folks,
probably been asked before but, will a yz250 barrel go on  to a dt250mx of a similar year (1978).
has anyone done this before?
any tips welcome.
cheers
les.

Doubt it.  Visually they are very dissimilar.  The closest relation to the last of the air cooled YZ250 motors was fitted in the TY250 Pinky and likely the IT250.  The DT is not even close.




 

Swaps like this have been done... not sure of that exact one. Might try searching forum.

The DT engine is almost always simply a detuned version of a YZ model... The challenge is on the YZ they made a lot of changes sometimes and the DT stayed more or less the same. Trick is finding the correct year YZ cylinder and head from the engine that was the model Yamaha used to build the monoshock DT... My guess is to look at 75-76 YZ or IT parts. (IT was a slightly detuned version of the YZ... sometimes cylinder and head was the same as YZ with maybe different pipe, carb and gearbox)

Hopefully one of the experts who may have done this will be along soon. Good luck and let us know how you make out.

MarkT,

i will let you know. the dt250mx barrel i have on it is on its last bore size (the bike i just got running) and it sounds like a can of marbles at tickover, so if i am replacing the barrel i thought i might try a yz250 as there is not much difference in price. on googling this it seems like it should fit, but it would be handy to have the advice of someone who has done it. if that does not come, i will give it a try for a few quid anyway.

cheers
les.
 

First off: there was no such bike as a 1978 DT250MX. Look on your engine and report back with the first three characters of the serial number.
The 1978 DT250E has a different base gasket and bolt pattern than the YZ250s of that era.
If it is a 1975 MX250B you have many options I will list.

International forum. “MX” was used to identify “mono shock” DT models in other parts of the world. 

 

rsteve56

OK, the engine is 1R7, i believe the last of  the dt250 over here anyway.

les.
 

Lots more info on a 400 swap...  but you don't have a 400. 

I did find that piston pin heights changed due to rod length changes in the first couple of years...  75-76 I think.

One thing I found is a UK seller of aftermarket gaskets in the UK...  you might try contacting them and seeing if they can give you any details of the base gasket differences, if any. ebay seller

In my opinion, one reason Yamaha is still such a viable model today is because they "recycled" parts and designs over many models and decades.  There are a few "one off" models but those are the exception...  I think your chances are good at making this work if you can ID the cylinder series...  it may be a 77 that will fit?   

Good luck and happy hunting! 

MarkT

Thanks Mark, i will give it a go and order the barrel. plenty of sources on ebay UK list the same base gasket for the dt/yz/it/ty, and if i have to use the dt piston to get the correct pin height, so be it, it will last longer anyway with the extra skirt. 
i will report back later!
cheers
les.