Can anyone tell me the diameter of the oil pump check valve ball for my 1970 DT1C 250? 

Part number is 93505-32002-00 and still available.

Diameter is 5/32".  (yes, not metric)

Great, thanks for your help, cheers

And for those that don't already know, the ball dimensions, as a fraction of an inch, are in the part number - last digit of the first batch of five numbers over the first two of the second batch. 

I would add a "usually" to Alan's comment about part number encoding.  

Yamaha did a similar "size encoding" thing with other parts like bolts but unfortunately it's not "always" true these days, possibly due to part supercessions.

The older parts books (many available in the Tech Library here) often show sizes in the part description.  I used a Yamaha Parts manual to confirm the ball size is 5/32"

Bit of trivia... the really early parts books really did a good job of identifying sizes of "generic" parts like bolts, seals, and bearings.  PLUS they even described what other Yamaha models a part would fit!  For example, the parts book for my 1964 MG1T shows which parts it shares with the YG, MJ, and other current Yamaha models at the time.