Hi all,

​​​​will a 77-79 dt400 cylinder fit onto a 77-79 dt250 bottom end?

Has anyone tried making their 250 a 400? with a cylinder, conrod and piston?

Lots to consider & i'd say No without researching.
First thing is look online at the base gasket & my bet is 400 is a lot bigger so it all ends there.
Sorry about that.
I made a 280 out of a 250. Then made a 325 out of a 360 swapping cranks "just because i wanted to".
But 250 cylinder stayed on 250 cases & same with 360 cylinder staying on 360 cases & just swapping cranks.
That's the short story but was more involved than 'that'. Like your train of thought though. Cheers.

The more I think about it, I'm getting curious how that 280 concoction worked out?  250 cylinder with the 360 crank?  Compression "could" stay the same, but the "charge" would be more forceful, in a sense boosting the intake charge.  Did you have to make a cylinder spacer, or get a custom piston made?
And on the topic of the later dt250/400's, I wonder if the 400 crank would come close to fitting in the dt250 cases?  Just like your 280cc.

I'd think "without being there" that the 400 crank wheels would be the same OD as the 250.
They just move the pin out further. which is why my older model interchange easily.
I made a cylinder spacer "i'm curtain".
Think they run the same 214 conrod but of course goes 6mm higher with the extra stroke.
Actualy mine was/is a DT1MX top end so i suppose 'much' more pressure from more stroke. I do a lot of guessing.
I think the ring came close to showing in the inlet at bdc but only visible not fully exposed so ring gap ends were safe.
Must admit i didn't run it much & had it in an R5 twin frame then just used to rip up & down our road experimenting.
Had a homemade chamber exhaust & fair turned on but started building power from lowdown.
Just did it to prove a point--someone said "can't do that" so i had to lol.
So the opposite was the case with the 250 crank in a 360.
I shortened the top of the cylinder 6mm.
Not sure what happend at bdc but think i have a 311 conrod in that but anyway it had 180degrees exhaust duration due to shortening the top. Had piston coming right to the top of course--as you do.
But although it ran sweet it idled like a trials motor & turned on at revs, but lacked compression pressure i guess.
Did plan to do something like get an RT2MX head or RT1MX head & take even get some milled off that & see what good pressure does to it.
Conversely my 280 might end up being too much pressure when used in anger & melt down.
Still plan oneday to fit the 280 in my rat race bike to try down the farm & give it a good workout.
Cylinder needed boring really but i'm sure i had a new piston kit in at the time.
One thing i didn't do is lower the ports on the 280 which i should have as piston goes past the exhaust & transfers at bdc.
But had in mind i might want to revbert it to a 250 oneday but gettin too old to worry now.
Be 20 odd years ago when i did that.
Don't think i ever took a video of that running, too long ago. Before easy videoing was invented haha.

I'm very intrigued by the 280 build to say the least.  Have you ever heard of good things, from a bore x stroke setup that is even?  70x70.  I've been told that's the best way, but I don't exactly know why.  Back to my comment on the compression staying the same..  the exhaust port doesn't change, and the piston height stays the same related to the cylinder, the cranking or "static" compression would stay the same.  It would use the extra stroke to boost the intake charge, if that makes any sense.

The longer crank would help extract more power from the power stroke I suppose too.  Could see where pipe design would be thrown right out the window though, cuz you'd probably exhaust more of the intake charge than stock.