Hello
Does anyone know the size and thread pitch for the nut that bolts the gas tank down - 1975 DT250
Thank you
 

It's a M10 x 1.25 on my 250A.

I have to take metric bolts/screws down to the local hardware store and thread them on a nut to see what size it truly is... I'm learning... Then when I buy what I need, I double it... nice to have some on hand... in case you "mis-place one.. or many"... 
Sneezles61

And if you are also missing the large washer that goes under that nut and sits on top of the rubber insulator, you can use one of these special washers that are used on automotive shocks and struts. It is cupped like the original washer and works nicely as a replacement. Unless you are looking for exact OEM stuff.
 

This is, " handier than pockets in yer underwear !! "

www.lowes.com/pd/IRWIN-Metric-Thread-Gau...fPD_BwE&gclsrc=aw.ds

I would have thought 8mm x1.25. 10mm sounds a bit overkill to hold the tank down.
Might have to withdraw that statement as found a pic of a nut with that number & looks big. Even if it 'is' 10mm the thread will be 1.25 still.
Edit 3 haha. Didn't see KennyV post, so 10x1.25 it is then.

You can see from the exploded view that the part number indicates M10.

If you're going to a hardware shop 10mm might be 1.5 thread pitch. I know it would be here.
Here is a 1.25 nut. Not sure about the price ha, but hope its not $999.99 it shows.

Here in the US when I was a kid it used to be...  if you could find a metric nut anywhere but a dealership...  the hardware store 10mm nut was 1.5 pitch too. 

But there are so many Japanese products here now and even back then Datsun and Toyota used a 1.25 pitch for a 10mm nut.  So I think it's actually harder to find a 10 x 1.5 nut here in the US than a 10 x 1.25.  Yamaha definitely used 10 x 1.25.

Wrench sizes have varied too... a 17mm wrench size was common for 10mm but Japanese tended to use a 14mm hex size.

This is from the Yamaha Enduro Tool Guide… it’s supposed to cover all bolt sizes and thread pitch’s used on these bikes over the years.