The only available options are 77-79 DT250 cylinders. There is no interchangability with YZ250.

Good way to check what fits is to get on partzilla or something & check the base gasket shape. Numbers can be different from only minor changes but if the shape & stud pattern looks the same then you're in with a chance. Pays to be sure the stroke it the same too as think IT250's went small bore long stroke in maybe 83 but not sure on earlier YZ's. Generally Yam's 250's stayed at 70x64 right through the early years. I had a new MX250B motocrosser & man that was a good motor. Be good if a top end from that would fit.

I'm not guessing. I have every year air cooled YZ250 cylinder made. The stud pattern is different from his lower end. Fact. Period.

Woops, sorry, my bad!!. I was probably on the page before & not read yours before jumping in. Your the boss & i can't argue my way out. All good.

Might be why there isn't posts on the topic like there is on the 400. 

I could not find definitive evidence that the 77-79 DT250 base gasket fits any of the YZ 250's...  which is why I suggested contacted a gasket manufacturer...  sometimes different part numbers are used but differences are insignificant or non-existent.  Sounds like that may not be the case this time.  Bummer

Me again,

Well, my yz250 barrel arrived and am very pleased with the condition. a DT250 base gasket fits perfectly except that the stud holes are just 2mm out, so they will be redrilled and Roberts your Aunties live-in lover!
i have figured that i can fiddle nuts onto the studs as the cylinder is lowered instead of using the large alloy standard ones.
to start with i am going to use a dt250 piston for longevity and the dt250 cylinder head so as i am not experimenting with timing etc.

i will post pics when there is something interesting to show.
cheers folks
les.

What cylinder did you get? Year and code?

For it to have a chance of performing you realy need the high compression head with the high exhaust port--me thinks!!.

RT325,
good point, i have one that i am looking at on ebay.
anybody know the timing btdc on a 1978-79 yz250E??
cheers
les.

 

Hi again,

Well, here we go. the yz250f barrel successfully grafted onto the dt250 bottom end. for anyone interested in doing this, the only difference between the two barrels is that the yz has the stud holes 2mm inward of where the dt ones are, so my stud holes are slotted in that direction. this gives the slight problem that the alloy sleeve nuts cannot be used because they are off centre in the drop holes, i got round this by removing the studs from the bottom end and then using 10mm allen bolts that have a slightly smaller head diameter that will sit off-centre and still be reached with a long allen key. 
The large alloy exhaust sleeve on the yz cylinder also fouls the upper front engine mount slightly, but there is well enough meat there to remove some, which i have done.
the carb is a lovely flat-slide with a clear float bowl and a dialable power jet that i hope i can adjust while riding the bike, kinda like a chain-saw.
still waiting on the cylinder head and have to modify the exhaust to fit slightly.

cheers
les.