stock form.
Not a fan of big bore kits either way. I put stage 1 hot cams, wiseco piston (stock compression) and new valves in last time i rebuilt mine, seemed to give it a little more bite.
A friend of mine bought an Athena 290cc kit back in the day to his '09 yz250f, he killed it twice before he realized that it wasnt a good idea.
Just like ddmx said, a larger piston means more mass and will eventually result in a rough running motor. You would need to get atleast a weighted crank to match the piston and a stronger rod to make sure everything is strong enough.
However, sometimes even a forged piston causes a rougher running engine. You'd be impressed what just minor changes can do to a motor. I highly suggest you to keep your fingers off these kits.
If i were you i'd rather keep it stock or spend the money to get my cylinder head done. Or just buy a new yz250f if you wanna stay on a yamaha, that thing is mint!
