84 xc 250 cylinder crank swap

can a 84 xc250 be made into a larger displacement motor by swapping parts like the 74 250 into a 400 using the 250 bottem end

The 82-up 250 cases are essentially identical to the 430/500 cases on the outside, and in the tranny and clutch compartments, so most of that stuff interchanges. The problem is in the crank bowl... the 430/500 have a larger crank bowl cast into the cases and you can't put the 430 or 500 crank into the 250 cases. So to make a 430 out of a 250 you'll need 430 cases, crank, cylinder, piston, head. Pretty much everything else can be used to complete the 430: 250 ignition, trans, clutch. You'd need a 430 exhaust, too.

Vintage Mike- I have a set of 430 cases plus clutch, gears etc. if you're interested. Might also have top end.

you can grind out the reinforcing spider webing on the inside of the 250 center cases so it will clear the larger crank but why bother when you caould easily get 430 centers.

Joe

thanks for the info

i have been checking out price and availability of parts and i think i will do some experimenting,i am friends with eric gorr,of eric gorrs forward motion.this winter after i am done riding i am going to send him my 250 motor and let him bore,stroke,and port it,i will give complete details of size and horse power we come up with and what parts and how much it would cost,it looks like we can make a 360 out of it

I was playing with the idea of putting a 83 250cr top-end on a 400 bottom end. Wrist pins are the same, gaskets match, just working on how to adjust deck height for the 10 mm longer stroke. Works out to almost 300cc.

The 400 has longer stroke? Then the 250's ports will not be in the right place no matter what you do with deck height.

10mm is a bit much, I agree, but it can be made to work with a little porting and playing with deck height.

I had a 1980 390cr that I bought from Pro Circuit . They bored it to 412cc, (called it 420), I had Mongo racing stroke it 4mm. Played with barrel position, ended up with a stock base gasket and a copper ring between the head and cylinder.

Only bike in 33 years of racing that I wish I had kept.

1980 390cr
Fox Forks(overrated), Yamaha twin-leading shoe front brake,Ashe pipe, Mossbarger reed with boost bottle, updated 81 style shocks & swingarm, U.S. Navy submarine insulation for a safety pad on the tank.