300ex crank and top end on 250x?

[ZSNOW]

well im looking into buying a 250x because i want the kickstart bike. well if i end up going to 250x route for racing next year. can i put a 300ex crank and top end on it? and will it be exactley the same as the 300ex power and performance wise(but it will be a sparks motor)

[ZSNOW]

come onnn. someone must have some knowledge on this.:confused: :ermm:

[maddmatt02]

I asked this once and was told no

[ZSNOW]

but the motors are identical. all thats different is the rod. the stroke is longer on the 300ex which means i just need to do a whole 300ex topend.:(

im confusedd lol can someone come in and explain this?

[maddmatt02]

I asked this before like I said and I stole this quote from 2muchquad -

"The 250x crank is NOT compatiple with the 300ex crank.The 300ex crank is longer for the starter gear behind the stator."

[ZSNOW]

so what about the 300ex head and cylinder on a 250x and a long rod? im just trying to get around the 250cc motor because it is not as fast as the 300 motor from what i hear?

[maddmatt02]

if you put a 300 top end on a 250 bottom end, your gonna have a 250 still, same bore between the two, its the stroke that got the extra cc's.

if you wanna stay kick and have a larger displacement, get a 330 kit(80mm), which will be a 288 on the 250x, which is 6cc larger than a 300(282)

or a stroker will bring it up to 299 on stock bore.

combine the two and you can get a 349

[Y2daT420]

im going to try the same lol........my 250x is locked up......i have a 300ex topend jug and cylinder but if a 300ex crank wont fit the 250x tranny ill prob need a whole 300 tranny that would save a lot of time my plan was to make it kick and electric start ..cuz im tired of kicking the %^$thing ....i guess i could use a complete 300 bottom end (tranny ) and put my kicker on it.......i havent tore it down yet but am assuming the 250 flywheel dosn't have the starter gears on it



or maby i should call in a air strike and start over....lol

[maddmatt02]

also, making the 300 a kicker isnt going to be a bolt on deal. Im pretty sure I remember that the 250x case, the oiling is different, as well as other problems.

if you search on here youll come up with a lot, I wanted to do it to mine before i found out I couldnt.

[dork]

300ex crank has larger diameter main journals and bearings.

[bwamos]

Originally posted by ZSNOW
so what about the 300ex head and cylinder on a 250x and a long rod? im just trying to get around the 250cc motor because it is not as fast as the 300 motor from what i hear?

Old post but, I'll reply anyway. ;)

The 250x stock is generally faster than the 300ex stock motor for many reasons. Better power curve, higher compression, and higher rev limit, lighter, etc...

The cylinder and head are virtually the same. Pretty much a direct swap and will make no difference.

If you want a faster 300... get an big bore sleeve and an 80mm piston. Port and polish the head and get oversized intake and exhaust valves. Leave the stroke nice and shallow.