250x cylinder vs 300ex

[schaefer]

i am putting together 250x chasis with what will be a 300ex motor.... anyway i can get a 250x cylinder for next to nothing and want to know if i can use it with the longer stroke of the 300ex bottom end. i was also got a 250x 12:1 piston with the chasis and parts. alltogether my question is 300ex botom end, 300ex rod, 250x topend; cylinder, head, cam, wiseco 12:1 piston. any help appreciated.

[enduro400rider]

all 250x and 300ex parts are compatible, they have the same motor just with a little longer stroke, the heads are the same i beleive so you could use a 250x jug

[schaefer]

i know they are compatible. i need to know if the 300ex cylinder is any taller.

[dork]

same height. the pistons have the same compression height but the stock 300ex piston has a slight dish for lower compression.

[schaefer]

thanks.

[250X_project]

Originally posted by enduro400rider
all 250x and 300ex parts are compatible, they have the same motor just with a little longer stroke, the heads are the same i beleive so you could use a 250x jug

NOT trying to flame, but want to set things straight for future readers, as I have found myself reading post years old. Not all parts are compatible. The top end should be completely the same besides the piston and cam, though both can be interchanged. Also the bottom end has some differences and can't all be interchanged. The cranks, cases and some misc. parts can't be switched.

But, yes your set-up will be fine. It is actually upgraded. You will be above the 12:1 compression and the 250X cam is a tiny bit more aggressive than the 300EX.

[schaefer]

i will be having the 12:1 milled down to lower the compression, yes, because of the differing wrist pin and my wife won't need the power, and the high $octane requirement.. all replies appreciated. thanks for the help.

[250X_project]

Yeah I've seen people do that. Premium gas coast enough, w/o needing race fuel. Good luck.

[Lonestar_R]

I have a couple questions about this subject. If I am wrong in saying that the 300EX and the 250X have the same bore. Correct? Just a different stroke, correct? Also What would my compression be if I use a stock 250X piston and a 250X cylinder?

[250X_project]

Yes, both are 74mm bores. The 300EX just has a longer stroke to the crank.

With a stock compression 250X piston I would guess you would have about 10.6:1 compression depending on your cam.

Also I calculate the 12:1 250X piston as about 13.6:1 in a 300EX.