300ex top end on 250X bottom end?

[400thacker]

I know I have to use the 250X rocker cover for the compression realease but my real question is this...

I know that the bore is the same and the stroke or rod is shorter on the 250 but I have 2 engines that need top ends and I found a 300ex top end with head, piston, cam ect for $75 thats in good usable condition.
Is the cylinder height the same?
is the wrist pin on the same location on the 250x and 300ex?

what do I need to do to use a 300ex top end and can it be done?

I need to know what the exact differences are.
thanks in advance guys!

Dan

[smr]

it will bolt right on. No difference that will be noticed.

You also don't have to use the 250x rocker cover. You will still be able to start it just fine without the decompression.

[stroked 400ex]

i had a fresh 300ex cylinder and wiseco piston laying in my garage and it fit right on into my 250x no problems with fitting it... so youll have no problem...

[400thacker]

so everything will bolt right on...what would be the actual CC of the motor? It would have to have less compression if the stroke is different correct?

if the piston on the 250x bottom end does not go up as far (stroke) then it would create less combustion pressure in the head right?

should I use the 250 piston or the 300 piston?

[dman66]

Dude that's a great price for those parts,if there all in good shape.I've been contenplating buying a head and rockers,but prices seem steep for what is offered on ebay. Good luck.You don't need the decomp. connected!!!! My x has 12:1 wiesco piston and kicking is not a problem,just don't let your shin get in the way.The 250x and 300ex piston should be the same diamensions - the 300ex rod is longer I believe.

[witech]

You can use the jug and head but the crank piston and rod are not the same . The wrist pin is in a different position on the 300ex to compensate for the stroked crank . Thats how they kept the cyllinders the same. Just keep the rotating assemblies together and you will be ok.

[stroked 400ex]

cc's will not change unless you get a bigger bore... just because you use a stock bore 300ex cylinder it wont give you any more power the difference is in the stroke but you can put a high compression piston in, i have a 12:1 and it kicks over fine without the decompression

[smr]

use the 250x piston with the 250x bottom end.

[400thacker]

thanks guys thats what I needed to know. I appreciate the help.