say, does anyone know if the 300 fourtrax crankshaft will fit in a stock 250sx motor without case mods?
i don't wanna buy another motor and i already have the crankshaft. also, were the gear ratios any different in the 300 motor if i do need to buy one? i wanna keep the gear ratios i have unless the other ratios were higher. plus, the 250sx cylinder and head bolt on to the 300 motor from what i'm told. is this true? i don't wanna have to buy any major parts to do this, i am on a limited budget. another guestion quick, can i bore out my 250sx cylinder .030 and put a .030 inch bigger piston from the 300 motor in my cylinder? i wanna bore it a little to give it a little more displacement than a stock 300ex.please respond, i'd appreciate any and all help. thanx guys.

 

Crankshafts will not interchange. Only took one time to find this out and it was long ago. The cases on the 300 engine have the clearance for the crank and the 250 cases do not.
You can't simply swap cases because every shaft and gear in the 300 tranny is different and a bearing swap doesn't allow it either. Also, the flywheel and stator from the 300 engine has to be used on a 300 crankshaft. 300 models are different in not only size, but will not match with the 250 stators.
So...if anything, you would want to swap the entire bottom end as one piece to get a stroker engine.

As for gearing, I don't have the stats with me, so if someone has them and I am wrong, please correct me....but if memory serves me correctly, 1st gear was the same or very close and 2nd,3rd,4th, & 5th gears are way lower than the 250SX and are even lower than the 250ES/TRX250 engine.

With all this, I wouldn't fool with doing this swap into an SX unless you intend to use it for BigRed purposes. 300 engine swaps are usually done to BigReds and TRX250 models.

The only difference in the topends between the 250SX, 250ES/TRX250, and 300 motors are in camshaft profiles. All 3 are different.

Ofcourse the exception is the piston. The 300 piston has a different piston pin location for the longer stroke. Cylinder bore size is the same.

As I said before, I could be wrong on the gear ratio thing, so dig deeper into it if you are still seriously considering this swap.
You wouldnt want a slower 250SX than you had before modding it.

Good Luck

 

knowsalot, thanx for the help. i wanted to make sure that the 300 cases, cranks and all other parts were the same before i went out and bought another motor. besides, i found a much easier route to make my bike a 300. powroll has a 300 big bore kit for the 250sx/es three wheeler. it retails for $150, but the are having a sale on them for $25. it requires the cylinder to be bored out, but other than that it just drops in. i need another cylinder though due to the fact i want a spare in case anything happens. does anyone have a spare cylinder for a 250sx they would be willing to part with? if so, e-mail me at Smd987@Hotmail.com. thanx.