300ex crank in 250x?

[300exazzkikr]

Is it possible to put a 300ex crank and topend in a 250x bottom end? I know the 300's rod is longer than the 250's but are the cranks interchangeable.? It would be easier to exchange a crank than split a crank. Thanks.

[300exazzkikr]

:huh

[250xman]

well im not quite sure what your asking but yes you can put a 300 crank in a 250x bottom end. top ends are identical except on 87's they had a hotter cam and more compression.

[kicken250x]

Thats a good idea...I never thought of that... but yea they will switch out I went and checked a few reliable sources and they yea that it would. That is the only way to get a 250 to be a true 330 without getting it stroked. Later...

[300exazzkikr]

250xman-
the cylinder on a 300ex is taller than that of a 250x. Thats how it's a 282cc instead of a 246cc. It's got more stroke. Thanks guys.

[300exazzkikr]

the 250x and the 300ex have teh same bore.

[danney f]

OK: a 250x and 300ex have the exact same cylinder, the bore is the exact same size, the head is exactly the same, the head cover on the 250x has a decompression release and the 300ex does not. The crank is differant, the 300ex has a longer stroke than the 250x. As far as putting a 300ex crank in a 250x, not sure on that one. I have been told that it will work and I have been told that it will not, something about the 300ex crank is bigger on the flywheel end. Never have tried it. But I do know for a fact that the cylinder and head can be interchanged between the two.

[300exazzkikr]

Ok if the 300ex and the 250x have the same exact head and cylinder except for the decompression. That would have to make the cranks different because it has a longer stroke besides teh rod being longer. make sense?

[danney f]

The 250x has a bore of 74.0mm with a stroke of 57.3mm and the 300ex has the same bore but has a stroke of 65.5. I just looked in my book and it says the cranks are differant. Also a stock 250x has a compression ratio of 9.4:1 and a 300ex has 8.9:1

[250xman]

yea what he said but the rod isnt longer, its shorter to compensate for the higher lifting of the crank. i know for a fact that a 300 crank WILL fit into a 250x because iput one in my freinds 91 250x last year.

[Moded86R]

Acctaully I would think the pistons would be different.Since they have the same top end.But the stroke on the 300 is longer.They would riase the wrist pin how ever much to compensate for the longer stroke.Do you get what I am saying?

[300exazzkikr]

i understand..Maybe i'll just have to rip the two motors apart and find out...hmmm..:devil

[250xman]

if you order a piston for a 300 and a piston for a 250x from weisco the part #'s are identical.

[300exazzkikr]

wow..ok..i guess we'll figure this out eventually.

[outlaw319]

The crank will work in the 250! You'll just need to order a 300 EX piston though! the 300 and 250 are the same motor the only difference is the stroke! They both have the same bore of 74 mm, but the stroked on the 250 is 57.3, and the 300 is 65.5. ( As I think it was Danney f who correctly put this)! Even though they have the same bore, the pistons are different! The pin location and I think the skirting is different too, to compensate for the stroke!
Thus if you do this mod, you're still building a 282cc (300EX) that is kickstarted instead of electric!
Hope this helps!:)

[300exazzkikr]

The pistons are the same. I also looked up the part numbers like 250xman did..and it's the same piston..This is mind boggling.

[quadfamily]

This reminds me of an age old question since we're on the subject of 250X and 300EX engine parts swaping. Does enyone know FOR SURE (you have actually done it yourself) whether or not you can put a kick starter from a 250X onto a 300EX so you can have electric start AND kick start backup?

[danney f]

I have "SEEN" one on a 300ex, I have never done one though. He said the side case had to be machined and he just used all the parts from a 250x.

[outlaw319]

The pistons are different! I have a Wiseco catalog right here! the part # for the 300 is (4574mo7400). the 250X part # is (4440mo7400)! you might have been looking at the part # for rings, they're the same.
Not to offend anyone!
As for putting a kick start on the 300, it can be done!
You need to get the kick start assembly from an old X, and have your cases machined out to fit! If you look on your 300's case it has the indention for the kickstart shaft!

[300exazzkikr]

WOW..Thanks..i went back and looked and i was looking at the rings. the pistons are different. Alright to figure this out....everything is the same except for the piston? So a 250x owner could put a 300 piston in theres and get the same displacement?

[250xman]

no see the only way to get more cc's is to bore it or stroke it. i guess i was looking at the ring #'s too, but the 300's piston is slightly shorter than a 250x's to help compensate along with the rod for the longer stroke. and ive too have seen a 300 with a kicker on it.

[outlaw319]

250xman is right to get your cc's you have to figure your bore and stroke! I have a little simple formula to help you figure it out!
I'll use a 300EX for example!
Here it is:
(bore x bore xstroke x .0007854 = cc displacement)
300EX
74 x 74 x 65.5 x .0007854 = 281.7057 (round the 7 up)=282cc's
Hope this helps!

[Moded86R]

I am not sure of the pistons are shorter or if just the wrist pin is slightly higher in the 300's.If you raise the wrist pin your lowering the point of tdc.Since the 300 has a longer stroke it acctually equals it out.